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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 i G e oE n g i n e er sFile No. 0153-031-00\030504 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 ii G e oE n g i n e er sFile No. 0153-031-00\030504 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 iii G e oE n g i n e er sFile No. 0153-031-00\030504 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 v G e oE n g i n e er sFile No. 0153-031-00\030504 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. 1 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 2 G e oE n g i n e er sFile No. 0153-031-00\030504 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 3 G e oE n g i n e er sFile No. 0153-031-00\030504 (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. 4 G e oE n g i n e er sFile No. 0153-031-00\030504 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 5 G e oE n g i n e er sFile No. 0153-031-00\030504 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 6 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 7 G e oE n g i n e er sFile No. 0153-031-00\030504 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 8 G e oE n g i n e er sFile No. 0153-031-00\030504 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). 9 G e oE n g i n e er sFile No. 0153-031-00\030504 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): 10 G e oE n g i n e er sFile No. 0153-031-00\030504 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 11 G e oE n g i n e er sFile No. 0153-031-00\030504 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 12 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 13 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 14 G e oE n g i n e er sFile No. 0153-031-00\030504 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 15 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 16 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 17 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 18 G e oE n g i n e er sFile No. 0153-031-00\030504 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 19 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. 20 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. 21 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 24 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. 25 G e oE n g i n e er sFile No. 0153-031-00\030504 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). A-1 G e oE n g i n e er sFile No. 0153-031-00\030504 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 G e oE n g i n e er sFile No. 0153-031-00\030504 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 B-3 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 B-4 G e oE n g i n e er sFile No. 0153-031-00\030504 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. B-5 G e oE n g i n e er sFile No. 0153-031-00\030504 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. C-1 G e oE n g i n e e r sFile No. 0153-031-00\030504 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 C-2 G e oE n g i n e e r sFile No. 0153-031-00\030504 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. C-3 G e oE n g i n e e r sFile No. 0153-031-00\030504 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. D-1 G e oE n g i n e er sFile No. 0153-031-00\030504 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. D-2 G e oE n g i n e er sFile No. 0153-031-00\030504 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 D-3 G e oE n g i n e er sFile No. 0153-031-00\030504 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 D-4 G e oE n g i n e er sFile No. 0153-031-00\030504 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. D-5 G e oE n g i n e er sFile No. 0153-031-00\030504 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 D-6 G e oE n g i n e er sFile No. 0153-031-00\030504 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 D-7 G e oE n g i n e er sFile No. 0153-031-00\030504 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. D-8 G e oE n g i n e er sFile No. 0153-031-00\030504 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 D-9 G e oE n g i n e er sFile No. 0153-031-00\030504 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. D 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 D-10 G e oE n g i n e er sFile No. 0153-031-00\030504 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. D-11 G e oE n g i n e er sFile No. 0153-031-00\030504 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. D-12 G e oE n g i n e er sFile No. 0153-031-00\030504 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. 1 Developed based on material provided by ASFE, Professional Firms Practicing in the Geosciences; www.asfe.org . E-1 G e oE n g i n e er sFile No. 0153-031-00\030504 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. E-2 G e oE n g i n e er sFile No. 0153-031-00\030504 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. E-3 G e oE n g i n e er sFile No. 0153-031-00\030504