HomeMy WebLinkAboutAppendix A Geotech & vol 1 cvr
KERSEY III PRELIMINARY PLAT
DRAFT
ENVIRONMENTAL IMPACT STATEMENT
TECHNICAL APPENDICES
VOLUME I
Appendix AReport, Geologic, Hydrologic, and Geotechnical Services – GeoEngineers, March 5, 2004
Appendix BAir Quality Analysis – MFG Consultants, March 2, 2004
Appendix CPlants and Animals Assessment – Raedeke Associates, Inc., Auburn, WA, May 17, 2004
Appendix DWetland Assessment – Raedeke Associates, Inc., May 17, 2004
VOLUME II
Appendix EKerseyIII– Average Monthly Volume Calculations (Wetland Hydration), June 2003
Appendix FTransportation Impact Analysis – Transportation Solutions, Inc., March 2004
Appendix GWater Alternatives Analysis – Apex Engineering, March 2004
Appendix HSewer Alternative Analysis – Apex Engineering, March 2004
Appendix IRoad Analysis, Apex Engineering, October 2003
Appendix JArcheological, Anthropological Analysis– Larson Anthropological/Archaeological
Services, November 2002.
Appendix KCity of Auburn Comprehensive Plan Objectives and Policies
Appendix LCrime Prevention Through Environmental Design Methods and Examples
APPENDIX A
REPORT, GEOLOGIC, HYDROLOGIC
AND GEOTECHNICAL SERVICES
GEOENGINEERS
MARCH 5, 2004
CONTENTS
Page No.
INTRODUCTION........................................................................................................................................1
SCOPE OF SERVICES.............................................................................................................................1
EARTH RESOURCES...............................................................................................................................2
TOPOGRAPHY2
Affected Environment2
Significant Impacts2
Mitigation Measures ? Topography3
UnavoidableSignificant Adverse Impacts3
VEGETATION3
SOILS AND GEOLOGY3
Affected Environment3
Significant Impacts7
Mitigation Measures ? Soils and Geology7
UnavoidableSignificant Adverse Impacts8
GEOLOGIC HAZARDS8
Affected Environment8
Significant Impacts 12
Impacts During Construction 12
Impacts During Operation 13
Mitigation Measures ? GeologicHazards 14
Construction Mitigation 14
Operational Mitigation 16
UnavoidableSignificant Adverse Impacts 16
WATER....................................................................................................................................................16
SURFACE WATER CONDITIONS 16
Affected Environment 16
Significant Impacts 17
Mitigation Measures ? SurfaceWater 17
UnavoidableSignificant Adverse Impacts 18
GROUNDWATER18
Affected Environment 18
Significant Impacts 20
Mitigation Measures ? Groundwater 21
UnavoidableSignificant Adverse Impacts 22
LIMITATIONS...........................................................................................................................................22
REFERENCES.........................................................................................................................................23
FIGURES Figure No.
VICINITY MAP 1
SITE PLAN ? ALTERNATIVE 481 2
SITE PLAN ? ALTERNATIVE 700 3
SITE AND EXPLORATION MAP 4
INTERPRETED GEOLOGIC MAP 5
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CONTENTS (Continued)
FIGURES (Continued) Figure No.
INTERPRETED CROSS SECTION A-A? 6
INTERPRETED CROSS SECTIONS B-B? AND C-C?7
STREAM RECONNAISSANCE8
SHALLOW GROUNDWATER EQUIPOTENTIAL MAP 9
SOIL EROSION HAZARD AREAS 10
LANDSLIDE HAZARD AREAS 11
SEISMIC HAZARD AREAS12
APPENDICES Page No.
APPENDIX A ? FIELD EXPLORATIONS AND LABORATORY TESTING PROCEDURES................A-1
FIELD EXPLORATIONS A-1
LABORATORY TESTING A-1
APPENDIX A FIGURESFigure No.
SOIL CLASSIFICATION SYSTEMA-1
KEY TO LOG SYMBOLS A-2
LOGS OF TEST BORINGS A-3...A-7
LOGS OF TEST PITS A-8...A-30
SIEVE ANALYSIS RESULTS A-31?A-39
Page No.
APPENDIX B ? WATER BUDGET EVALUATION.................................................................................B-1
INTRODUCTION....................................................................................................................................B-1
DATA SOURCES...................................................................................................................................B-1
EXISTING CONDITIONS.......................................................................................................................B-1
Land Type Distribution B-1
Surface Water Runoff B-1
Evapotranspiration B-2
Groundwater Recharge B-2
Shallow Groundwater Flow to Wetlands B-2
DEVELOPED CONDITIONS (ALTERNATIVE 481)..............................................................................B-3
GENERALB-3
Land Type Distribution B-3
Surface Water Runoff Estimate B-3
Evapotranspiration Estimate B-4
Groundwater Recharge Estimate B-4
Shallow Groundwater Flow to Wetlands B-4
REFERENCES.......................................................................................................................................B-5
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CONTENTS (CONTINUED)
APPENDIX B TABLES Table No.
SELECTEDWATER BUDGET PARAMETERS B-1
SUMMARYOF AVERAGE ANNUAL WATER BUDGET FOR LAND TYPESB-2
WATER BUDGET FOR EXISTING CONDITIONSB-3
SHALLOW GROUNDWATER FLOW TO WETLANDS UNDER EXISTING CONDITIONSB-4
WATER BUDGET FOR ALTERNATIVE 481 B-5
SHALLOW GROUNDWATER FLOW TO WETLANDS UNDER ALTERNATIVE 481B-6
APPENDIX B FIGURESFigure No.
ANNUAL PRECIPITATION B-1
MONTHLY PRECIPITATIONB-2
SHALLOW GROUNDWATER LEVEL MONITORING B-3
Page No.
APPENDIX C ? SEDIMENT BUDGET ANALYSIS................................................................................C-1
INTRODUCTION....................................................................................................................................C-1
DATA SOURCES...................................................................................................................................C-1
SOIL LOSS ESTIMATES.......................................................................................................................C-1
Rainfall/Runoff Erosivity (R) C-2
Soil Erodibility (K) C-2
Hillslope Length and Steepness (LS) C-2
Cover-Management (C) C-2
Support Practices (P) C-2
Average Annual Soil Loss (A) C-3
REFERENCES.......................................................................................................................................C-3
APPENDIX D ? GEOTECHNICAL RECOMMENDATIONS..................................................................D-1
GEOTECHNICAL RECOMMENDATIONS............................................................................................D-1
GENERALD-1
EARTHWORKD-2
GeneralD-2
Clearing andSite Preparation D-2
Subgrade Preparation D-3
Structural Fill D-3
Temporary Cut Slopes D-4
Permanent Cut and Fill Slopes D-5
Utility Trenches D-5
Sedimentation and Erosion Control D-6
FOUNDATIONSD-6
GeneralD-6
Foundation Design D-6
Foundation Settlement D-7
Lateral Resistance D-7
Footing Drains D-7
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CONTENTS (Continued)
Page No.
BELOW GRADE WALLS ANDRETAINING WALLSD-8
GeneralD-8
Design ParametersD-8
Backdrainage D-8
Construction ConsiderationsD-9
FLOOR SLAB SUPPORT D-9
SEISMICITYD-10
GeneralD-10
Ground ShakingD-10
UBC Site CoefficientsD-10
Liquefaction Potential D-10
Ground RuptureD-10
Landslides D-11
PAVEMENT RECOMMENDATIONS AND SUBGRADE PREPARATION D-11
Subgrade PreparationD-11
Asphalt Concrete PavementD-11
Asphalt-Treated BaseD-11
STORMWATERPONDS D-12
DRAINAGE CONSIDERATIONSD-12
LIMITATIONSD-12
APPENDIX E ? REPORT LIMITATIONS AND GUIDELINES FOR USE.....................................E-1?E-3
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REVISED REPORT
GEOLOGIC, HYDROGEOLOGIC AND
GEOTECHNICAL ENGINEERING SERVICES
PROPOSED KERSEY III SUBDIVISION
AUBURN, WASHINGTON
FOR
APEX ENGINEERING
INTRODUCTION
This report provides a summary of our geologic,hydrogeologic and geotechnical engineering services
for the proposed Kersey III subdivision. This reportis presented in support ofan Environmental Impact
Statement (EIS) for the proposed Kersey III subdivision. The site is located on the side of an upland
plateau, between the White River and Lake Tapps, inAuburn,Washington (Figure 1). The site consists
of an approximate 170-acre undeveloped parcel located in the southern half of Section 32, Township 21
North, Range 5 East, Willamette Meridian (W.M.). There is a Bonneville Power Administration (BPA)
transmission line easement through the eastern portion of the site. The site is accessible via Kersey Way
th
Street SE.
and 49
Alternative 481 (Preliminary Plat) for the Kersey III subdivision includes the construction of 481
single-family homes and 72 multi-familyunits. Thelayout of Alternative 481 is shown on Figure 2.
Alternative 700 (PlannedUnit Development [PUD])includes the construction of 700 single-family homes
and 72 multi-family units. The layout of Alternative 700 is shown on Figure 3. Alternative 481 and
Alternative 700 both include the construction of two stormwater detention ponds in the northern portions
of the site. Alternatives 481 and 700 both include theoption of installing a sanitary sewer system within
Kersey Way(Apex Engineering, 2003). The No Action Alternative would allow development under the
existing Comprehensive Plan, andexisting zoning and subdivision regulations. The No Action
Alternative assumes subdivision of the site into approximately 34 individually developed 5-acre lots.
SCOPE OF SERVICES
Our role has focused on the Earth and Water elements of the EIS. We provided geologic,
hydrogeologic and geotechnical services to evaluate the followingcomponents of thealternatives analysis:
1) sanitary sewer line; 2) surface water and groundwater; 3) stormwater detention and discharge;and
4) landslide, seismic and erosion potential.
Our specific scope of services included the following tasks:
1.
Review of existing information regarding surface and subsurface conditions in the vicinity of the
proposed subdivision. Our review included the following information: documentation of previous
on-site subsurface explorations, aerial photographs, geologic maps and reports, hydrogeologic
reports, water well records, and climatologic data.
2.
Site and stream reconnaissance.
3.
Supplemental subsurface soil and groundwater data were obtained from the following subsurface
explorations: 1) excavation of 23 test pits throughout the site to characterize shallow soil and
groundwater conditions; and 2) drilling of five borings to maximumdepths of approximately 60 feet
and installing three piezometers to evaluate soil and groundwater conditions.
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4.
Laboratory testing on selected soil samples to characterize hydrogeologic and engineering properties.
5.
Six site visits over a 10-month period to measure groundwater levels within the three piezometers.
6.
Water budget calculations to evaluate surface and subsurface flow to streams and associated wetlands
in the site vicinity. Water budgets were calculatedfor the site under existing conditions and under
Alternative 481 (developed conditions that would cause the largest potential impact to surface water
and groundwater conditions).
7.
Stream evaluation and sediment budget analysis. The stream evaluation is a qualitative (not
quantitative) assessment of potential impacts fromstormwater discharge. The sediment budget uses
the Revised Universal Soil Loss Equation.
8.
Geotechnical and engineering geology evaluation that includes landslide potential, erosion and
sedimentation, and the potential impacts of a proposed sewer line along Kersey Way and Bowman
Creek.
EARTH RESOURCES
TOPOGRAPHY
Affected Environment
The site is located along the northeastern side of an upland plateau between the White River and Lake
Tapps. The site consists of an alternating series of three ravines and two ridges that generally slope
downward from south to north. Site topographyis shown on Figures 1 and 4.
The western ravine intersects the southern site boundary at an elevation of approximately 470feet and
slopes down toward the northeast where it intersects the northern site boundary at an elevation of about
220 feet. The westernmost portion of the site includes slopes that are generally greater than 30 percent.
The central portion of the site is occupied by a ridge that has a maximum elevation of approximately
570 feet at the southern site boundary, and slopes down toward the north withan average grade of about
15 percent. Two relativelynarrow north-northwest trending ravines, separated by a ridge, are located
within the eastern portion of the site. Slopes along the eastern ridge are commonly greater than
25 percent.
Significant Impacts
The design of the development will be heavily influenced by the existing topography. In general, site
development will attempt to followexistingcontours,but cuts on the order of 25 feet and fills on the
order of 20 feet are still anticipated for construction of the main roadway through the proposed
subdivision under Alternatives 481 and 700. Secondary roads and cul-de-sacs willalso require
modification of the existing topography to achieve smooth transitions between the main roadway and
building lots. Total grading volumes are expected to be on the order of 450,000 cubic yards.
Most of the building lots will require some grading to establish the building pads and to connect to
the roadways and utilities. Alternative 700 has a higher lot density, but smaller footprint than
Alternative481. The smaller footprint of Alternative 700 may result in smaller impacts to topography
than Alternative 481. The No Action Alternative is assumed to have smaller topographic impacts than
Alternative 481 and Alternative 700 because of the significantly smaller number of anticipated building
lots.
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Mitigation Measures ?Topography
Cuts and fills planned for Alternatives 481 and 700 will likelybequite extensive because of the
slopingterrain caused bythe series of ridges and ravines. Cuts on the orderof 25 feet and fills up to
about 20 feet are anticipated based on the conceptual plans. The steep slopes resulting from cuts that
range from 5 to 15 feet high can typicallybe held in place using conventionalgravity retaining walls or
by gradingthe slopes 2H:1V (horizontal:vertical) or flatter. Cuts in excess of 15 feet that cannot be
sloped back could require tieback, soil nail, or similar engineered walls.
Fills compacted in place to support the new roadwaysand homes should be designed to accommodate
the type of fill material used and the underlying soilconditions. Permanent fill slopes will generally be
inclined at 2H:1V or flatter. Retaining walls can be used to limit the lateral extent of the fills. Potential
retaining wall options for fill applications include concrete cantilever walls, concrete masonry unit
(CMU) block walls, mechanically stabilized earth (MSE) walls, and soldier pile walls. Mitigation
measures to control settlement as a result of embankment construction are discussed below.
No mitigation measures are currently proposed forthe No Action Alternative, but similarmeasures
proposed for Alternatives481 and 700 could be expected, depending on specific constructions plans for
the No Action Alternative.
Unavoidable Significant Adverse Impacts
Constructionof roadways and utilities under Alternative 481 or Alternative 700 will result in the
modification of slopes that are greater than 40 percent. The extent of slope modification under the No
Action Alternative would be dependent uponspecific designs and construction plans.
VEGETATION
Approximately 90 percent of the site consists of forested areas with second growth conifers (primarily
Douglas fir) and deciduous species (e.g., alder and big leafmaple). Undergrowth includes salmonberry,
sword fern, salal, Oregon grape, blackberry, grass, and other species typical of forested areas in the Puget
Sound region. Undergrowth in wetland areas located in the western portion of the site also includes
nettles, skunk cabbage and vine maple.
The remaining 10 percent of the site includes approximately 12.5 acres of grass and shrubs within the
cleared BPAtransmission line easement, and about 4.5 acres of grassland in the northwestern portion of
the site (near B-1, B-2, TP-2, TP-3 and TP-4, as shown on Figure 4). Additional descriptions of the
existing vegetation, including significant impacts,mitigation measures and unavoidable significant
adverse impacts are presented in the report by Raedeke Associates, Inc. (2003a).
SOILS AND GEOLOGY
Affected Environment
Soils.
Thecharacteristics of surficial soils are the result of the combined influence of the following
six factors:(1) the parent material fromwhich the soil was derived; (2) climate; (3) living organisms;
(4)topographic effects; (5) the length of time thatthe soil has been developing; and (6) modification by
humans. The surficial soils throughout the Puget Lowland, including the site, have developed on
materials that were deposited, or exposed by erosion, during themost recent glaciation of this area
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(referto the following Geology section). The soils have been forming for about 13,000years, since the
retreat of theglacier, which is a relatively short timeby geologic standards. The soil-forming processes
generally have not greatlymodified the characteristics of the geologic materials from which the surficial
soils are derived.
All on-site soils have beenmapped by the USDA Natural ResourceConservation Service
(NRCS,formerly knownas the USDA Soil Conservation Service) as part of the Alderwood series
(Snyder et al., 1973), which consists of moderatelywell drained soils formedin glacial deposits. The
entire site is mapped as an Alderwood gravelly sandy loam, with a 6 to 15 percent slope designation
(AgC) over the majority of the site, and a 15 to 30 percent slope designation (AgD) in two areas with
steeper slopes (Snyder et al., 1973).These two areasmapped as AgD by NRCS are located in the
northeastern portion of the site along Kersey Way, and along the southern half of thewestern site
boundary (shown as reference erosion hazard areasonFigure 10). Soil typedistributions mapped on
Figure 10 (AgC and AgD) are a refinement of the NRCSmap,based on site-specific topographic and
geologic information.
The permeability of Alderwood gravelly sandyloam is moderately rapid in the surface layer and
subsoil, and very slow in the substratum (Snyder et al., 1973). Runoff is slow to medium in areas mapped
as AgC, and medium in areas mapped as AgD. The erosion hazard is moderate in areas mapped as AgC
(Figure 10), and severe in areas mapped as AgD (designated as “erosion hazard area” on Figure 10).
Geology.
In general, the site is underlain by a ½- to 1-foot thickness of forest duff and topsoil,
which in turn overlies one of four distinct geologic units – namely fill, ice contact deposits, glacial till and
advance outwash. The fill is undocumented, and some of it was observed to contain organicmatter. The
ice contact, glacial till, and advance outwash deposits are primarily sandy materials with variable, clay,
silt, gravel, cobble and boulder content.
The native glacial soil will provide suitable bearing support for the new infrastructure (homes, roads,
retaining walls, embankments, etc.). The organic soils (forest duff and topsoil) and non-engineered fill
are often relatively compressible, and large settlements can occur when new loads are placed over these
materials. Mitigation will be required to address the potential settlement associated with these materials.
Subsurface explorations completed for this evaluation included the drilling of five borings
(B-1throughB-5), and the excavation of 23 test pits (TP-1 through TP-23).Three of the five borings
werecompleted as piezometers. The exploration locations are shown on Figure 4. A description of the
fieldmethodology and logs of the exploration borings and test pits are presented in Appendix A.
Laboratory testing results of selected soil samples from the borings and test pits are alsopresented in
Appendix A.
Figures 5, 6 and 7 consist of a surficial geologic map and three cross sections of the site based on the
existing literature (Mullineaux, 1965a; Luzier, 1969; Woodward et al., 1995), previous subsurface
explorations in the site vicinity (Hart-Crowser &Associates, Inc. [Hart-Crowser], 1982; Anderson Design
Consultants, Inc. [Anderson], 1999; Earth Consultants Inc., 2000; Washington State Department of
Ecology [Ecology], 2002a) and the subsurface explorations completed for this evaluation.
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The geologic map (Figure 5) identifies four primary units, fromyoungest to oldest: 1) Fill (Hf);
2)Vashon ice contact deposits (Qvi); 3) Vashon till (Qvt); and 4) Vashon advance outwash deposits
(Qva). The ice contact, till, and advance outwash units were deposited during themost recent glaciation
of the region, which occurred 13,000 to 15,000yearsago (during the Quaternaryperiod), and is known as
the Vashon stade of the Fraser glaciation. Recently (Holocene epoch) placed fill (artificially placed soil)
is present over glacial deposits (Qvt or Qvi) in the southwestern portion of the site. Only those areaswith
an observed or inferred fill thickness of approximately5 or more feet are mappedon Figure 5.
Fill located within the southwestern portion of the site includes subgrade material for an existing
unpaved roadway, and dumped yard waste. The subgrade fill generally consists of medium dense silty
sand with gravel and occasional cobbles, and appears to be re-worked till (Qvt) and ice contact deposits
(Qvi) that originated on site. Subgrade fill encountered in test pits TP-11 and TP-12 was approximately
10 feet thick. A previously completed test pit along this roadway (Earth Consultants Inc., 2000) also
encountered fill with a minimum thickness of 11 feet (ECTP-2, located on Figure 4). It is our opinion
that the thickness of fill may exceed 11 feet in the vicinity of these test pits, based on surface conditions.
An approximate 15 to 20-foot high mound of yard wastewith a diameter of about 30 feet was observed
along the western site boundary, near Evergreen Way SE. The yard waste is a loosemixture of grass
clippings and debris. Minor amounts of fill (less than 4 feet thick) consisting of loose to medium dense
silty sand were observed in test pits TP-4 and TP-20.
Vashon ice contact deposits (Qvi) at the site generally consist of amedium dense to dense mixture of
sand, silt and gravel, with occasional cobbles. Qvi was encountered beneath fill in test pits TP-4 and
TP-20. Qviwas also encountered at or just below the ground surface in all five borings and six of the test
pits (TP-2, -6, -10, -13, -15- and -16).These sediments were deposited by meltwater on or adjacent to
glacial ice. Vashon ice contact deposits are mapped within the western, central, and lower-elevation areas
of the site (Figure 5).The on-site Vashon ice contact deposits are interpreted to have a maximum
thickness of approximately 40 feet, and are underlain by Vashon till or Vashon advance outwash
(Figures6 and 7). Our subsurface explorations indicate thatQvi is morewidely distributed at the site
than indicated by existing geologic maps (Mullineaux, 1965a; Luzier, 1969). The relatively low
permeability of the observed ice contact deposits would limit the utility of a stormwater infiltration pond
at the locations of the proposed detention ponds (near borings B-1 and B-4). This finding of limited
infiltration potential is consistent with those presented by Earth Consultants Inc. (2000).
Vashon till (Qvt) at the site generally consists of dense, gray to gray-brown silty sand with occasional
cobbles and boulders that was deposited at the base of the glacier and overridden by thousands of feet of
ice. Qvt was identified beneath fill in test pit TP-12, and beneath Vashon ice contact deposits in borings
B-1 and B-2, and three of the test pits (TP-4, -10 and -15). Vashon till was also encountered at or just
below the ground surface with a minimum thickness of six feet in 10 of the test pits (TP-1, -3, -5, -7, -8,
-9, -14, -17, -22 and –23). Qvt is thicker than 40 feet at boringsB-1 and B-2, and is inferred to have a
maximum thickness greater than 100 feet beneath the westernmost portion of the site (based on well logs
of PW-1 and PW-2 [Ecology, 2003a],located on Figure 1). Till was also identified in four previously
completed on-site test pits (Earth Consultants Inc.,2000; ECTP-6, -7, -8, and -9, located on Figure 4).
Vashon till is generally mapped within the higher-elevation areas of the site (Figure 5). The Vashon till
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unit is likelyunderlain byVashon advance outwash or Salmon Springs drift, as shown on Figures 6 and 7
(based on well logs of PW-1, PW-2, TW-1 and T-6 [Ecology, 2003a], located on Figure 1). The
relatively low permeabilityof till observed at the surface and shallow subsurface would limit the utilityof
a stormwater infiltration pond at the location of the proposed detention pond near boring B-1. This
finding of limited infiltration potential is consistent with those presented by Earth Consultants Inc.
(2000).
Vashon advance outwash (Qva) at the site generally consists of dense, gray-brown gravelly sand with
cobbles and silt. Qva was encountered beneath an approximate 33-foot thickness of Vashon ice contact
deposits in boring B-4. Boring B-5, and test pits TP-18 through TP-21 encountered Qva within five feet
of the ground surface. The log of a 320-foot deep on-site well (TW-1, located on Figures 1, 4 and 5)
drilled during 1979 (Ecology, 2002a) is interpreted to have encountered an approximate 200-foot
thickness of Qva in the southern portion of the site. These sediments were deposited by streams flowing
from the advancing ice sheet during the early part of the Vashon stade. Vashon advance outwash is
mapped within the north-central portionof the site (Figure 5). The Vashon advance outwash is inferred to
have a maximum thickness of about 200 feet, and is underlain by Salmon Springs drift (Figures 6 and 7).
Inferred contacts with Qva in cross section B-B’ (Figure 7) are based on the logs of two off-site wells
(PW-1 and PW-2, located on Figure 1) obtained from Ecology (2002a). Zones of relatively low
permeability observed within the advance outwash deposits at boring B-4 could cause significant
groundwater mounding that may limit the function of a stormwater infiltration pond at the location of the
proposed detentionpond near B-4.
Salmon Springs drift (Qss) is not exposed at the site and was not encountered in our subsurface
explorations(borings B-1 through B-5, and test pits TP-1 through TP-23).Existing geologic maps
(Mullineaux,1965a; Luzier, 1969) indicate that Qss is exposed between the approximate elevations of
100 and 300feet within one mile north and west ofthe site. The log of a 320-footdeep on-site well
(TW-1, located on Figures 1, 4 and 5)drilled during 1979 indicates that Qss beneath the site consists of
gray toyellow “hardpan”, gravel, sand and minor amounts of clay (Ecology,2002a). These sediments
were likely deposited during the Salmon Springs glaciation, prior to the Fraser glaciation.The Salmon
Springs driftconsists of glacial and interglacialsediments, including fluvial sand and gravel deposits.
The contact between Qss and the overlying Qva is interpreted to occur at an elevation of about 320 feet in
the vicinity of TW-1. The Puyallup Formation is inferredto underlie Qss beneath the site at an
approximateelevation of 110 feet (Figure 6).
The Puyallup Formation (Qpy) is not exposed at the site and was not encountered in our subsurface
explorations. Existing geologic maps (Mullineaux, 1965a; Luzier, 1969) indicate that Qpyis exposed
between the approximateelevations of 100 and 125 feet within onemile north of the site. The log of a
500-foot deep test well located near the eastern site boundary (T-6,located on Figure 1) indicates that
Qpy consisting of interglacial clay, silt and sand was encountered between the approximateelevations of
80 and 120 feet (Hart-Crowser, 1982).
Regional geologic maps and cross sections (Luzier, 1969; Woodward et al., 1995) indicate that Qss
and Qpy are underlain bya sequence of older glacial and interglacial deposits that extend to an inferred
elevation of approximately–1,000 feet in the site vicinity. These older glacial and interglacial deposits
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are underlain by Tertiary sedimentary bedrock with andesite intrusions (Mullineaux, 1970).Bedrock and
the overlying sequence of older glacial and interglacial deposits were not encountered in any of the
on-site borings and are not exposed in the site vicinity.
Significant Impacts
We anticipate that the project will attempt to balancecuts and fills at the site to reduce the amount of
import or export of materials. The existing ice contact and glacial till materials contain an appreciable
fines content rendering their workability very sensitive to the soil moisturecontent. If themoisture
content of these soils is more than a few percent above their optimummoisturecontent, theybecome very
difficult, if not impossible to compact to structural fill specifications. Operation of equipment on these
soils will also be very difficult during wet weatherconditions. The native advance outwash deposits
contain fewer fines and are generally less sensitive to moisture.
Zones and lenses of relatively permeable sedimentswere observed in the ice contact, till and advance
outwash deposits. There is a potential for significant stormwater leakage into the subsurface if detention
ponds are constructed in these deposits. The proposed stormwaterdetentionpond near boring B-1 is also
in the vicinity of steep slope areas. Significant stormwater leakage in the vicinity of steep slopes could
potentially affect slope stability.
Similar impacts to soils and geology are expected fromAlternatives 481 and 700. The smaller
footprint of Alternative 700 may result in smaller impacts to soils and geology than Alternative 481 The
No Action Alternative is assumed to have smaller soil and geologic impacts than either Alternative 481 or
700becauseof the significantly smaller number of anticipated building lots.
Mitigation Measures ? Soils and Geology
The potential impacts of the existing soil and geology conditions on site development generally fall
into one of three categories, namely:
1.
Settlement due to placing new loads (structures or fill embankments) over potentially compressible
materials, such as forest duff and undocumented, existing fill.
2.
Earthwork constraints associated with excavating,hauling, placing and compacting moisturesensitive
soils, such as the native ice contact and glacial till materials.
3.
Excessive leakage of stormwater into the soils below detention ponds if zones or lenses of relatively
free-draining materials are exposed in the pond bottoms.
The mitigation measures described below are appropriate for Alternatives 481 and 700.
The potential settlement issues can be partially mitigated by using proper site preparation techniques
that include removal of all surficial organicmaterials(vegetation, forest duff, topsoil, and large roots)
from below proposed infrastructure and new fill locations. The existing fill on site is undocumented and
likely non-engineered (not placed to specified compaction criteria). In addition, we observed some of the
existing fill to contain an appreciable organic content. Unless subsequent exploration and testing
indicates portions of the existing fill meet structural fill specifications, all existing fill should be removed
from below proposed infrastructure or new fill embankment locations.
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Impacts associated with earthwork using the on-site, moisture sensitive soils can be mitigated by
limiting earthwork activities to the dry season, typically considered to extend from June through October
in the greater Puget Sound region. Even during the “dry season”, periods of wet weather are likely, and it
may be necessary to limit earthwork activities during those times. Also, it may be necessary to moisture
condition (dryback) soils if theybecome too wet during inclement weather or their natural moisture
content is appreciably above their optimummoisture content. If earthwork activities occur during the
“wetseason”, those earthwork activitiesmay need to be limited to windows of good weather, or
all-weather fill may need to be imported to the site. Alternatively, admixtures such as lime or cement can
be used to improve the workability of on-site, moisture sensitive soils during wet weather conditions.
Depending on the specific design and location with respect to steep slopes, and if relatively free-
draining materials, such as permeable zones or lenses of sand and gravel, are exposed in the bottom of the
proposed stormwater detention ponds, it will be necessary to line those ponds with relatively impermeable
materials. The liners could consist of natural soil liners or geosyntheticmembranes. The on-site ice
contact and glacial till soils may be suitable for use as natural soil liners and detention pond embankment
fill, provided theypossessa suitably high fines content and can be compacted to the requisite compaction
levels.
Unavoidable Significant Adverse Impacts
No unavoidable significant adverse impacts to soils or geology are anticipated from Alternative 481,
Alternative 700 or the No Action Alternative.
GEOLOGIC HAZARDS
Affected Environment
Erosion Hazards.
Erosion of soil is a natural, ongoing physical process by which sediment is
removed from topographic high points and transported down gradient by a variety of geomorphic
processes. The erosional processes most commonly encounteredwithin and adjacent to the site include
soil creep and sheet wash, slope ravel, and rill and gully erosion.Erosional processes may be accelerated
during construction by removing vegetation andexposingnative soils. Removal of vegetation,
modification of topography and unmanaged stormwater runoff commonly contribute to increased erosion
rates. Somesoils are particularly susceptible to erosion because of particle sizegradation and/or density.
The rates of various erosion processes can be significantly reduced during and following construction by
implementing conventional constructionpractices, designed and constructed to reduce erosion impacts.
Erosion hazard areas are defined by the Auburn City code as those “areas identified by the USDA
Soil Conservation Service as having a severe rill and inter-rill erosion hazard” (City of Auburn, 1996a).
Erosion hazard areas, as identified by the City of Auburn Erosion Hazard Areas map (City of Auburn,
1996b), are shown on Figure 10 as reference erosion hazard areas.
Erosion hazard areas are defined by King County as those areas underlain by soils which are subject
to severe erosion while disturbed, and include those classified as having a severe to very severe erosion
hazard, including Alderwood gravellysandy loam on slopes 15 percent or steeper (King County, 1993).
This definition is consistent with the definition used by the Cityof Auburn. The entire site has been
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mapped as Alderwood gravelly sandy loam by the NRCS (Snyder et al., 1973). King County’s definition
of erosion hazard areas has been incorporated into our evaluation as a reference.
Erosion hazard areas mapped on Figure 10 havebeen identified on all slopes with a grade of
15percent or more. Theerosion hazard areas distribution shown on Figure 10 is a refinement of those
areas identified by the NRCS (Snyder et al., 1973) and Cityof Auburn (City of Auburn,1996b), based on
site-specific topographic and geologic information.
Based on a sediment budget analysis using the Revised Universal Soil Loss Equation (RUSLE), it is
estimated that approximately 5.5 tons per year of soilis eroded within the site under existing conditions.
Note that RUSLE estimates only the gross amount of soil moved from its original position; it does not
estimate net soil erosion (gross erosion minus deposition). The actual volume of soil eroded from a site is
generally much smaller than calculated by RUSLE. The sediment budget analysis is summarized in
Appendix C.
Landslide Hazards.
Landsliding is the slow to rapid, downslope movement of amass that
includes rock, soil and/or vegetative cover. The failuresmay occur as planar slides, block slides,
rotational slumps, debris avalanches and mudflows. Landslidingusually occurs on steep slopes and is
commonly initiated during periods of intense or prolonged rainfall when the water table is high.
Landsliding also can be initiated by removinglateral support from the toe of a slope or byoverloading the
slope with fill material or water.
The City of Auburn classifies landslide hazard areas as follows (City of Auburn, 1996b):
Class I – Known landslide hazard
Areas of known landslide hazard will be identified using the following criteria:
A combination of slopes greater than 15% underlain by silt or clay.
Evidence of movement during the Holocene Epoch (from 10,000 years ago to present), or the
occurrence of mass wastage debris
Areas designated by UGSG and/or DNR as Quaternary slumps, earthflows or landslides
Canyons potentially subject to inundation bydebris flows or catastrophic flooding
Slopes which could potentiallybecomeoversteepened and unstable as a result of streamerosion
Slopes greater than 40% with a vertical relief of 10 or more feet.
Class II – No landslide hazard
Areas of no landslide hazard will consist of slopes less than 15% and not meeting anyof the criteria
for Class I.
Class III – Unknown landslide hazard
Areas of unknown landslide hazards will be those hillslopes between 15% and 40% which are not
underlain by silt or clay.
Class I landslide hazard areas, as identified by the Cityof Auburn (Cityof Auburn, 1996), are shown
on Figure 11 as referenceClass I landslide hazard areas. The remainder of the site is identified as Class II
(no landslide hazard) or Class III (unknown landslide hazard) areas by the City of Auburn (City of
Auburn, 1996).
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Landslide hazard areas are also defined by the Auburn City code as those “areas potentially subject to
landslides based on a combination of geologic, topographic and hydrologic factors” (Cityof Auburn,
1996a). As an additional reference, landslide hazardareas are defined by King County as: (1) any areas
with slopes greater than 15percent that are underlain by impermeable soils and that include springs or
groundwater seepage; (2) landslides that have movedduring the last 10,000 years; (3) areas that are
potentially unstable as a result of rapid stream incision, stream bank erosion or undercutting by wave
action; (4) areas which show evidence of or is at risk from snow avalanches; or (5) areas located on
alluvial fans that are presently subject to or potentially subject to inundation bydebris flows or deposition
of stream-transported sediments (King County,1993).
Landslide hazard areas have been mapped on Figure 11 for areas where groundwater seepage was
observed on slopes greater than 15 percent that are underlain by impermeable soils, and all areas with
slopes greater than 40 percent. The landslide hazard areasdistribution shown on Figure 11 is a
refinement of those areas mapped bythe City of Auburn as Class I landslide hazard areas (City of
Auburn,1996b), based on site-specific field observations and topographic information. The remainder of
the site is classified as Class II (no landslide hazard), based on site-specific field observations and
topographic information.
Seismic Hazards.
The Puget Lowland area is a seismically active region that has experienced
thousands of earthquakes in historical time. Based on past earthquake activity, the UniformBuilding
Code assigns the Puget Lowland region a Zone 3 rating for seismicactivityon a scale of 1 (lowest)
to4(highest). Seismic hazards represent risk of injury or damage to humans and property resulting
directly fromearthquakes.Seismichazardmechanisms include surface fault rupture, ground shaking and
associated ground failure such as liquefaction and landsliding.Liquefaction is the loss of strength by
loose, saturated soil when subjected to vibration or shaking.
A review of the geologic map for Auburn (Mullineaux, 1965a) indicates that no faults have been
mapped in the immediatevicinity of the site. The closestmapped fault is located approximately six miles
east of the site (Mullineaux, 1965b). This north-south trending fault is exposed near the Green River,
where it is down-thrown on the east side and cuts through pre-Vashon glacial deposits (Mullineaux,
1965b). Recent scientific articles (e.g., Bucknam et al., 1992) suggest that fault movement in the southern
Puget Sound area may have occurred between 500 and 1,500 yearsago. Based on the available data,
surface fault rupture is, in our opinion, unlikely to occur at the site.
Historicalevidence collected bythe U.S. Geological Survey suggests that the number and location of
seismically triggered landslides are related to other known factors affecting landsliding, such as material
type, slope inclination, and groundwater conditions (Keefer, 1984). Therefore, areas at risk for
seismically triggered landslides are the same areas identified as landslide hazard areas.However, the
U.S. Geological Survey data indicate that seismic triggering of landslides is less common in the Pacific
Northwest than in other seismically active areas partly because of the typically greater focus depth of
earthquakes in the Pacific Northwest (Keefer, 1984).
The City of Auburn classifies seismic hazard areas as follows (City of Auburn, 1996b):
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Class I – Known seismichazard
Areas of known seismic hazard will be identified using the following criteria:
The presence of Holocene alluvium
Class I – Known landslide hazard areas.
Class II – No seismic hazard
Areas of no seismic hazard will include terrain not included in Class I – Known Seismic Hazard areas
or Class III – Seismic Hazard Unknown areas.
Class III – Unknown seismic hazard
Areas of unknown seismic hazard will include terrain comprising of recessional deposits not included
in Class I – Known Seismic Hazard areas.
Class I seismic hazard areas, as identified by the City of Auburn (City of Auburn, 1996), are shown
on Figure 12 as referenceClass I seismic hazard areas. The remainder of the site ismapped as Class II
(no seismic hazard) areas by the City ofAuburn (City of Auburn, 1996).
Seismic hazard areas arealso defined by the Auburn City code as those “areas subject to severe risk
or damage as a result of earthquake induced ground shaking, slope failure, settlement, soil liquefaction or
surface faulting” (City of Auburn, 1996a). As an additional reference, seismic hazard areas are defined
by King County as those areas “subject to severe riskof earthquake damageasa result of soil liquefaction
in areas underlain by cohesionless soils of low density and usually in association with a shallow
groundwater table or of other seismically induced settlement” (King County, 1993).
Seismic hazard areas on Figure 12 have been identified for areas designated as landslide hazard areas
on Figure 11. The seismic hazard areas distribution shown on Figure 12 is a refinement of those areas
mapped by the City of Auburn as Class I seismichazard areas(City of Auburn, 1996b), based on site-
specific field observations and topographic information.The remainder of the site is classified as ClassII
(no seismic hazard), based on site-specific field observations and topographic information.
Volcanic Hazards. Volcanic hazard areas are defined by the City of Auburn Comprehensive Plan
(City of Auburn, 2002) and generallyby King County (King County, 1993) as areas having the potential
for floods (which could include mudflows or lahars) resulting from volcanicactivity on Mount Rainier.
No volcanic hazard areas are identified on or adjacent to the site in the potential hazardsmap for
MountRainier (Crandell, 1973), which evaluates risk with respect to the Electron Mudflow.No volcanic
hazard areas are identified on or adjacent to the siteby the Cityof Auburn (City of Auburn, 1996). The
nearest volcanic hazard areas are identified as low risk areas locatedmore than half a mile north and
northwest of the site, within the White River valley(Crandell, 1973). The site is also located outside of
lahar inundation zones, pyroclastic-flow hazard zones, and post-lahar sedimentation areas identified in
another volcano hazards evaluation for Mount Rainier (Hoblitt et al., 1998).
Coal Mine Hazards.
The principal issues regarding public safety and property damagerelated to
abandoned coal mines include: (1) sinkholes and related gas emissions or concentrations; (2) trough
subsidence; and (3) coal spoils. Coal mine hazard areasare defined as those areas “underlain or directly
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affected by operative or abandoned subsurfacecoalmine workings” (King County, 1998). The Sensitive
Areas Map Folio for King County (King County, 1990) shows no coal mine hazard areas in the vicinityof
the site.
Significant Impacts
Erosion.
Much of the site soils are categorized as an erosion hazard, asshown on Figure 10.
Erosion considerations will be a primary factor during construction and are discussed in more detail
Impacts During Construction
below in the section of this report.
Landslides.
Based on the results of our site explorations and reconnaissance, we conclude there is
a potential for landsliding of existing, steep, landslide-prone slopes. This can be triggered by a seismic
event, an increase in pore water pressure fromexcessive rainfall or uncontrolled surface water, or
construction that traverses or cuts into a steep slope. As shown on Figure 12, landslide prone areas have
been identified along some of the steep slopes adjacent to existing wetland areas. With the exception of
the western limits of Evergreen Way Southeast, the conceptual designs for Alternatives481 and 700
suggest that new infrastructure will not traverse or be built over landslide hazard areas. However, some
roads and perhaps the northwest stormwater detention pond (near boring B-1) might be built adjacent to
landslide hazard areas.
The impacts are anticipated to be roughly the same for Alternative 481 and Alternative 700. Impacts
to the No Action Alternative are assumed to be similar to Alternatives 481 and 700, butmay vary
depending on specific construction plans for the No Action Alternative.
Seismicity.
The entire sitemay be subjected to earthquake shaking and should be considered to
have a moderate to high seismic riskfrom shaking forces. Liquefaction, lateralspreading, and fault
rupture are not anticipated to impact the proposed development.
The impactsare anticipated to be roughlythe same for the No Action Alternative, Alternative 481 and
Alternative 700.
Volcanic and Coal Mine Hazards.
We do not anticipate that volcanoes or existing coal mines
will adversely impactAlternative 481,Alternative 700 or the No Action Alternative.
Impacts During Construction
Erosion.
Much of the site soils are categorized as an erosion hazard, as shown on Figure 10. Some
of these sensitive soils will be disturbed during construction, increasing the erosion potential.
Construction activities that typically affect erosionpotential include vegetation removal, grading, fill
placement, and spoils removal or stockpiling. Erosion could lead to silt-laden runoff being transported
off-site, resulting in water quality degradation oflocal surface waters. Based on a sediment budget
analysis using RUSLE (Appendix C), it is estimated that approximately 2,000 tons per year of soil could
be eroded within the site under unmitigated construction conditions. This soil loss rate conservatively
assumes that 100 acres of the site will be completely clearedand exposed during construction. Note that
RUSLE estimates only the gross amount of soil moved from its original position; it does not estimate net
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soil erosion. The actual volume of soil eroded from a site is generally much smaller than calculated by
RUSLE.
Alternative 700 may have a smaller potential to cause erosion than Alternative 481 because of its
smaller footprint, assuming that mass clearing and grading is implemented during construction.
However, the larger number of building lots underAlternative 700 may result in increased vegetation
removal, grading, utilityservices connections (excavated trenches) and soil stockpiling, depending on
specificconstruction plans for either alternative. TheNo Action Alternative is assumed to have a smaller
potential to cause erosion than Alternative 481 because of the anticipated smaller number of lots.
Grading.
Large volumes of cut and fill will be required for the project. Excavated soils to be used
as fill would need to be stockpiled, and unsuitable or excess materials would be removed from the site
which increases erosion potential. Fill materialmay be required in excess of that available from on-site
excavations.It is anticipated that several sources offillmay be available in the Auburn area in volumes
sufficient enough to meet project demands without adversely impacting those local sources. It is also
anticipated that several soil disposal sites will also be available.
Depending on the locations of the aggregate sourcesand disposal sites, heavy trucks would be
required to transport the fill and waste materials.The smaller footprint of Alternative 700 may result in
less grading than Alternative 481. Less grading is assumed for the No Action Alternative than for
Alternative 481 or Alternative 700; thus grading impacts are anticipated to be lower for the No Action
Alternative.
Vibrations.
Earthwork activities at the site, such as excavating dense glacial materials, compacting
fill soils, or simply running trucks and construction equipment,may result in vibrations that could damage
nearby structures or disturb nearbybusinesses, residents or wildlife. Housing developments are located
westand south of the site, and several homesare located within the exclusion area in the central portion
of the site. Vibrations associated with earthwork activities cancause cracks in nearby structures and
settlement if those structures are founded over loose soils. However, there are few existing structures
immediatelyadjacent to the proposed development. Thus, vibration impacts fromearthwork are
anticipated to be moderateto low.
Since Alternative 700 may result in slightly less earthwork than Alternative 481, the potential
constructionvibration impacts associated with this alternativemay also be slightly smaller. The
NoAction Alternative is assumed to require less earthwork than Alternative 481 or Alternative 700,
so potential construction vibration impacts are also expected to be less for the No Action Alternative.
Impacts During Operation
Operation of Alternative 481, Alternative 700 or the No Action Alternative is not expected to have
substantial impacts to topography, soil conditions, or soil geologic hazards, with the exception that
sufficient engineering controls will be required to ensure that the developments do not increase the
potential for landsliding in those hazard areas.
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Mitigation Measures ? Geologic Hazards
Erosion.
See the section below regarding construction mitigation(erosion)for mitigation measures
associated with erosion. Additional geotechnical recommendations are included in Appendix D.
Landslides.
Themitigation measures described belowareappropriate for Alternatives 481 and
700. Mitigation for construction in a landslide hazardarea (westend of Evergreen Way) or adjacent to
landslide hazard areas (portions of planned roads and possibly a detention pond in the northwest portion
of the site) will need to be a primary design consideration. Mitigation could include use of retaining
structures, enhanced drainage, and/or setbacks to limit the potential for impacts of development proximate
to the landslide hazard areas. Typically, buffers andbuilding setbacks from the edges of landslide hazard
areas are approximately 50 feet and 15 feet, respectively. However, specific recommended buffers and
setbackscould be provided once the specific locations and designs of on-site structures are determined.
Buffers and setbacks based on site specific studies and designs could be less (or more) than those
indicated above.
Other engineering controls (mitigation) include designing, constructing and maintaining features that
limit uncontrolled surface water or groundwater flow in landslidehazard areas. In pond areas, it may be
necessary to line the ponds so that infiltration of stormwater does not adversely impact the stabilityof
adjacent slopes. New permanent cut and fill slopes should be designed and constructed using accepted
standards of practice. Additional geotechnical recommendations are included in AppendixD.
Seismicity.
There is a risk of earthquake induced shaking at the site, as with all sites in the Puget
Sound region, and the intensity of the shaking could be severe. Where practical, construction activities in
seismic hazard areas should be avoided. The impact of strong ground shaking can be mitigated by
designing the proposed improvements in general accordance with the seismic provisions of the applicable
editionof the building code at the time of design and construction.Additional geotechnical
recommendations are included in Appendix D.
Construction Mitigation
Erosion.
The mitigation measures described below are appropriate for Alternatives 481 and 700.
During and after construction, an Erosion and Sediment Control Plan (ESCP) should be followed which
would provide for the interception and treatment of potential silt-laden runoff that could occur during
clearing, grading, construction of infrastructure, and site stabilization. The ESCP should provide
measures to ensure that no silt-laden runoff leaves the construction site. The project ESCP would
describe general requirements, soils and ground-cover protection measures, conveyance systems,and
sedimentation facilities. To the extent practical, the ESCP would be in accordance with the requirements
of the “Stormwater Management Manual for Western Washington” (Ecology, 2001). In addition, the City
of Auburn’sdesign and construction standards manuals(Cityof Auburn, 1998 and 2003) outline several
measures to be implemented duringearthwork and grading activities.
Site-specificmeasures that would be employedfor the proposed project to mitigate short-term
impacts to the earth environment during construction include:
Limit clearing and grading to construction, lay-down, and staging areas to minimize exposed soil.
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Completesite preparation, excavations, and fill placement during the drier summer and early fall
months where practical.
Route surface water through temporary drainage channels around and away from disturbed soils or
exposed slopes.
Cover exposed soil stockpiles and exposed slopes with plastic sheeting, as appropriate.
Usestrawmulch and erosion control matting to stabilize graded areas and reduce rain and runoff
impacts to slopes.
Usemobilesedimentation tank trucks to collect and contain turbid water, if needed. Alternatively,
polymersmay be used to reduce sedimentation impacts.
Construct sedimentation ponds, check dams, and filterfences to remove as muchsediment as possible
prior to returning runoff to natural drainages.
Intercept and drain water from any surface seeps when they are encountered.
During wet periods, stabilize disturbed areas using mulch and/or hydroseeding within an appropriate
time interval of final grading (in emergency situations, the project engineer could require coveringof
any exposed slopes at anytime).
Construct a stabilized construction entrance and tire cleaning area to clean vehicles prior to leaving
the site.
Designate practices to be used to dispose of wood wastes and/or any soil spoil materials that cannot be
re-used at the site.
Conduct routine inspection of the construction site to ensure effectiveness of measures and to
determine need for maintenance or further measures.
Identify individual ESCP measures as separate bid items in construction plans and specifications.
Incorporate provisions allowing temporary cessation of work under certain limited circumstances,if
weather conditions warrant.
Following construction, the side slopes of embankments and cut slopes should be protected against
erosion. Erosion could be caused by uncontrolled surface water. As a minimum, side slopes should be
re-vegetated (i.e., hydro-seeded) to protect against erosion. Additional geotechnical recommendations are
included in Appendix D.
Grading.
Disposal or re-use of the excavated soils as fill for this project will depend upon whether a
number of factors such as the type of soil (coarse-grained or fine-grained) and its moisturecontent These
determinations require site-specific analysis, construction planning and sequencing and an economic
evaluation. It is anticipated that a significant portion of the excavated soils will be reused as fill on the
project and that several aggregate sources and disposal sites will be available for additional fill
requirements and disposal of unsuitable materials, respectively.Additionalgeotechnical
recommendations are included in Appendix D.
Vibrations.
As a minimum, vibration mitigation should include a precondition surveyofadjacent
structures located within 100 feet of proposed work areas and a vibration monitoring program. The
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purpose of the vibration monitoring program will be to establish the buffer distances required between
vibration-sensitive structures and construction equipment such that the measured vibration levels are lass
than published threshold levels that can cause damage. If the initial vibration monitoring evaluation
indicates vibration levels at vibration-sensitive structures is greater than the threshold level, additional
mitigation measures will be implemented, such as increasing the buffer distance or using smaller
constructionequipment. Selection of the appropriate earthwork equipment (equipment used for
excavating dense soils, compacting fill soils, and hauling materials) should balance the potential impacts
associated with vibrations with the economics of rate of progress and cost of the equipment.
Operational Mitigation
Operation of Alternative 481, Alternative 700 or the No Action Alternative is not expected to have
substantial impacts to topography, soilconditions, or geologic hazards, with the exception that sufficient
engineering controls will be required to ensure that the developments do not increase the potential for
landsliding in those hazard areas. Engineering controls (mitigation) include designing, constructing and
maintaining features that limit uncontrolled surface water or groundwater flow in these areas. Additional
geotechnical recommendations are included in Appendix D.
Unavoidable Significant Adverse Impacts
No unavoidable significant adverse impacts from geologic hazards are anticipated from
Alternative 481, Alternative 700 or theNo Action Alternative.
WATER
SURFACE WATER CONDITIONS
Affected Environment
The site is located within Water ResourcesInventory Area (WRIA) 10, the Puyallup-White
watershed, which drains approximately 1,050 square miles in King and Pierce counties (Hashim et al.,
2003). Five wetlands (Wetlands A, B, C, D and 1, located on Figures 2 and 3) and two unnamed
intermittent streams have been identified in the western portion of the site. The headwaters of the two
intermittent streams are located in the vicinity of Wetlands 1 and B, respectively (Figures 2 and 3). An
evaluation of streamand wetland conditions is presented in a report by Raedeke Associates, Inc. (2003b).
Surface water runoff fromthe western portion of the site flows into unnamed tributary number 0043,
th
shown on Figure 8. Significant blockage of culverts beneath 49 Street SE (stream station 3+00) and
Kersey Way(stream station 17+00) were observed duringour September 2002 stream reconnaissance.
After an approximate 3-foot high cascade from the culvert beneath Kersey Way, the unnamed tributary
discharges into Bowman Creek near stream station 45+50.
Surface water runoff from the eastern portion of the site flows through a culvert beneath Kersey Way,
near stream station 14+00 of Bowman Creek (Figure 8). Significant erosion was observed near the outlet
of this culvert. An approximate 15-foot wide erosional bowl with a 7.5-foot vertical drop was observed
approximately 20 feet north and downstream of the culvert.This erosional feature appears to be
progressing in an upstream direction, toward Kersey Way.
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As authorized by Chapter 90.48of theRevised Code of Washington (90.48 RCW) (Water Pollution
Control Act), Chapter 173-201A of the Washington Administrative Code (173-201A WAC) (Water
Quality Standards for Surface Waters of the State ofWashington)has established water quality standards
for surface water in Washington. Surface water quality criteria include fecal coliform organisms;
dissolved oxygen; total dissolved gas; temperature;pH; turbidity; toxic, radioactive or deleterious
materials; and aesthetic values. Specific concentrations or threshold values for these surface water quality
criteria vary according to the classification of a specific water body. Surface water body classifications
include Class AA (extraordinary), Class A (excellent), Class B (good) and Class C (fair). Ecology has
prepared a list, as specified by Section 303(d) of the federal Clean Water Act,which identifies impaired
surface waters that do not meet the water quality standards specified by173-201A WAC.
The White River in the site vicinity and Bowman Creek are classified asClass A surfacewaters
according to 173-201A WAC. There are five sitesalong the White River in Ecology’s303(d) list of
impaired and threatened waterbodies, within a 2-mile radius of the site. The listed water quality
parameters from these five sites are instreamflow, pH and temperature (Ecology,2000). Additional
information regarding surface water quality impacts is presented in a report by Raedeke Associates, Inc.
(2003b).
Significant Impacts
The creation of impervious surfaces will cause a net increase in surface water runoff. Alternative 481
and Alternative 700 would add approximately 60 and 56 acres of impervious surfaces to the existing site,
respectively (DBM, 2004). The potential impact of the No Action Alternative will be directly related to
the amount of added impervious surfaces. Increases in surface water runoff have the potential to increase
on-site erosion and increase the rate of off-site streamchannel erosion.
Increases in stormwater flow volumes would likely cause accelerated erosion along some portions of
the stream banks of Bowman Creek and unnamed tributary 0043. Sediment deposition in the proposed
stormwater detention ponds (under Alternatives 481 and 700) is expected to cause a reduction in the
volume of sediment exiting the project site. This reduction in sediment could increase the erosive
potential of streams by causing a sediment-starved condition, and thus accelerate erosion downstream of
the site
Constructionactivities such as clearing and gradingwill increase the potential for soil erosion, which
may impactwater quality if significant amounts of sediment are allowed to enter the surface water
drainage system.
Mitigation Measures ? Surface Water
Similarmitigation measures are expected for Alternatives 481 and 700. Mitigation measures for the
No Action Alternative will be dependentupon its specific design.
Runoff directed to stormwater detention ponds willbe routed through pipes under Alternatives 481
and 700, andnot through the natural open channels on site. The use of open channels to route stormwater
is not feasible, given the steep slopes present on site. Use of pipes to convey stormwater will reduce the
potential for erosion.
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Surface water discharge from the proposed stormwater detention ponds under Alternatives481 and
700 will be designed to match 50 percent of the existing peak flow rate for the 2-year storm event under
existing conditions. Discharge fromthe proposed ponds will also be designed to match (100 percent) the
peak flow rates for the 10-year, 25-year, and 100-year storm events under existing conditions. These
restricted discharge rates will reduce the potential for increased stream channel erosion. Partial
infiltration of stormwaterto mitigate reductions in shallow groundwater flow (refer to the following
Groundwater
section) will also reduce some of the increased runoff caused by impervious surfaces.
Geologic
Additional mitigation measures regarding erosion by surface water runoff are presented in the
Hazards
section and in Appendix D (Geotechnical Recommendations).
The existing off-site erosional feature observed at the outlet of the culvert beneath Kersey Way and
near Bowman Creek (near stream station 14+00) is an existing problemthat should be mitigated
concurrent with development of the proposed KerseyIII project. This erosional feature has the potential
to undermine Kersey Way. We recommended that a supplemental evaluation of stream channel
conditions along Bowman Creek in the vicinity of stream station 14+00 be completed during final design.
Mitigation for the existing condition could consist of a properlydesigned and constructed energy
dissipater, and stream channel and bank protection.
Mitigationmeasures to prevent surface water quality degradation from soil erosion during
Geologic Hazards
constructionactivities were discussed in the section.
Unavoidable Significant Adverse Impacts
No unavoidable significant adverse impacts to surface water are anticipated from Alternative481,
Alternative 700 or the No Action Alternative. With proper mitigation of the existing erosional feature
along Bowman Creek, it is our opinion that no significant adverse changes to the channel will result from
Alternative 700, Alternative 481, or the No Action Alternative.
GROUNDWATER
Affected Environment
ShallowGroundwater.
Localized zones of shallow groundwater were encountered within
Vashon-ageglacial deposits in four of our on-site borings (B-1 through B-4) and one test pit (TP-4).
Results of shallow groundwater level monitoring at B-1, B-2 and B-4 are summarized in Appendix B.
Three test pits (SL-7, -15 and -25, located on Figures 2 and 7) completed during a previous on-site
investigation(Anderson, 1999) also encountered shallow groundwater. Shallow lenses of wet soil were
also encountered in one of our other test pits (TP-6)and three test pits (ECTP-2, -7, and -8, located on
Figures 2 and 7) completed during another previous on-site investigation (Earth Consultants Inc., 2000).
These shallow zones of groundwater were typicallyencounteredat depths of 5 to 30 feet beneath ground
surface, in 2- to 10-foot thick lenses of sand or silty sand that were underlain by sediments with low
permeability.
Mottled soils observed in five of our other test pits (TP-1, -3, -5, -10 and -17) and three test pits
(ECTP-6, -7 and -13, located on Figure 2) froma previous investigation (Earth Consultants Inc., 2000)
indicate that shallow groundwater also may be present at these locations during wetter periods of the year.
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These shallow groundwater zones are perched on localized deposits of low permeabilitysediments.
Groundwaterflow within these shallow perched zones is expected to mimicthe topographyandtravelin
down-slope directions, as shown on Figure 9. Shallow groundwater in the western portion of the site
flows toward Wetlands A, B, C, D and 1 (locatedon Figures 2 and 3). Shallow groundwater in the
eastern portion of the site generally flows toward the north. A portion of shallow groundwater at the site
is also expected to migratedownward and provide recharge to the deeper regional aquifers.
Regional Aquifers.
Based on existing studies of the area, the shallowest regional or laterally
extensive aquifer beneath the site occurs within the Vashon advance outwash (Qva) sediments at an
inferred elevation of approximately 300 to 350 feet (Luzier, 1969; Woodward et al., 1995). However,
existing geologicmaps (Mullineaux, 1965a; Luzier, 1969), cross sections (Hart-Crowser, 1982), and well
logs (Ecology, 2002a) inthe site vicinity also indicate that the presence of the Qva aquifer is uncertain
beneath the southern portion of the site and is absent beneath the northwestern portion of the site. The log
of on-site test well TW-1 (Ecology,2002a) indicates that no water was encountered in Qva sediments
during drilling. Figures 6 and 7 illustrate the inferred subsurface distribution of Qva sediments. Where
present, groundwater within the Qva aquifer beneath the site is inferred to flow north, toward the White
River.
Existing geologic maps (Mullineaux, 1965a; Luzier, 1969), cross sections (Hart-Crowser, 1982;
Woodward et al., 1995), and well logs (Ecology, 2002a)in the site vicinity indicate that the Salmon
Springs drift (Qss)aquifer is present beneath the entire site. Qss sediments areinterpreted to be present
directly beneath Qva sediments, or directlybeneath Qvt sediments (where Qva is absent), as shown on
Figures 6 and 7. The contact between Qva and Qsssediments is inferred between the approximate
elevations of 280 and 320 feet. The log of on-site test well TW-1 (Ecology, 2002a) indicates that
groundwater was encountered in Qss sediments between the approximate elevations of 260 and217 feet
during drilling, with a measured static water level elevation of about 304 feet. Groundwater within the
Qss aquifer beneath the site is also inferred to flow north, toward the White River.
One additional aquifer, identified as the “Q(B)c” aquifer (Woodward et al., 1995), is inferred to be
present in the site vicinity, beneath Salmon Springs drift (Qss). The Q(B)c aquifer is located in an older
sequence of pre-Vashon glacial and interglacial sediments. The bottom of the Qss aquifer (inferred
elevation of 110 to 120 feet)and top of the underlying Q(B)c aquifer are inferred to be separated by
approximately 50 feet of low-permeability sediments of the Puyallup Formation (Qpy). The Q(B)c
aquifer is inferred to be about 60 to 70 feet thick,with a basal elevation of about 50 feet. The Q(B)c
aquifer is inferred to be underlain by an additional sequence of low-permeability sediments until
sedimentary bedrock is encountered at an inferred elevation of –1,000 feet. Bedrock in the site vicinity is
not expected to be a significant source of groundwater because of its fine-grained and cemented nature
(Woodward et al., 1995; Mullineaux, 1970).
A review of records on file with the WashingtonState Department of Ecology (2002b and 2003)
identified two groundwater right certificates and one surface water right certificate within half a mile of
the site. The two groundwater right certificates are held bythe City of Auburn for wells located
approximately800 to 2000 feet west of the site, located within the Lakeland Hills development. These
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G e oE n g i n e er sFile No. 0153-031-00\030504
wells extract water from the Salmon Springs drift (Qss) aquifer. The surface water right certificate is for
an unnamedspring located about 600 to 2600 feet north of the site. Water from the spring is used for
domestic and irrigation purposes.
Water Budget.
A water budget evaluation was completed to estimate surface water runoff,
evapotranspiration, recharge to groundwater, and shallow groundwater flow to wetlands at the site under
existing conditions. The methodology and results of this evaluation are presented in AppendixB.
Based on an averageannual precipitation rate of 42.4 inches per year (in/yr), 20.4 in/yr are estimated
to provide groundwater recharge under existing conditions. Existing average annual evapotranspiration is
estimated to be 21.6 in/yr, or about50percent of precipitation.This evapotranspiration rate is consistent
with informationpresented in Bauer and Mastin, 1997, Bidlake and Payne,2001, and Orr et al., 2002.
The remaining 0.4 in/yris estimated to provide direct surface water runoff.
Shallow groundwater flow to Wetlands A, B, C, D and 1 is estimated to be approximately 4,400 cubic
feet per day (cfd), or about 23 gallons per minute (gpm) under existing conditions, on an average annual
basis.
GroundwaterQuality.
A 1995 study of groundwater in southwesternKing County (Woodward
etal., 1995) concluded that there were no significant chemicaldifferences in water quality among the
Quaternary aquifers (which include the Qva, Qss and Q[B]caquifers). Based on water quality data from
223 wells in southwestern King County, this study alsoconcluded that there is no widespread degradation
of groundwater quality.
A comparison of water quality data from this 1995 study for wells completed in Quaternary aquifers
with groundwater qualitystandards established by 173-200 WAC indicates that all of the samplesmet the
criteriafor total dissolved solids,nitrate, heavy metals (arsenic, barium, cadmium, chromium, copper,
lead, mercury, selenium silver and zinc), and organiccompounds (including benzene, bromoform, carbon
tetrachloride, chloroform, ethylbenzene,methylene chloride, toluene, trichloroethylene and vinyl
chloride). Fecal coliformconcentrations exceeded the established criterion in less than one percent of the
samples. Iron and manganese were the only parameters that exceeded the established criteria at a
significant frequency (greater than 10 percent of the samples).
Significant Impacts
The creation of impervious surfaces will cause a net reduction in groundwater recharge and shallow
groundwater flow. Alternative 481 would introduceapproximately60 acres of impervious surfaces and
could potentially reduce groundwater recharge to 16.1in/yronan average annual basis.This recharge
rate is about 21 percent lower than estimated under existingconditions. As a result of this reduction in
recharge, a similar reduction in shallow groundwater flow can be expected under Alternative 481.
Alternative 700 would introduce approximately 56 acres of impervious surfaces. The impact to
shallow groundwater flow under Alternative 700 would be similar, but slightly less than under
Alternative481. The No Action Alternative is expected to have a similar, but slightly smaller impact to
groundwater recharge and shallow groundwater flow, in a direct relationship with the extent of introduced
impervious surfaces.
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G e oE n g i n e er sFile No. 0153-031-00\030504
Installation of a sanitary sewer system within Kersey way (an option under Alternatives 481 and 700)
could alter shallow groundwater flowpaths by diverting shallow groundwater toward permeable backfill
material in the trench excavation for the sewer line. This potential alteration to shallow groundwater may
reduce flow toward Bowman Creek.
The potential reduction of groundwater recharge to regional aquifers is not considered to be
significant because of therelatively small area of the site (less than 0.3 square mile)with respect to the
Puyallup-White watershed (greater than 1,000 square miles). However, a reduction in shallow
groundwater flow could adversely affect nearby wetlands.
Potential impacts on groundwater quality include surface spills of fuels, lubricants, and other
chemicals used during construction and operation of the proposed development. Turbidity and suspended
solids from construction activities generally do not affect groundwater. Near-surface sources of potential
groundwater contaminants are less likely to affect the regional aquifers, which occur at greater depths and
are typicallyoverlain byone or more sequence of low permeability sediments.
Mitigation Measures ? Groundwater
Mitigationmeasures recommended to address potential reductions in the quantityof shallow
groundwater flow under each of the alternatives include partial infiltration of stormwater runoff generated
on site.Thewater budget evaluation indicates that infiltrationofrunoff from approximately 3.44 acres of
impervious surfaces fromAlternative 481 could restore average annual shallow groundwater flow rates to
WetlandsA,B, C, D and 1 to those calculated under existing conditions. Required impervious area
runoff contributions for Alternative 700 wouldbe approximately 6 percent less than under
Alternative481, because of the approximate 6 percent reduction in imperviousareas (56 versus 60 acres).
Impervious area runoff contributions for the No Action Alternative could be estimated based on proposed
impervious surface areas. This mitigation could be achieved by diverting roof runoff from selected areas
to infiltration trenches.
It is recommended that these infiltration trenches have a depth of 2 to 4 feet, and be placed near the
upslope ends of the wetlands. Based on grain-size distribution data from shallow soil samples near the
existing wetlands, a preliminary design infiltration rate of 1.0 inch per hour is estimated. This design
infiltration rate is based on the ASTM Gradation Testing table (Table 7.2) presented in Ecology’s
StormwaterManagementManual for Western Washington. Based on the water budget evaluation, ideal
impervious area runoff contributions for Alternative 481 should be approximately2.03,1.14, 0.13 and
0.14 acres for Wetlands A and 1 (combined), B, C and D, respectively.
Mitigation measures recommended to address the potential diversion of shallow groundwater along
the sewer system within Kersey Way (an optionunder Alternatives 481 and 700) include the installation
of backfill seepage barriers. Seepage barriers installed at approximate 100-foot intervals would prevent
shallow groundwater from flowing along the trench ofpermeable backfill material, thus preventing the
diversionofexisting shallow groundwater flowpaths. Specific design recommendations for the backfill
seepage barriers are presented in Appendix D.
Mitigation measures recommended to address groundwater quality impacts include groundwater
quality protection techniques such as construction bestmanagement practices, spill prevention plans, and
monitoring of any stormwater discharged to groundwater.
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REFERENCES
Anderson Design Consultants, Inc. (Anderson), 1999, Soil logs for Lakeridge Development, Kersey Way
project, March 11, 1999.
Apex Engineering PLLC, 2003, KerseyIII Sewer Alternatives, dated June 27, 2003.
Bauer, H.H., M.C. Mastin, 1997, Recharge fromprecipitation in three small glacial-till-mantled
catchments in the Puget Sound lowland, Washington: U.S. Geological Survey, Water Resources
InvestigationWRI 96-4219, 119 p
Bidlake, W.R., K.L. Payne, 2001, Estimating recharge to ground water from precipitation at Naval Base
Bangor and vicinity, Kitsap County,Washington: U.S. Geological Survey, Water Resources
InvestigationWRI 01-4110, 33 p
Bucknam, R.C., Hemphill-Haley, E., and Leopold, E.B.,1992, Abrupt uplift within the past 1700 years at
southern Puget Sound, Washington: Science, V. 258, p. 1611-1614.
City of Auburn, 1995, Topographic maps 1411 and1412, SW ¼ and SE ¼, Section 32, T21N, R5E,
W.M., based on aerial photographydated 4-24-95.
City of Auburn, 1996a, Auburn CityCode, Title 16, Environment, Chapter 16.06, Environmental Review
Procedures, Section 16.06.030, Additional definitions.
Cityof Auburn, 1996b, Erosion Hazard Areas, Landslide Hazard Areas, Seismic Hazard Areas, and
Volcanic Hazard Areas maps, dated January18 and 19, 1996.
Cityof Auburn, 1998, City of Auburn Design and Construction Standards 1998,dated October 1998.
City of Auburn, 2002, City of AuburnComprehensive Plan, originally adopted August 1986, amended to
comply withGrowth Management Act April 1995, includes revisions throughDecember 2002.
Cityof Auburn, 2003, Cityof AuburnConstructionStandards, comprised of Standard Specifications &
Standard Details, dated December 2003.
Crandell, D.R., 1973, Potential hazards from future eruptions of Mount Rainier, Washington.
U.S. Geological Survey,Miscellaneous Geologic Investigations Map I-838.
DBM Consulting Engineers, 2000a, Preliminary Plat of the Kersey Three, Auburn, WA, drawings dated
August 16 and 23, 2000.
DBM Consulting Engineers, 2000b, Downstreamanalysis for theKersey Three project sites located south
thrd
of Kersy Way between the intersections of 49 Street SE and 53 Street SE, Auburn,
Washington,report dated August 21, 2000.
DBM Consulting Engineers, 2003, Kersey III stormwater calculations, provided to the City of Auburn on
February 20, 2003.
Earth Consultants Inc., 2000, Infiltration Evaluation and Arterial Roadway Study, Kersey Three
th
ResidentialDevelopment, Kersey Way and 49 Street Southeast, Auburn, Washington.
Consultant report for Lakeridge Development, Inc. dated August 17, 2000.
23
G e oE n g i n e er sFile No. 0153-031-00\030504
Hart-Crowser & Associates, Inc. (Hart-Crowser), 1982, Ground Water Study, Auburn,Washington.
Consultant’s report prepared for Pool Engineering Inc., dated July21, 1982.
Hashim, B., Green, B., and Phillips, A., 2003, Washington's Water Quality Management Plan to Control
Nonpoint Source Pollution, Washington State Department of Ecology Publication Number 99-26
(Revised).
Hoblitt, R.P., J.S. Walder, C.L. Driedger, K.M. Scott, P.T. Pringle, and J.W. Vallance, 1998, Volcano
Hazards from Mount Rainier, Washington, Revised 1998: USGS Open-File Report 98-428.
Keefer, D.K., 1984, Landslides caused by earthquakes:Geological Society of America Bulletin, V. 95,
p. 406-421.
King County, 1990, Sensitive Areas Map Folio, King County, Washington.
King County, 1993 and 1998, King County Code Title 21A, Zoning.
Luzier, J.E., 1969, Geology and Ground-Water Resources of Southwestern King County, Washington.
WashingtonState Dept of Ecology, Water Supply Bulletin 28.
Mullineaux,D.R., 1965a, Geologic Map of the Auburn Quadrangle, King & Pierce Counties,
Washington.U.S. Geological Survey, Geologic Quadrangle Map GQ-406.
Mullineaux,D.R., 1965b, Geologic Map of the Black Diamond Quadrangle, King County, Washington.
U.S. Geological Survey, Geologic Quadrangle Map GQ-407.
Mullineaux,D.R., 1970, Geology of the Renton, Auburn, and Black DiamondQuadrangles,King County,
Washington.U.S. Geological Survey Professional Paper 672.
Orr, L.A., H.H. Bauer, J.A. Wayenberg, 2002, Estimates of ground-water recharge fromprecipitation to
glacial-deposit and bedrock aquifers on Lopez, San Juan, Orcas, and Shaw Islands, San Juan
County, Washington: U.S. Geological Survey, Water Resources Investigation WRI 02-4114,
114 p
Palmer, S.P., 1992, Preliminary maps of liquefaction susceptibility for theRenton and Auburn7.5’
quadrangles, Washington, WashingtonDivision of Geology and Earth Resources, Open File
Report 92-7.
Raedeke Associates, Inc., 2003a, Plants and Animals Assessment for the Kersey III Plat, City of Auburn,
Washington,Preliminary Draft EIS Report to the City of Auburn, Washington, September 29,
2003.
Raedeke Associates, Inc., 2003b, Wetland Assessment of the Kersey III Preliminary Plat, Cityof Auburn,
Washington,Preliminary Draft EIS Report to the City of Auburn, Washington, September 29,
2003.
Snyder, D.E., Gale, P.S., and Pringle, R.F., 1973, Soil Survey of King County area, Washington.
U.S. Department of Agriculture, Soil Conservation Service.
WashingtonState Department of Ecology (Ecology), 2001, Stormwater Management Manual for Western
Washington.Water Quality Program, Publication Numbers 99-11 through 99-15.
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G e oE n g i n e er sFile No. 0153-031-00\030504
WashingtonState Department of Ecology (Ecology), Northwest Regional Office, 2002a, review of well
records and boring logs onfile; information providedon January 24, 2002.
WashingtonState Department of Ecology (Ecology), Northwest Regional Office, 2002b, review of water
rights information on file; information provided on January 28, 2002.
WashingtonState Department of Ecology (Ecology), Southwest Regional Office, 2003, review of water
rights information on file; information providedon February 5 andJune 3, 2003.
WashingtonState Department of Ecology,2000, The303(d) list of impaired and threatened waterbodies,
list for 1998 ().
Woodward,G.G., Packard, F.A., Dion, N.P., and Sumioka, S.S., 1995, Occurrence and qualityof ground
water in southwestern King County,Washington: U.S. Geological Survey Water-Resources
InvestigationReport 92-4098, 69 p., 4 pl.
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APPENDIX A
FIELD EXPLORATIONS AND LABORATORY TESTING PROCEDURES
FIELD EXPLORATIONS
Subsurface conditions at the proposed Kersey III subdivision were explored by excavating 23 test pits
(TP-1 through TP-23) and drilling five test borings (B-1 through B-5). The approximate locations of the
explorationsare shown on the Site and Exploration Plan, Figure 4.
The 23 test pits were excavated with a CAT 312B trackhoe provided by the WelcomeCompanies of
Monroe, Washington on June 26 and 27, 2002. The five test borings were drilled using a CME-85 track-
mounted, hollow-stemauger rig provided by Gregory Drilling of Redmond, Washington between July8
and 22, 2002.
Relatively undisturbed samples were obtained from the borings using a 2.5-inch inside-diameter split-
barrel Dames and Moore sampler driven into the soil with a 300-pound hammer free-falling 30 inches.
The 300-pound hammer on this drill rig was operated using a CME automatic hammer. The number of
blows required to drive the sampler the last 12 inches, or other indicated distance, is recorded on the
boring logs.
The field explorations were continuously monitoredby a geologist from our firm who examined and
classified the soils encountered, obtained representative soil samples, and observed groundwater
conditions. Soils were classified in general accordance with the classification systemdescribed in
FigureA-1.A key to theboring log symbols is presented in Figure A-2. The logs of the test borings are
presented in Figures A-3 through A-7. Logs of the test pits are presented in Figures A-8 through A-30.
The logs are based on our interpretation of the field and laboratory data and indicate the various types of
soils encountered. Theyalso indicate the depths at which these soils or their characteristics change,
although the change might actually be gradual. Ifthe change occurred between samples in the borings,
the depth of the change was interpreted.
LABORATORY TESTING
All soil samples were brought to our laboratory for further examination.Selectedsamples were
tested to determine their grain size distribution, percent fines and organic content. The sieve analyses
were performed in accordance with the ASTM D-136 test procedure. Results of the sieve analyses are
presented in Figures A-31 through A-39. The result of the percent fines test is presented on the log of test
pit TP-6 (Figure A-13).
The organic content test was performed in accordancewith the ASTM D 2974-87 test procedure. The
result of the organic content test is presented on the log of test pit TP-12 (Figure A-19).
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APPENDIX B
WATER BUDGETEVALUATION
INTRODUCTION
A water budget evaluation was completed to estimatesurface water runoff, evapotranspiration,
recharge to groundwater, and shallow groundwater flow to wetlands at the proposed Kersey III
subdivision in Auburn, Washington.Water budgets were calculated for the site under existing conditions
and under Alternative 481.
DATA SOURCES
The water budgets were completed using daily and monthly climatic data for the McMillin Reservoir
obtained from the National Oceanic AtmosphericAdministration (NOAA) for January1980 through
December 1994, and January 1996 through December 2001. Data from 1995 were incomplete. Four
NOAA stations (McMillin Reservoir, Tacoma, Auburnand Kent) are located within approximately
12miles of the Kersey III site. The McMillin Reservoir station was selected for use in the water budget
evaluation because of its proximity and similar elevation tothat of the site, and its extensive historic data
set. Figures B-1 and B-2 summarize the annual and monthlyprecipitation at the McMillin Reservoir
station, respectively. The average annual precipitation is 42.4 inches per year (in/yr).
The distributions of land types were estimated from aerial photographs (Walker and Associates,
2002), topographicmaps(City of Auburn, 1995), the King County soil survey (Snyder et al., 1973),
existing and proposed land use areas provided byDBM Consulting Engineers (2003, 2004), and field
investigations at the site. Shallow groundwater flow conditions were characterized from the on-site field
investigations and an analysis of the grain-size distribution of selected soil samples.
EXISTING CONDITIONS
Land TypeDistribution
The site under existing conditions was delineated into two soil types (B and C) and two vegetation
types (forest and grassland), which resulted in a combination of four different land types (forest on soil
type B, forest on soil type C, grassland on soil type B, and grassland on soil type C). Soil type B was
assigned to areas mapped as Vashon advance outwash in Figure 5 (see main text of report), and soil
typeC (less permeable) was assigned to areas mappedas artificial fill, Vashon ice contact deposits, and
Vashon till.Grasslands were designated to areas along the BPA transmission line easement and within
the northwestern portion of the site. The estimated distribution of the four land types is summarized in
Table B-1.
Surface Water Runoff
The daily surface water runoff was estimated using the SCS Runoff Curve Number procedure
described in Viessman and Lewis (1996). Runoff curve numbers of 55 and 70 were assigned to forested
areas in soil types B and C, respectively. These numbers are from the USDA (1986), and assumea
wooded area in good condition (protected from grazing) with adequate soil cover by litter and brush.
B-1
G e oE n g i n e er sFile No. 0153-031-00\030504
Runoff curve numbers of 61 and 74 were assigned to grassland areas in soil types B and C, respectively.
These numbers are also from the USDA (1986), and assume grassland areas in good condition (greater
than 75 percent ground cover), with light oroccasional grazing. The assigned runoff numbers are
summarized in Table B-1.
Monthly surface water runoff was calculated byadding the daily values for each month. Annual
surface water runoff was calculated by adding the monthly values for each year. Average annual surface
waterrunoff was calculated by taking the average of the annual values. Average annual surface water
runoff for the various land types is summarized in Table B-2.
Table B-3 summarizes the area-weighted monthlyaveragesand average annual surface water runoff
for the site under existing conditions. The average annual surface water runoff from the site under
existing conditions is estimated to be 0.4 in/yr, or about one percent of the average annual precipitation.
Area-weighted averages were calculated with the land typedistribution (for existing conditions)
summarized in Table B-1. Monthly averages were calculated by taking the average value for each of the
12 months in a year.
Evapotranspiration
The amountof monthly evapotranspiration was estimated using the Thornthwaite/Mathermethod, as
outlinedinDunne and Leopold (1978). The monthly precipitation used to estimate the monthly
evapotranspirationfor the Thornthwaite/Mather methodincorporates the amount of water lost to surface
water runoff. Evapotranspiration was adjusted to account for variations in the water capacity and rooting
depth of soils for each land type. Forested areas in soil types B and C are estimated to have an available
water capacity (AWC) of 30 centimeters (cm) in the root zone (Dunne and Leopold, 1978). Grassland
areas in soil types B and C are estimated to have an AWC of 15 cm in the root zone. AWC values for the
various land types are summarized in Table B-1.
Averageannual evapotranspiration for the various land types is summarized in Table B-2. Table B-3
summarizes the area-weighted monthly averages and average annual evapotranspiration for the site under
existing conditions.Theaverage annual evapotranspiration from the site under existing conditions is
estimated to be 21.6 in/yr,or about 51percent of the average annual precipitation.
Groundwater Recharge
Groundwater recharge at the site was calculated by subtracting the amount of precipitation lost to
evapotranspiration and runoff from total precipitation, and accounts for soil moisture deficits. The
remaining precipitation was assumed to infiltrate to groundwater.
Averageannual groundwater recharge for the various land types is summarized in Table B-2.
TableB-3 summarizes the area-weighted monthly averages and averageannual groundwaterrecharge for
the site under existing conditions. The average annual groundwater recharge from the site under existing
conditions is estimated to be 20.4 in/yr,or about48percent of the average annual precipitation.
Shallow Groundwater Flow to Wetlands
Shallow groundwater flow to Wetlands A, B, C, D and 1 (locations shown onFigures 2 and 3 of the
main text) was calculated by estimating the hydraulicconductivity, hydraulicgradient, and cross-sectional
B-2
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flow area of the shallow aquifer to each of the wetlands. Based on sieve analyses of shallow soil samples
from test pits TP-2, TP-4, TP-6 and TP-10 (Appendix A), the hydraulic conductivity was estimated to be
-3
centimeters per second (8.5 feet per day) byusing the empirical Breyer and Hazen equations (as
3 x 10
presented in Kresic, 1997). Hydraulic gradients, ranging fromabout 0.11 to 0.15, were estimated from
the shallow groundwater equipotential map (Figure 9 of the main text). Cross-sectional flow areas were
calculated from estimatedflowpath widths (from Figure 9), and an estimated aquifer thickness ranging
from one to four feet (based on test pit logs). Figure B-3 summarizes shallow groundwater levels
measured at B-1, B-2 and B-4.
The cumulative average annual shallow groundwater flow to the wetlands is estimated to be
4,400cubic feet per day (23 gallons perminute)under existing conditions. Table B-4 summarizes the
estimatedmonthly and annual distributions of shallow groundwater flow to each of the wetlands. Note
that Wetlands A and 1 were combined because of their close proximityand overlapping shallow
groundwater flow paths.
DEVELOPED CONDITIONS (ALTERNATIVE 481)
GENERAL
A water budget was evaluated for the design alternative with the largest area of impervious surfaces,
Alternative 481. Alternative 700 and the No Action Alternativewill have lesser amounts of impervious
surfaces, therefore impacts will be less than those under Alternative 481. Our evaluation of Alternative
481 represents the “worst-case scenario.”
Land TypeDistribution
Under Alternative 481, the site was delineated into seven different land types. Approximately
69acres of the site would be essentially unchanged, and would maintain the four land type designations
identified forthe site under existing conditions. Alternative 481 would include approximately 62 acres of
impervious surfaces (DBM, 2003), which was delineatedas another land type. The remaining 39 acres of
the site were delineated as developed open space, consisting of pervious lawns and landscaped areas.
Open space areaswerefurther delineated into two land types, based on the underlying soil type (B or C).
The estimated distribution of the resulting seven land types is summarized in Table B-1.Note that a
subsequent recalculation indicates that approximately60 acres of impervious surfaces would be
introduced by Alternative 481 (DBM, 2004). This slight modification of impervious surfaces area (about
3%) is not expected to cause a measurable difference in the water budget. Any differences in the water
budget evaluation caused by using a larger impervious surfaces area (62 acres) will be conservative
(i.e., evaluated impacts to groundwater would be less with a smaller area of impervious surfaces).
Surface Water Runoff Estimate
Runoff curve numbers of 69 and 79 were assignedto developed open space areas in soil typesB
andC, respectively. These numbers are from the USDA (1986), and assumethat the areas are in fair
condition (grass cover of 50 percent to 75 percent). A runoff curve numberof 98 was assigned to all
impervious areas. This number is also from the USDA (1986), and is applicable to paved streets, parking
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G e oE n g i n e er sFile No. 0153-031-00\030504
lots, roofs and driveways. The assigned runoff numbers are summarized in Table B-1. Average annual
surface water runoff for the various land types is summarized in Table B-2.
Table B-5 summarizes the area-weighted monthlyaveragesand average annual surface water runoff
for the site under Alternative 481. The average annual surface waterrunoff from the site under
Alternative 481 is estimated to be 8.8 in/yr, or about 21 percent of the average annual precipitation.
Evapotranspiration Estimate
Open space areas in soil types B and C are estimated to have an AWC of 10 cm in the root zone. This
value is within the expected range presented for soils covered with a combination of grass and shallow-
rooted vegetation (Dunne and Leopold, 1978). Impervious surfaces are estimated to have a minimal
AWC of 1 cm. AWC values for the various land types are summarized in Table B-1.
Averageannual evapotranspiration for the various land types is summarized in Table B-2. Table B-5
summarizes the area-weighted monthly averages and average annual evapotranspiration for the site under
Alternative 481. The average annual evapotranspiration from the site under Alternative 481 is estimated
to be 17.0 in/yr, or about 40 percent of the average annual precipitation.
Groundwater Recharge Estimate
Averageannual groundwater recharge for the various land types is summarized in Table B-2.
TableB-5 summarizes the area-weightedmonthlyaveragesand average annual groundwater recharge
forthe site under Alternative 481. The average annual groundwater recharge from the site under
Alternative 481 is estimated to be 16.6 in/yr, or about 39 percent of the average annual precipitation. This
representsa19 percent decrease (3.8 in/yr) in groundwater recharge when compared to the site under
existing conditions. Thisgroundwater recharge estimate includes approximately0.5 in/yrof stormwater
infiltrated to mitigate shallow groundwater flow to wetlands.
Shallow Groundwater Flow to Wetlands
All of the on-site wetlands (Wetlands A, B, C, Dand 1) are located within the western 82-acre parcel
(Dutyparcel) of Kersey III. Separate water budgets were calculated for the Dutyparcel to estimate
groundwaterrecharge in the vicinity of the wetlands. Under existing conditions, the Duty parcel was
estimated to consist of 77.5 acres of forest on soil type C, and 4.5 acres of grassland on soil type C.
Under Alternative 481, the land type distribution was estimated to be 40.5 acres of forest on soil type C,
1.5 acres of grassland on soil type C, 14 acres of open space on soil type C, and 26 acres of impervious
surfaces.
The area-weighted average annual groundwater recharge from the Duty parcel is estimated to be
20.2in/yrunder existing conditions, and 16.4 in/yrunder an unmitigatedAlternative 481. This represents
an 18.8 percent reduction in groundwater recharge in the vicinityof the wetlands under an unmitigated
Alternative 481. If all ofthe shallow groundwater flowing to the wetlands is conservatively assumed to
originate from the Duty parcel, then shallow groundwater flow rates under an unmitigatedAlternative 481
can be approximated by reducing the rates presented in Table B-4 by18.8 percent. Approximately
830cubic feet per day would need to be added to the shallow groundwater system to compensate for the
reduced groundwater recharge caused by impervioussurfaces introduced byAlternative 481. Based on
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an estimatedsurface water runoff rate of 24.2 in/yr from impervious surfaces (Table B-2), the surface
water runoff fromapproximately 3.44 acres of impervious surfaces would need to be added to the shallow
groundwater system to approximateexisting shallow groundwater conditions. This 3.44-acre area
consists of 2.03, 1.14, 0.13 and 0.14 acres for Wetlands A and 1 (combined), B, C and D, respectively.
With mitigation, the cumulative average annual shallow groundwater flow to the wetlands under
Alternative 481 would match the existing flow. Table B-6 summarizes the estimated monthly and annual
distributions of shallow groundwater flow to the wetlands under Alternative 481 (with mitigation).
REFERENCES
City of Auburn, 1995, Topographic maps 1411 and 1412, SW¼ and SE¼, Section 32, T21N, R5E, WM,
based on aerial photography dated 4-24-95.
DBM Consulting Engineers, 2003, Land use breakdown and total site areas for the Kersey Three project,
February18 and May 27, 2003.
DBM Consulting Engineers, 2004, Kersey III – Impervious Areas Estimates, January 22, 2004.
Dunne, T. and L.B. Leopold,1978, Water in Environmental Planning, W.H. Freeman and Company,
New York.
Kresic, N., 1997, Quantitative Solutions in Hydrogeology and Groundwater Modeling, Lewis Publishers,
Washington, D.C.
Maidment, D.R., 1993, Handbook of Hydrology, McGraw-Hill, New York.
Snyder, D.E., Gale, P.S., and Pringle, R.F., 1973, Soil Survey of King County area, Washington.
U.S. Department of Agriculture, Soil Conservation Service.
U.S. Department of Agriculture (USDA), 1986, Urban Hydrology for Small Watersheds, Technical
Release 55, Soil Conservation Service, EngineeringDivision.
Viessman, W., and G. L. Lewis, 1996, Introduction toHydrology, Fourth Edition, Harper Collins College
Publishers, New York.
Walker and Associates, 2002, Aerial photographs 8-28 and 8-29,KC-99, 8-23-99.
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APPENDIX C
SEDIMENT BUDGET ANALYSIS
INTRODUCTION
A sedimentbudget was completed to estimate soil loss at the proposed Kersey III subdivision in
Auburn, Washington. Sediment budgets were calculated to evaluate variations in soil loss for the site
under existing conditions, during construction, and under developed conditions (Alternative 481).
Alternative 700 may have a smaller potential to cause erosion than Alternative 481because of its smaller
footprint, assuming that similar mass clearing and grading is implemented during construction. The No
Action Alternative is assumed to have a smaller potential to cause erosion than Alternatives 481 or 700
because of its anticipated smaller lot density. Our evaluation of Alternative 481 is assumed to represent
the “worst-case scenario.”
DATA SOURCES
The sediment budget was completed using average monthly climatic data for the McMillin Reservoir
obtained from the National Oceanic AtmosphericAdministration (NOAA) for January1980 through
December 1994 and January 1996 through December2001. Data from 1995 were incomplete. McMillin
Reservoir station was selected for use in the sediment budget because of its proximity to the site, its
elevation is similar to that of the site, and its extensive historic data set.
Soil characteristics were determined from the King County Washington Soil Survey and field
investigations at the site. The surface soilsconsist of the Alderwood Gravelly Sandy Loam.
SOIL LOSS ESTIMATES
The sediment budget was performed using the Revised Universal Soil Loss Equation (RUSLE).
RUSLE can be used to estimate soil loss from lands (undisturbed, disturbed and reclaimed) due to
raindrop impact, overland flow and rill erosion. Soilloss occurs when material from a segment of land is
removed. However, estimates of soil loss do not account for net erosion of the segment of land
(i.e.,deposition of material on the segment of land is not included). RUSLE is an exceptionally well-
validated and documented equation (Toy and Foster, 1998). The following is the equationused to
estimate the average annual soil loss (A, tons per acre per year):
A = RKLSCP
where,
Rrainfall/runoff erosivity, and is a function of the amount and intensity of rainfall.
Ksoil erodibility, and is a functionofparticle-size distribution,organic matter content, soil
structure and permeabilityof the surface material
LShillslope length and steepness, and is a functionof topography
Ccover-management, and is a function of surface cover and roughness, soil biomass and soil-
disturbing activities
Psupportpractice, and is a function of conservation practices.
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Rainfall/Runoff Erosivity (R)
The rainfall/runoff erosivity at the site is estimated to be 40 hundreds of feetton-
-1
forceinches(acrehouryear) from the Isoerodent Map of Oregon and Washington in the Agriculture
Handbook Number 703 (Agricultural Research Service, 1996). This same value was used for the site
under existing, construction,and developed conditions.
Soil Erodibility (K)
Soil erodibility is a function of particle-size distribution, organicmatter content, soil structure and
permeability of the surface material. The site soils are mapped as Alderwood gravelly sandy loam (SCS,
1973). Alderwood gravelly sandy loam is generallycomprised of 25 to 35 percent silt and very fine sand
and 10 to 30 percent sand. The permeabilityof Alderwood gravelly sandy loam is generally between
2.0 to 6.3 inches per hour and is classified as moderate.
The soil erodibility was evaluated using the Soil-Erodibility Nomograph (Wischmeier et al., 1971;
Agricultural Research Service, 1996; Dunne and Leopold, 1978; Selby, 1982).The soil erodibility in the
vicinity of the site under existing and developed conditions is estimated to range between 0.09 to
-1
0.20 tonacrehour (hundreds of acre-feetton-forceinches), with an average value of 0.145. During
construction, disturbance of the soil structure and removal of organic matter is estimated to increase the
soil erodibility to 0.20 foran estimated disturbed area of 100 acres under Alternative 481.
Hillslope Length and Steepness (LS)
Hillslope length and steepness are a function of topography. A total of 11 hillslope segments on the
site were selected for analysis to estimate a range of LS values to adequately represent the site
topography.The length and steepness of the hillslope segmentsweremeasured from a two-foot contour
map of the site under existing conditions. The LS values for the hillslope segments were evaluated by
inputting the horizontal lengths and steepnesses into RUSLE. The LS values for the site under existing
conditions range between approximately 1.67 and 9.58, with an average value of 5.55. During
construction and after development, the hillslope segments are expected to be slightlyshorter with
slightly smaller slopes, with an average estimated LS value of 5.0 for the 100 acres of disturbed area.
Cover-Management (C)
The cover-management factor is a function of surface cover and roughness, soil biomass and soil-
disturbing activities. A value of 0.001 was used for the site under existing conditions.This value is
applicable for lands comprising 75 to 100 percent tree canopy and 90 to 100 percent of the area covered
by at least 2 inches of forest litter (Dunne and Leopold, 1978).
During construction, a value of 0.45 was used for 100 acres of disturbed area. This value assumes
that there is no tree canopy or ground cover (Dunne and Leopold, 1978). After development of
Alternative 481, the disturbed pervious areas wereassigned a value of 0.013. This value assumes an
80percent grass ground cover with 25 percent canopycover of trees and brush (Dunne and Leopold,
1978).
Support Practices (P)
The support practices factor is a function of conservation practices. The site is undisturbed under
existing conditions and no conservation practices have beenimplemented. Therefore, the support
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practices factor was assigned a value of 1.0. During construction, a value of 1.3 was estimated for the
100 acres of disturbed area. After development, the disturbed pervious areas were assigned an estimated
value of 1.2 because ground surfaces are likely to be smoother and more compactedthan during
construction.
Average Annual Soil Loss (A)
The average soil loss under existing conditions is estimated to be 0.0322 tons per acre per year, or
approximately5.5 tons per year for the entire 170-acre site. The average annual soil loss for disturbed
areas during construction is estimated to be 19.8 tons per acre per year, which results in about 1,980 tons
per year for the entire site. This estimate is for unmitigated construction conditions. After development
of Alternative 481, 62 acres of impervious surfaces(DBM, 2003) are assumed to provide no sediment,
and 38 acres of disturbed pervious areas have an estimated soil loss of 0.452 tons per acre per year, which
results in approximately 19 tons per year for the entire site. This estimate does not include the effects of
any tight-lined conveyances or the stormwater detention ponds. Note that a subsequent recalculation
indicates that approximately 60 acres of impervioussurfaces would be introduced by Alternative 481
(DBM, 2004). This slight modification of impervioussurfacesarea (about 3%)is not expected to cause a
measurable difference in the sediment budget. The actual volume of soil eroded from a site is generally
much smaller than calculated by RUSLE.
REFERENCES
DBM Consulting Engineers, 2003, Land use breakdown and total site areas for the Kersey Three project,
February18 and May 27, 2003.
DBM Consulting Engineers, 2004, Kersey III – Impervious Areas Estimates, January 22, 2004.
Dunne, T. and L.B. Leopold, 1978. Water in Environmental Planning. W.H. Freeman and Company,
New York.
Selby, M.J., 1982. Hillslope Materials and Processes. Oxford University Press, Great Britain.
Toy, T.J., and G.R. Foster, 1998. Guidelines for the Use of the Revised Universal Soil Loss Equation
(RUSLE) Version 1.06 on Mined Lands, Construction Sites, and Reclaimed Lands. Western
Regional Coordinating Center, Colorado.
U.S. Department of Agriculture, Soil Conservation Service (SCS), 1973. Soil SurveyofKing County
AreaWashington.SCS, Washington.
U.S. Department of Agriculture, Agricultural Research Service, 1996. Predicting Soil Erosion by Water:
A Guide to Conservation Planning With The Revised Universal Soil Loss Equation (RUSLE),
Agriculture Handbook Number 703. Agricultural Research Service, Washington.
Wischmeier,W.H., C.B. Johnson and B.V. Cross, 1971. A Soil Erodibility Nomograph for Farmland and
Construction Sites. Journal of Soiland Water Conservation, Vol.26, No.5, Pp. 189-192.
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APPENDIX D
GEOTECHNICAL RECOMMENDATIONS
GENERAL
GeoEngineers presents preliminary geotechnical recommendations herein for the proposed Kersey III
developmentin Auburn,Washington. At the time our geotechnical services were provided, the design
was only in a conceptual stage; so our recommendations should be consideredpreliminary and should be
expected to evolve as the project elements become better defined. Additional explorations and analyses
may be required in supportof final design.
Based on the conceptual layout, our literature review, explorations and evaluation, we conclude that
developmentof the site can generallybe accomplished as proposed and that shallow foundation support
will be suitable for the planned residences. A summary of primary geotechnical considerations for the
site development is provided below. The summary is presented for introductory purposes only and
shouldbe used in conjunction with the complete recommendations presented in this report.
The proposed buildings may be supported on shallow spread foundations bearing on (1) undisturbed
dense to very dense glacial deposits (ice contact, tillor advance outwash), or (2) properly compacted
structural fill extending down to the dense or very dense glacial deposits. We recommend an
allowable bearing pressure of 2,000 pounds per square foot (psf) for design of spread footings on the
undisturbed dense glacial deposits or on properlyplaced and compacted structural fill overlying the
dense glacial soils.
A relatively large mound of yard waste was observed along the western siteboundary. Removal of
this fill should be planned as part of the project.
Ten or more feet of fill was observed beneath the unpaved roadway in the southwestern portion of the
site. In general, removal of non-engineered fill situated beneath proposed structures or roadways
shouldbe planned as part of the project. The existing fill may remain in place if subsequent testing
verifies it has been compacted to structural fill specifications.
On-grade slabs for the houses should be underlain bya capillarybreak layer consisting of at least a
4-inch thickness of gravel overlain by a vaporretarder consisting of plastic sheeting.
Site soils that will be encountered during construction and may be considered for use as structural fill
include the medium dense to very dense glacial deposits. The ice contact and till soils contain a high
percentage of fines and will be sensitive to changes in moisturecontent and difficult to handle and
compact during wet weather. We expect that operation of equipment on these soils will not be
difficult when the soils are at their natural moisturecontent. However, site preparation and earthwork
should be completed during the drier summer months to avoid the increased cost of importing fill if
the on-site soils become wet and unsuitable for use as structural fill.
The verydense glacial depositsmay be verydifficultto excavate in the planned deeper cuts and large
excavators and/or dozers equipped with rippers may be needed.
We anticipate that shallowgroundwatermay be present as perched layers within the glacial deposits
during the wet winter and spring months. We do not expect that groundwater other than relatively
small quantities of perched or trapped groundwaterwillbe encountered during excavation.
Groundwater may also be encountered at the contact between the fill soils and native glacial soils.
We expect that seepage water can typicallybehandled bydigging interceptor trenches in the
excavations and pumping from sumps.
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Cobbles and boulders were encountered in our explorations, and the contractor should be prepared to
deal with themwhen encountered.
Effective erosion and sedimentation controls must be implemented duringconstructionso that
potential impacts to the adjacent areas areminimized. The erosion potential of the on-site soils is low
to high. The erosion and sedimentation control measures used for this project shouldbe in
accordance with the requirements of the City of Auburn.
EARTHWORK
General
Based on the subsurface soil conditions encountered in the explorations, we expect that the soils at the
sitemay be excavated using conventional construction equipment. Very dense glacial deposits in the
deeper cuts may require a large, heavy-duty excavator or dozer with rippers to accomplish the
excavations. Glacial deposits on site commonly contain cobbles and boulders that will be encountered
during excavation. Accordingly, the contractor should be prepared to deal with cobbles and boulders.
The till and ice contact soils contain sufficient fines (materialpassing the U.S. standard No. 200
sieve) to be highly moisture-sensitive and susceptible to disturbance, especially when wet. Ideally,
earthwork should be undertaken during extended periods of dry weather when the subgrade soils will be
less susceptible to disturbance and provide better support for construction equipment.Dry weather
construction will help reduce earthwork costs and increase the potential for using the native soils as
structural fill.
Trafficability on the site is not expected to be difficult during dryweather conditions. However, the
native soils will be susceptible to disturbance fromconstruction equipment during wet weather conditions
and pumpingand rutting of the exposed soils under equipment loads may occur.
We recommend that a representative from our firmbe present during proofrolling and/or probing of
the exposed subgrade soils in building and pavement areas and during placement of structural fill as
described below. Our representative will evaluate the adequacy of the subgrade soils and existing fill
soils, identify areas needing further work; perform in-placemoisture-density tests in the fill to determine
if the compaction specifications are being met, and provide advice on any proceduralmodifications that
may be appropriate for the prevailing conditions.
Clearing and Site Preparation
Areas to be developed or graded should be cleared of surface and subsurface deleterious matter
including any debris, underbrush, trees and associated stumpsand roots. Graded areas shouldbe stripped
of organic soils. Based on our explorations and site observations, we estimate that stripping depths at the
site will generally be on the order of 12 inches to remove the topsoil and organic soils. Deeper
excavations may be needed to remove root balls associated with large trees. The mound of yard waste
located along the western site boundaryalso should be removed.
The organic strippings can be stockpiled and processed for landscaping purposes to revegetate
disturbed areas following completion of grading. If spread out to revegetate disturbed areas, the organic
strippings should be placed in a layer less that 1 foot thick, should not be placed on slopes greater than
3H:1V (horizontal to vertical) and should be track-walked to a uniformly compacted condition. Materials
that cannot be used for landscaping or revegetation of disturbed areas should be removed from the project
site.
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We recommend that any unsuitable soil or fill encountered in the proposed building areas and within
two feet of pavement subgrades be removed and replaced with properlycompacted structural fill.
Excavations to remove any unsuitable soil or fill should extend horizontally beyond the building
perimeters and edges ofpavement for a distance equal to the excavation depths. We anticipate that
excavations greater than or equal to 10 feet deep may be required under the proposed roadway extension
of Evergreen Way Southeast in the southwestern portion of thesite, where buried topsoil and fill soils
were encountered (TP-11 and TP-12).
Subgrade Preparation
Prior to placing new fills, pavement base course materials or capillarybreak materials below on-grade
floor slabs, all subgrade areas should be evaluated by proofrolling to locate any soft or pumping soils.
Proofrollingcan be completed using a piece of heavy, tire-mounted equipment such as a loaded dump
truck. During wet weather, the exposed subgrade areas should be probed to evaluate the presence and
determine the extent of soft soils. If soft or pumping soils are observed they shouldbe removed and
replaced with structural fill.
If deep pockets of soft or pumping soils are encountered in areas to be developed outside the building
areas, it maybe possible to limit the depth of overexcavation byplacing a woven geotextile fabric such as
Mirafi 600X (or approved equivalent) on the overexcavated subgrade prior to placing structural fill. The
geotextile will provide additional support by bridging over the soft material and will help reduce fines
contamination into the structural fill.We anticipate that no more than 2 feet of structural fill placed over
a geotextile fabric will be needed to support pavement areas over soft subgrade conditions at this site.
After completing the proofrolling, the subgrade areas shouldbe recompacted to a firm and unyielding
condition, if possible.The degree of compaction that can be achieved will depend on when the
construction is performed. If the work is performedduringdry weather conditions, we recommend that
all subgrade areas be recompacted to at least 95 percent of the maximum drydensity (MDD) in
accordance with the American Society for Testing and Materials (ASTM) D 1557 test procedure. If the
work is performed during wet weather conditions, it may not be possible to recompact the subgrade to
95percent of MDD (ASTM D 1557). In this case,werecommend that the subgrade be compacted to the
extent possible without causing undue weaving or pumping of the subgrade soils.
Subgrade disturbance or deterioration could occur if the subgrade is wet and cannot be dried. If the
subgrade deteriorates during proofrolling or compaction, it maybecome necessary to modify the
proofrollingor compaction criteria or methods.
Structural Fill
All fill, whether existing on-site soil or imported soil, that will support floor slabs, pavement areas or
foundations, or will be used for fill slopes, or placed against retaining walls or in utility trenches should
generally meet the criteria for structural fill presented below. The suitability of soil for use as structural
fill depends on its gradation and moisture content. The existing dense glacialsoils encountered in some
of the explorations are expected to be suitable for structural fill in areas requiring compaction to at least
95percent of MDD, as determined by the ASTM D 1557 testmethod,provided the work is accomplished
during the normally dryseason (July through September)and that the soil can be properly moisture
conditioned. However, for wet weatherconstruction, we believe that it may be necessary to importsand
and gravel with a low fines content to achieve adequate compaction for support of pavement areas, floor
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slabs and structures. Alternatively, admixtures, such as cement or lime, could be used toimprove the
workability of the native fine-grained soils to permituse as structural fill during wet weather conditions.
We recommend importedstructural fill consist of either crushed gravel or well graded sand and
gravel containing less than 5 percent fines (materials passing U.S. Standard No. 200 sieve) by weight
relative to the fraction of thematerial passing the ¾-inch sieve. This imported fill material should be free
of rock fragments larger than 4 inches, debris and organicmaterial. We recommend that the suitabilityof
structural fill material from proposed borrow sources be evaluated by the geotechnical engineer before the
earthwork contractor is allowed to transport any material to the site.
Structural fill should be mechanically compacted to a firm, non-yielding condition. Structural fill
placed in building areas to support footings and floor slabs should be compacted to at least 95 percent of
MDD (ASTM D 1557). Pavementarea fill, including utility trench backfill, should be compacted to at
least 90 percent of MDD, except for the upper 2 feet below finished subgrade surface, which should be
compacted to at least 95 percent of MDD. Structural fill to support walkways should be placed after the
subgrade is evaluated and be compacted to at least 90percent of MDD. Structural fill used to construct
fill slopes should be compacted to at least 90 percent of MDD. Structural fill that is compacted by heavy
equipmentshould be placed in loose lifts generallynot exceeding 10 inches in thickness. Each lift should
be conditioned to the proper moisture content and compacted to the specified density before subsequent
lifts are placed.
We recommend that fill placed against below grade walls and retaining walls be compacted to
between 90 and 92percent MDD, using hand operatedcompaction equipment within 5 feet of the wall.
Overcompaction should be avoided to prevent the buildupof excessive lateral pressures on the wall.
Temporary Cut Slopes
Temporary cut slopes maybe required for rough grading, construction of retaining structures,
basement excavations, or to install utilities, such as the planned sewer main along Kersey Way.
The stabilityof open cut slopes is a function of soil type, groundwaterseepage, slope inclination,
slope height and nearby surface loads. The use of inadequately designed open cuts could impact the
stability of adjacent work areas, existing utilities, and endangerpersonnel. In our opinion, the contractor
will be in the best position to observe subsurface conditions continuously throughout theconstruction
process and to respond to variable soil and groundwater conditions. Therefore, the contractor should have
the primary responsibilityfor deciding whether or not to use open cut slopes rather than some form of
temporary excavation support, and forestablishing the safe inclination of the cut slope. Although, we
anticipate shoring such as braced trench boxes will be required for installation of the proposed sewermain
along Kersey Way because of the desire to reduce traffic impacts and disturbance to adjacent
infrastructure.
Acceptable slope inclinations should be determined during construction. All open cut slopes and
temporary excavation support should be constructed or installed, and maintained in accordance with the
requirementsof the appropriate governmental agency. For planning purposes, temporary unsupported cut
slopesmore than 4 feet high may be inclined at 1-1/2H:1Vmaximum steepness within the surficial soils,
medium dense glacial deposits, and within properly compacted structural fill. Cut slopes can be
steepened to 0.75H:1V within the dense to very dense glacial deposits if groundwater seepage is not
present and as approved by the geotechnical engineer.If seepage is present on the cut face of the dense to
verydense glacial deposits then the cut slope should be inclined no steeper than 1H:1V. We recommend
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that a representative from our firm observe the cuts in the glacial deposits to assess stability prior to
making final temporary cuts.
The above guidelines assume that surface loads such as equipment loads and stockpile loads will be
kept a sufficient distance away fromthe top of the cutso that the stability of the excavation is not
affected. We recommend that this distance be not less than half the height of thecut.
Some sloughing and raveling of the cut slopes should be expected. Temporary covering, such as
heavy plastic sheeting with appropriate ballast, should be used to protect these slopes during periods of
wet weather. Surface water runoff fromabove cut slopes should be prevented from flowing over the
slope face byusing berms,drainage ditches, swales or other appropriate methods.
If temporary cut slopes experienceexcessivesloughing or raveling during construction, it may
become necessary to modify the cut slopes to maintain safe working conditions. Slopes experiencing
problems can be flattened, regraded to add intermediate slope benches, or additional dewatering can be
provided if the poor slope performance is related to groundwater seepage.
Permanent Cut and Fill Slopes
We recommend that permanent cut or fill slopes be constructed at inclinations of 2H:1V or flatter,
and be blended into existing slopes with smooth transitions. To achieve uniform compaction, we
recommend that fill slopes be overbuiltslightly and subsequently cut back to expose well compacted fill.
To reduce the risk of erosion, newly constructedslopes should be planted or hydroseeded shortly after
completion of grading. Until the vegetation is established, somesloughing and raveling of the slopes
should be expected. This may necessitate localized repairs and reseeding. Temporary covering, such as
clear heavyplastic sheeting, jute fabric, loose straw or excelsior matting could be used to protect the
slopes during periods of rainfall.
Utility Trenches
General
Trench excavation, pipe bedding, and trench backfilling should be completed using the general
procedures described in the 2002 Washington State Departmentof Transportation (WSDOT) Standard
Specifications or other suitable procedures specified by the project civil engineer.
Utility trench backfill should consist of structural fill and should be placed in lifts of 10 inches or less
(loose thickness) such that adequate compaction can be achieved throughout the lift. Sand backfill,
containing less than 5 percent fines, maybe compacted in loose lifts not exceeding 12 inches when placed
below five feet of the finished ground surface. Each lift must be compacted prior to placing the
subsequent lift. Prior to compaction, the backfill should be moisture conditioned to within 3 percent of
the optimum moisture content, if necessary. The backfill shouldbe compacted in accordance with the
criteria discussed above in the structural fill section of this report.
SanitarySewerMain
Alternatives481 and 700both includethe option of installing a sanitary sewer systemwithin Kersey
Way, with an average sewer depth of approximately12 feet (Apex Engineering, 2003). Inour opinion,
normal pipe bedding and manhole leveling course requirements will besatisfactory. This would
ordinarily include at least 6 inches of pea gravel, crushed rock or sand on each side of the pipe and below
manholes.
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Where the pipe is bedded and/or backfilled with free-draining pea gravel, crushed rock, sand or
gravel, and where the pipe invert slopes more than 5 percent, we recommend that backfill seepage barriers
be constructed at appropriate intervals to prevent movement of ground water through the beddingor
backfill soils. Seepage barriers are typically spaced 100 feet apart for grades of 20 percent or less.
Steeper utility trench grades will require closer spacing, probably 30 feet or less.
We recommend that seepage barriers consist of CDF (controlled densityfill). Alternatively,
impermeablesoils such as silt or clay with or without cement or bentonite could be used, but we believe
that it would be easier and more economical to use simple forming to place CDF barriers prior to placing
trench backfill. Each seepage barrier should be notched at least 12 inches into the base and sides of the
trench to key the barrier into the native soils. Although it is desirable to also notch into the trench
sidewalls, we understand that this would be difficult to do if a trench box is used as temporary shoring. If
the contractor can suggest an alternative, we should be contacted to evaluate the suggested option. Care
must be taken during barrier construction to avoid pipe damage.
The barriers should extend from the base of the key trench, surround each pipe, and extend a distance
of at least 2 feet or one pipe diameter, whichever is greater, above the top of the pipe bedding. They
shouldbe at least 2 feet long measured parallel to the pipes. At manhole locations, the seepage barriers
could be constructed using CDF backfill on all sides of the manhole.
Sedimentation and Erosion Control
In our opinion, the erosion potential of the on-site soilsis slight to very severe (i.e., low to very high).
Construction activities including stripping and grading will expose soils to the erosional effects of wind
and water. The amount and potential impacts of erosion are partly related to the timeofyear that
constructionactually occurs. Wet weather construction will increase the amount and extent of erosion
and potential sedimentation.
Erosion and sedimentation controlmeasuresmaybe implemented byusing a combination of
interceptor swales, strawbale barriers, silt fences and straw mulch for temporary erosion protection of
exposed soils. All disturbed areas should be finish graded and seeded as soon as practical to reduce the
risk of erosion. Erosion and sedimentation control measuresshould be installed and maintained in
accordance with the requirements of the City of Auburn.
FOUNDATIONS
General
We recommend that the planned residences be supported on spread footings founded on the medium
dense to very dense native glacial deposits or on properly compacted structural fill extending down to
dense to verydense glacial deposits. If structural fill is used to support foundations then the zone of
structural fill should extend beyond the faces of the footing a distance at least equal to the thickness of the
structural fill.
Foundation Design
For shallow foundation support, we recommend perimeter footing widths of at least 12, 15 and
18 inches for one-, two-, and three-story homes, respectively. Interior, isolated column footings should be
at least twice as wide as the perimeterfooting widths described above for the representative number of
floors in the home. Provided that footings are supported as recommended above, an allowable bearing
value of 2,000psf may be used for footings supported on the medium dense to very dense glacial deposits
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or on structural fill compacted to at least 95 percent MDD placed over the glacial soils. This allowable
bearing pressure applies to the total dead and long-term live loads and may be increased up to one-third
for short-term live loads such as wind or seismic forces.
The design frost depth for the Puget Sound area is 12 inches, therefore, we recommend that exterior
footings forstructures be founded at least 18 inchesbelow lowest adjacent finished grade. Interior
footings should be founded at least 12 inches below bottom of slab or adjacent finished grade.
Foundation Settlement
We estimate that the post-construction settlement of footings founded as recommended above will be
less than one inch. Differential settlement between comparably loaded column footings or along a
25-foot section of continuous wall footing should be less than ½inch. We expect most of the footing
settlementswill occur as loads are applied. Loose or disturbed soils not removed from the footing
excavation prior to placing concrete will result in additional settlement.
Immediatelyprior to placing concrete, all debris and soil slough that accumulated in the footings
during forming and steel placement must be removed. Debris or loose soils not removed from the footing
excavations will result in increased settlement.
If wet weather construction is planned,we recommend that all footing subgrades be protected using a
lean concrete mudmat. The mudmat should be placed the same day thatthe footing subgrade is
excavated and approved for foundation support.
We recommend that all footing excavations be evaluated by a representative of our firm immediately
before any structural fill, mud mat, steelor concrete is placed, to evaluate if the work is being completed
in accordance with our recommendations and that subsurface conditions are as expected.
Lateral Resistance
Lateral loads may be resisted by a combination of friction between the footing and the supporting
soil, and by the passive lateral resistance of the soilsurrounding the embedded portions of the footings.
For shallow foundations constructed as recommended above, the allowable frictional resistance may be
computed using a coefficient of friction of 0.35 applied to the vertical dead load. The allowable passive
resistance on the sides of the footingsmay be computed using an equivalent fluid density of 300 pounds
per cubic foot (pcf) if the footings are surrounded bymedium dense to very dense native soil or structural
fill. The structural fill should extend out from the face of the foundation element for a distance at least
equal to three times the depth of the element and be compacted to at least 95 percentof the MDD
(ASTM D-1557). The above values include a factor of safety of about 1.5.
Footing Drains
We recommend that perimeter footing drains be installed around each house.The perimeter drains
shouldbe installed at the base of the exterior footings. The perimeter drains shouldbe provided with
cleanouts and should consist of at least 4-inch-diameter perforated pipe placed on a 3-inch bed of, and
surroundedby, 6 inches of drainage material enclosed in a non-wovengeotextile fabric such as
Mirafi140N (or approved equivalent) to prevent fine soil frommigrating into the drain material. We
recommend that the drainpipe consist of either heavy-wall solid pipe (SDR-35 PVC, or equal) or rigid
corrugated smooth interior polyethylene pipe (ADS N-12, or equal). We recommend against using
flexible tubing for footing drainpipes. The drainagematerial should consist of pea gravel or “Gravel
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Backfill for Drains” per WSDOT Standard Specifications, Section9-03.12(4). The perimeter drains
should be sloped to drain bygravity, if practicable, to a suitable discharge point, preferably a storm drain.
We recommend that the cleanouts be covered, and be placed in flush mounted utility boxes. Water
collected in roof downspout lines mustnot be routedto the footing drain lines.
BELOW GRADE WALLS AND RETAINING WALLS
General
The concept level design suggests that cuts and fills at the site may range up to 25 and 20 feet,
respectively. At this time it is not known if these grade transitions will be supported by retaining walls or
by permanent inclined slopes. If retaining structures are selected, we can provide recommendations for
suitable retaining wall types and design parametersafter the geometry is better defined (location, layout,
height, etc.). There is a strong likelihood that below-grade foundation walls and conventional concrete
walls or concrete masonryunit (CMU) block walls will be used at thesite, so recommendations for these
types of retaining walls are provided below.
Design Parameters
Lateral earth pressures for design of conventional below-grade walls and retaining structures should
be evaluated using equivalent fluid densities of 35 pounds per cubic foot (pcf) and 53 pcf for level
backfill conditions and backfill inclined at 2H:1V, respectively, provided that the walls will not be
restrained against rotation when backfill is placed.Linear interpolation can be usedtoassess the design
lateral earth pressures for intermediateslopeinclinations.If thewallswill be restrainedfrom rotation, we
recommend usingequivalent fluid densities of 55 pcfand 80 pcf for level backslope and 2H:1V backslope
conditions, respectively.Walls are assumed to be restrained if top movement during backfilling is less
than H/1000, where H is the wall height. These lateralsoil pressures do not include the effects of
surchargessuch as floor loads, traffic loads or othersurface loading. Surcharge effects should be included
as appropriate.
If vehicles can approach the tops of exterior walls to within 3/4 the height of the wall, a traffic
surcharge should be added to the wall pressure. For car parking areas, the traffic surcharge can be
approximated by the equivalent weight of an additional 1 foot of soil backfill behind the wall. For
delivery truck parking areas and access driveway areas,the traffic surcharge can be approximated by the
equivalent weight of an additional2feet of soilbackfill behind the wall. Surcharge loading from
earthquakes can be modeled as a uniform lateral earthpressure of 7H, where H represents the wall height,
for both restrained and unrestrained walls.
These recommendations are based on the assumption that all retaining walls at this project will be
provided with backdrainage, as discussed below. The values for soil bearing, frictional resistance and
passive resistance presented above for foundations are applicable to retaining wall design. Footings for
walls located in level ground areas should be founded at a depth of 18 inches below the adjacent grade.
Embedment for footings founded in areas with sloping ground should be evaluated on a case-by-case
basis.
Backdrainage
To reduce the potential for hydrostatic water pressure buildup behind the retaining walls, we
recommend that the walls be providedwith backdrainage. Backdrainage can be achieved by using free
draining material with perforated pipes to discharge the collected water.
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Free draining material should consist of a mixtureof about 30 to40 percent clean medium to coarse
sand and 60 to 70percent fine gravel, with negligible fine material (smaller than no. 200 sieve.)
Alternatively, the free draining materialcan consist of clean gravel (as discussed above for footingdrains)
surrounded by a non-woven geotextile fabric such as Mirafi 140N (or approved equivalent). The zone of
free draining material should be at least 2 feet wide and should extend from the base of the wall to within
1 foot of the ground surface. The free draining material should be covered with 1 foot of less permeable
material, such as the on-site ice contact and till soils.
A 4-inch-diameter perforated collector pipe should be installed within the free-draining material at the
base of each wall. We recommend using either heavy-wall solid pipe (SDR-35 PVC, or equal) or rigid
corrugated smooth interior polyethylene pipe (ADS N-12, or equal). We recommend against using
flexible tubing for wall backdrain pipe. The footing drain recommended above can be incorporated into
the bottom of the backdrainage zone and used for this purpose.
The pipes shouldbe laid with minimumslopes of one percent and discharge into the stormwater
collection system to conveythe water off site. The pipe installations should include a cleanout riser with
cover located at the upper end of each pipe run. The cleanouts could be placed in flush mounted access
boxes. The roof downspouts should not discharge into the perforated pipes intended for providing wall
backdrainage.
Alternatively, where seepage at the face of a wall isnot objectionable, the walls can be provided with
weepholes to discharge water from the free draining wall backfill material.The weepholes should be
2-inch diameter, and spaced about every8 feet center-to-center along the base of the walls. The
weepholes shouldbe backed with galvanized heavy wiremesh to prevent loss of the backfill material.
Construction Considerations
Careshould be taken by the contractor during backfilling to avoid overstressing the retaining walls.
Zones of wall backfill not supporting structural elementsshould be compacted to between 90 and
92percent of maximum drydensity. Compaction of at least 95 percent will be needed in the upper 2 feet
and at least 92percent below that where the backfill supports structural elements such as slabs or
driveways.Heavy compaction equipment should not be operated within 5feet of below-grade walls or
retaining structures to avoid overstressing the walls. Hand-operated equipment shouldbe used in this
zone. In addition, the contractor should keep all heavy construction equipment away from the top of
retaining walls a distance equal to 3/4 the height of the wall, or at least 5 feet, which ever is greater.
FLOOR SLAB SUPPORT
We expect that floor slabs can be supported on the medium dense to very dense native soil
encountered in our explorations or on properly compacted structural fill extending down to these soils.
However, we recommendthat an appropriate capillarybreak and vapor retarder be installed below the
floor slab inthe proposed buildings to reduce the risk of moisture migration through the on-grade floor
slab. This is especially important since zones of groundwater seepagemay be present below the planned
floor slab level in morepermeable layers within the native soil. If zonesof significant seepage are
observed during construction, then additional drainage protection such as a system of perforated
drainpipes located below the floor slab withinthe capillary break materialmay be necessary.
We recommend that floor slabs be constructed on a gravel layer to provide uniform support, drainage,
and to act as a capillary break. The gravel layer should consistof at least 4 inches of clean gravel with a
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maximum size of ¾ inches and negligible sand or silt. If prevention of moisturemigration through the
slab is essential in the buildings, (e.g., in portions of the building areas where an adhesive will be used to
attach tile or carpeting, for storage areas, and for enclosed areas), a vapor retardant such as plastic
sheeting should be installed between the slab and the gravel layer. It may also be prudent to apply a
sealer to the slab to further retard the migration of moisture through the floor.
SEISMICITY
General
The Puget Sound area is a seismically active region and has experienced thousands of earthquakes in
historical time. Seismicity in this region is attributed primarilyto the interaction between the Pacific,
Juan de Fuca and North American plates. The Juan de Fuca plate is subducting beneath the North
American Plate. Each year 1,000 to 2,000 earthquakes occur in Oregon and Washington. However, few
of these are typically felt because the majority of the earthquakes are relatively minor,smaller than
Richter magnitude3.
Potential seismic hazards fromearthquakes include ground shaking, liquefaction, ground rupture from
lateral spreading and surface fault rupture, and landslides. Our opinions regarding the likelihood of these
seismic hazards occurring at the siteare presented below. These opinions are based on the seismicity
criteria recommended in the 1997 edition of the Uniform Building Code (UBC).
Ground Shaking
There is a risk of earthquake induced ground shakingat the site, as with all sites in the Puget Sound
region, and the intensityof the ground shaking could be severe. The severity of ground shaking will
primarily be a function of the earthquake magnitudeand proximity to the site. In our opinion, strong
ground shaking should be considered in the design of the structures and improvements at this site.
We recommend that the seismic ground shaking at the site be evaluated in accordancewith the
applicable edition of the UBC.
UBC Site Coefficients
The Puget Sound region is designated as a Seismic Zone 3 in the 1997 edition of theUniform
Building Code (UBC). For Zone 3 locations, a Seismic Zone Factor (Z) of 0.30 is applicable based on
UBC Table 16-I. Based on UBC Table 16-J, the soil profile at the site is best characterized as Type S.
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Liquefaction Potential
Liquefaction is a phenomenon where soils experience a rapid loss of internal strength as a
consequenceof strong ground shaking. Ground settlement, lateral spreading and/or sand boilsmay result
from soil liquefaction. Structures supported on liquefied soils could suffer foundation settlement or
lateralmovement that could be severely damaging to the structures.
Conditions favorable to liquefaction occur in looseto medium dense, clean to moderately silty sand
that is below the groundwater level. Based on ourevaluationof the subsurface conditions observed in the
explorationscompleted at the site, it is our opinion that the potential for liquefaction at the site is low.
Ground Rupture
Ground rupture from lateral spreading is associatedwith liquefaction. Lateral spreading involves
lateral displacements of large volumesof liquefied soil, and can occur on near-level ground as blocks of
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surface soils displace relative to adjacent blocks.In our opinion, ground rupture resulting from lateral
spreading at the site is unlikely because the potential for liquefaction is low.
Because of the thickness of the Quaternary sediments below the site, which are inferred to bemore
than 1,000 feet thick, the potential for surface fault rupture is considered remote.
Landslides
On-site areas at risk for seismically triggered landslides (Figure 12) are the sameareas identified as
landslide hazard areas on Figure 11. Designs for infrastructure adjacent to landslide areas should include
appropriate engineering controls (buffers, setbacks, drainage enhancements,retaining walls, etc.) to
reduce the impact of the landslide hazard areas on the infrastructure, and visa versa.
PAVEMENT RECOMMENDATIONS AND SUBGRADEPREPARATION
Subgrade Preparation
Parking area and access drive pavement subgrades should be prepared as described previously in the
Earthwork section of this report. In addition to these requirements, we recommend that the prepared
subgrade be proofrolled thoroughly prior to paving tolocate any soft or pumping soils. If proofrolling is
not practical, GeoEngineers should evaluate the prepared subgrade with a hand probe rod. If soft or
pumping soils are encountered, such unsuitable subgrade soils should be overexcavated and replaced.
The depth of overexcavation should be determined by GeoEngineers.
It may be possible to limit the depth of overexcavation of unsuitable subgrade soils by placing a
geotextile reinforcement fabric such as Mirafi 600X (or approved equivalent) on the overexcavated
subgrade and covering the geotextile with base coursematerial or rock spalls. The geotextile will provide
additional support by bridgingover the soft material, and will help reduce fines contamination into the
gravel or rock spalls. The combination of geotextile and gravel or rock should provide a stable base on
which to place and compact other pavement base course materials.
Asphalt Concrete Pavement
In automobile parking areas, we recommend a minimum pavement section consisting of 2 inches of
ClassB asphalt concrete (AC) over a 4-inch thickness of densely compacted crushed rock base course. In
truck traffic areas, we recommend a minimum pavement section consisting of at least 3 inches of Class B
asphalt concrete (AC) over a 6-inch thickness of densely compacted crushed rock base course. The base
course should be compacted to at least 95 percent of the maximum dry density (ASTM D 1557). Thicker
asphalt sections may be needed based on the actual traffic data.
Asphalt-Treated Base
Because pavementsmay be constructed during the wet seasons, consideration may be given to
covering the areas to be paved with asphalt-treated base (ATB) for protection. Parking areas should be
surfaced with 3 inches ofATB, and truck driveways for materials delivery should be surfaced with 6
inches of ATB. Prior to placement of the final pavement sections, we recommend that areas of ATB
pavement failure be removed and the subgrade repaired. If ATB is used and is serviceable when final
pavementsare constructed, the crushed surfacing base course can be eliminated, and the design asphalt
concrete pavement thickness can be placed directly over the ATB.
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STORMWATER PONDS
The conceptual layout forthe development includes two stormwater detention ponds, one near the
northwest corner of the site and one near the north central portion of the site.At this time the specific
pond locations, elevation and geometryhave not been identified. In general, the native glacial till soils
and ice contact materials are relatively impermeable,and ponds cut into these materialswould probably
not require liners. We anticipate the glacial till and ice contact materials will also be suitable for use as
embankmentfill to construct detention ponds, provided adequate compaction can be achieved.
The native advance outwash soils arerelatively permeable and generally would not be suitable for
construction of detention pond embankments. Ponds excavated into advance outwash materialswould
need to be lined.
As currentlyenvisioned, the pond in the northwest portion of the site would be situated near landslide
hazard areas. The impact of the pond on the landslide hazard areas, and visa versa, will need to be a
primary design consideration.
DRAINAGE CONSIDERATIONS
We anticipate shallow groundwater seepage may enter deep excavations depending on the time of
year construction takes place, especially in the wintermonths.However, we expect that this seepage
watercan be handled by digging interceptor trenches in the excavations and pumping fromsumps. The
seepage water if not intercepted and removed from the excavations will make it difficult to place and
compact structural fill and may destabilize cut slopes.
All paved and landscaped areas should be graded so that surface drainage is directed away from the
buildings to appropriate catch basins.
Watercollected in roof downspout linesmust not be routed to the footing drain lines. Roof
downspout water should be routed to appropriate discharge points in separate pipe systems.
LIMITATIONS
We have prepared this report for use by Apex Engineering PLLC, the City of Auburn, and other
members of the design team for the Kersey III project.
Within the limitations of scope, schedule and budget, our services have been executed in accordance
with generally accepted practices in the fields of geology and geotechnical engineering in this area at the
time this report was prepared. No warranty or otherconditions, express or implied, should be understood.
Please refer to Appendix E titled Report Limitations and Guidelines for Use for additional
information pertaining to use of this report.
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APPENDIX E
1
REPORT LIMITATIONS AND GUIDELINES FOR USE
This appendix provides information to helpyou manageyour risks with respect to the use of this
report.
GEOTECHNICAL SERVICESARE PERFORMED FOR SPECIFIC PURPOSES,
PERSONS AND PROJECTS
This report has been prepared for use by Apex Engineering PLLC, the City of Auburn, and other
members of the project teamfor design of the proposed development. This report may be made available
to prospective contractors for bidding or estimating purposes; but our report, conclusions and
interpretations should not be construed as a warranty of the subsurface conditions. This report is not
intended for use by others, and the information contained herein is not applicable to other sites.
GeoEngineers structuresour services to meet thespecific needs of our clients. For example,a
geotechnicalor geologic study conducted for a civil engineer or architectmay not fulfill the needs of a
construction contractor or even another civil engineer or architect that are involved in the same project.
Because each geotechnical or geologic study is unique, each geotechnical engineering or geologic report
is unique, prepared solely for the specific client and project site. No one except Apex Engineering PLLC
and the Cityof Auburn should relyon this report without first conferring with GeoEngineers. This report
shouldnot be applied for any purpose or project except the one originally contemplated.
A GEOTECHNICAL ENGINEERING OR GEOLOGIC REPORT IS BASED ON A UNIQUE
SET OF PROJECT-SPECIFIC FACTORS
This report has been prepared for the proposed Kersey III subdivision located in Auburn,
Washington. GeoEngineers considered a number of unique,project-specific factors when establishing the
scope of services for this project and report. Unless GeoEngineers specifically indicates otherwise, do not
rely on this report if it was:
notprepared for you,
notprepared for yourproject,
not prepared for the specific site explored, or
completed before important project changes were made.
For example,changes that can affect the applicabilityof this report include those that affect:
the function of the proposed structure;
elevation, configuration, location, orientation or weight of the proposed structure;
compositionof the design team; or
project ownership.
If importantchanges are madeafter the date of this report, GeoEngineers should be given the
opportunity to review our interpretations and recommendations and provide written modifications or
confirmation, as appropriate.
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Developed based on material provided by ASFE, Professional Firms Practicing in the Geosciences; www.asfe.org .
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SUBSURFACECONDITIONS CAN CHANGE
This geotechnical or geologic report is based on conditions that existed at the time the study was
performed. The findings and conclusions of this report may be affected by the passage of time, by
manmade events such as construction on or adjacent to the site, or by natural events such as floods,
earthquakes, slope instabilityor groundwater fluctuations. Always contact GeoEngineers before applying
a report to determine if it remains applicable.
MOST GEOTECHNICAL AND GEOLOGIC FINDINGS ARE PROFESSIONAL OPINIONS
Our interpretations of subsurface conditions are based on field observationsfrom widely spaced
sampling locations at the site. Site exploration identifies subsurface conditions only at those points where
subsurface tests are conducted or samples are taken. GeoEngineers reviewed field and laboratory data
and then applied our professionaljudgment to render an opinion about subsurfaceconditions throughout
the site. Actual subsurface conditions may differ, sometimessignificantly, from those indicated in this
report. Our report, conclusions and interpretations should not be construed as a warranty of the
subsurface conditions.
GEOTECHNICAL ENGINEERING REPORT RECOMMENDATIONS ARE NOT FINAL
Do not over-rely on the preliminary construction recommendations included in this report. These
recommendations are not final, because they were developed principally from GeoEngineers’professional
judgment and opinion.GeoEngineers’ recommendations can be finalized only byobserving actual
subsurfaceconditions revealed during construction. GeoEngineers cannot assume responsibility or
liability for this report's recommendations if we do not perform construction observation.
Sufficientmonitoring, testing and consultationby GeoEngineers should be provided during
constructionto confirm that the conditions encountered are consistent with those indicated by the
explorations,to provide recommendations for design changes should the conditions revealed during the
work differ from those anticipated, and to evaluatewhether or not earthwork activitiesarecompleted in
accordancewith our recommendations. Retaining GeoEngineers for construction observation for this
project is the most effective method of managingthe risks associated with unanticipated conditions.
A GEOTECHNICAL ENGINEERING OR GEOLOGIC REPORT COULD BE SUBJECT TO
MISINTERPRETATION
Misinterpretation of this report byother design team memberscan result in costlyproblems. You
could lower that risk by having GeoEngineers confer with appropriate members of the design team after
submitting the report. Also retain GeoEngineers to review pertinent elements of the design team's plans
and specifications. Contractors can alsomisinterpret a geotechnical engineering or geologic report.
Reduce that risk by having GeoEngineers participatein pre-bid and preconstruction conferences, and by
providing construction observation.
DO NOT REDRAW THE EXPLORATION LOGS
Geotechnical engineers and geologists prepare final boring and testing logs based upon their
interpretation of field logs and laboratory data. Toprevent errors or omissions, the logs included in a
geotechnical engineering or geologic report should neverbe redrawn for inclusion in architectural or other
design drawings. Onlyphotographic or electronic reproduction is acceptable, but recognize that
separating logs from the report can elevate risk.
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GIVE CONTRACTORS A COMPLETE REPORT ANDGUIDANCE
Some owners and design professionals believe they can makecontractors liable for unanticipated
subsurface conditions by limiting what they provide for bid preparation. To help prevent costlyproblems,
give contractors the complete geotechnical engineering or geologic report, but preface it with a clearly
written letter of transmittal. In that letter, advise contractors that the report was not prepared for purposes
of bid development and that the report's accuracy is limited; encourage them to confer with GeoEngineers
and/or to conduct additional study to obtain the specific types of information they need or prefer. A pre-
bid conference can also bevaluable. Besure contractors have sufficient time to perform additional study.
Only then might an owner be in a position to givecontractors the best information available, while
requiring them to at least share the financial responsibilities stemming from unanticipated conditions.
Further, a contingency for unanticipated conditions should be included in your project budget and
schedule.
CONTRACTORS ARE RESPONSIBLE FOR SITE SAFETY ON THEIR OWN
CONSTRUCTION PROJECTS
Our geotechnical recommendations are not intended to direct the contractor’s procedures, methods,
schedule or management of the work site. The contractor is solely responsible for job site safety and for
managing construction operations to minimize risks to on-site personnel and toadjacent properties.
READ THESE PROVISIONS CLOSELY
Some clients, design professionals and contractors may not recognize that the geoscience practices
(geotechnical engineering or geology) are far less exact than other engineering and natural science
disciplines. This lack of understanding can create unrealistic expectations that could lead to
disappointments, claimsand disputes.GeoEngineers includes these explanatory “limitations” provisions
in our reports to help reduce such risks. Please confer with GeoEngineers if you are unclear how these
“Report Limitations and Guidelines for Use”apply to yourproject or site.
GEOTECHNICAL, GEOLOGIC AND ENVIRONMENTAL REPORTS SHOULD NOT BE
INTERCHANGED
The equipment, techniques and personnel used to performan environmentalstudy differ significantly
from those used to perform a geotechnical or geologic study and vice versa. For that reason, a
geotechnical engineering or geologic report does not usually relate any environmental findings,
conclusions or recommendations; e.g., about the likelihoodof encountering underground storage tanks or
regulated contaminants. Similarly,environmentalreports are not used to address geotechnical or geologic
concerns regarding a specific project.
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