The URL can be used to link to this page

21-101125-UP-GeoTech-07-26-21-V2 REVISED GEOTECHNICAL ENGINEERING REPORT Proposed Mixed-use Development South 348th Street & 1st Avenue South Federal Way, Washington Project No. 1536.01 22 July 2021 Prepared for: FNW, Inc. Prepared by: TABLE OF CONTENTS Page INTRODUCTION ........................................................................................................................................... 1 SITE DESCRIPTION ..................................................................................................................................... 1 PROJECT UNDERSTANDING..................................................................................................................... 1 SURFACE CONDITIONS ............................................................................................................................. 1 SUBSURFACE CONDITIONS...................................................................................................................... 2 Regional Geology ............................................................................................................................................. 2 Soil Conditions ................................................................................................................................................. 2 Groundwater Conditions ................................................................................................................................. 4 CONCLUSIONS AND RECOMMENDATIONS ............................................................................................ 4 General 4 Regulated Geologic Hazard Environmentally Critical Areas ............................................................................ 5 Seismic Design Considerations ........................................................................................................................ 6 Site Preparation ............................................................................................................................................... 6 Structural Fill ................................................................................................................................................... 9 Utility Trenches ............................................................................................................................................. 11 Temporary and Permanent Slopes ................................................................................................................ 12 Shallow Foundations ..................................................................................................................................... 13 Backfilled Permanent Retaining Walls .......................................................................................................... 14 Rockeries ....................................................................................................................................................... 15 MSE and Gravity Block Retaining Walls ......................................................................................................... 17 On-Grade Concrete Slabs .............................................................................................................................. 18 Drainage Considerations ............................................................................................................................... 18 Asphalt Pavements ........................................................................................................................................ 19 Stormwater Management Considerations .................................................................................................... 20 CLOSURE ................................................................................................................................................... 22 FIGURES Figure 1 – Site and Exploration Plan Figure 2 – Topographic Survey City of Federal Way Drawing No. 3-22 – Rock Facing, Cut Section City of Federal Way Drawing No. 3-23 – Rock Facing, Fill Section APPENDICES Appendix A – Subsurface Exploration Procedures and Logs Appendix B – Laboratory Testing Procedures and Results Page 1 REVISED GEOTECHNICAL ENGINEERING REPORT PROPOSED MIXED-USE DEVELOPMENT SOUTH 348TH STREET & 1ST AVENUE SOUTH FEDERAL WAY, WASHINGTON Project No. 1536.01 22 July 2021 INTRODUCTION This revised report documents the surface and subsurface conditions encountered at the site and our geotechnical engineering recommendations for the proposed Federal Way mixed-use development. The project description, site conditions, and our geotechnical conclusions and design recommendations are presented in the text of this report. Supporting data including detailed exploration logs and field exploration procedures, and results of laboratory testing are presented as appendices. SITE DESCRIPTION The project site is a largely undeveloped parcel located in the southeast quadrant of the 1st Avenue South and South 348th Street intersection and occupies approximately 8.3 acres. Undeveloped properties border the site to the east and south. The adjacent undeveloped properties have been mapped as wetlands on the City of Federal Way Critical Areas Map (May 2016). The site and proposed improvement locations are illustrated on the Site and Exploration Plan, Figure 1. PROJECT UNDERSTANDING We understand that the proposed site improvements will include constructing one seven-story mixed-use building occupying the northwestern portion of the site (Podium), one single-story wood-framed retail building on the northeastern portion of the site (Retail), and eight wood-framed residential apartment buildings (Quads 1- 8) spread across the southwestern, center, and eastern portions of the site. We understand that each quad will be three-stories and the bottom two levels of the podium will be below- grade parking. The buildings will be serviced by asphalt paved parking and access drives. Stormwater management is expected to include use of the existing detention ponds. Moderate cuts and fills will be required to achieve site grades for all structures except the Podium, which will require significant excavation to reach the anticipated foundation grade of approximately 126 feet. SURFACE CONDITIONS The property is partially developed in that three detention ponds occupying approximately the southeastern 1.5 acres were constructed a few years ago. The detention pond area is bordered by chain- link fence and is accessed by about a 275-foot long gravel access road extending east from 1st Avenue Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 2 South. The site gradually slopes downward from the northwest corner to the south and west at an approximate overall 10H:1V gradient. However, the western border of the site and approximately the northwestern 275 feet of the site slopes down to 1st Avenue South and 348th Street with approximately 8 to 12-foot tall slopes inclined as steeply as approximately 45 to 50 percent. The slopes appear to be cut slopes, probably graded in association with construction of South 348th Street and 1st Avenue South. Total relief on the site is about 50 feet from north to south. Vegetation is varied and includes stands of mature trees, blackberry bushes, brush, and grass. Scattered debris (pieces of pipe, ecology blocks, and small piles of gravel), the obvious cut made for the existing access road, and slightly irregular topography indicate that some limited grading occurred in portions of the site other than the detention ponds. SUBSURFACE CONDITIONS Regional Geology We assessed the geologic setting of site and the surrounding vicinity by reviewing the following publication: • Booth, DB, Waldron, HH, and Troost, KG, Geologic map of the Poverty Bay 7.5’ Quadrangle, King and Pierce Counties, Washington, US Geological Survey Scientific Investigations Map 2854, 1:24,000, 2004. The published geologic mapping indicates the site and vicinity are underlain by Vashon glacial till (Qvt). Glacial till is typically composed of silt, sand, gravel, cobbles, and boulders. The till is glacially consolidated and when intact is characterized by a loose to medium dense weathered horizon on the order of about 2 to 4 feet thick underlain by denser unweathered material. Unweathered till typically has a relatively high density, relatively low permeability, and is generally well-suited for support of shallow foundations. The partially developed nature of the site suggests that fill material may be present as well. Both native till and some fill material were encountered in our test pits. Soil Conditions The subsurface evaluation for this project included excavating twelve test pits located approximately as shown on the Site and Exploration Plan, Figure 1. Descriptive logs of the subsurface explorations and the procedures utilized in the subsurface exploration program are presented in Appendix A. A generalized description of soil conditions encountered at the exploration locations is presented below. Detailed descriptions of soils encountered are provided on the descriptive logs in Appendix A. The soil descriptions presented below have been generalized for ease of report interpretation. Please refer to the test pit logs for more detailed soil descriptions. Variations in subsurface conditions may exist in proximity to exploration locations and the nature and extent of such variation may not become evident Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 3 until construction. If variations then appear, it may be necessary to reevaluate the recommendations of this report. The test pits disclosed subsurface conditions that were generally consistent with the published mapping. Subsurface conditions at the exploration locations are summarized below. Undocumented Fill Probable fill material was observed at six of the exploration locations (test pits 3, 4, 6, 7, 8, and 12). The fill depth in five of the test pits generally ranged from approximately 1 to 2 feet and was located either near the detention ponds or the adjacent roadways. The fill composition varied, but generally consisted of sandy silt with gravel with a varied organic content. The fill was underlain by a relic topsoil horizon at the locations of test pits TP-3 and TP-12. It should be noted that the character and depth of undocumented fill may vary over relatively short distances. In addition to the fill observed at and below grade as described above, a 6 to 10-foot tall stockpile is present to the northwest of the detention pond and at the location of TP-7. The material in the stockpile consisted of brown silty sand with some gravel. Based upon the observed composition and location it is likely that the stockpile was generated by stripping topsoil during construction of the detention pond area, in our opinion. The fill observed at the test pit locations and in the stockpile is considered undocumented in that it appears to have been placed in a non-engineered condition and due to its composition and density is considered inadequate for support of structures, pavements, and utilities without mitigation. Topsoil / Forest Duff All of the explorations disclosed a surficial horizon of organic topsoil that ranged in thickness from approximately 6 inches to 1 foot. In general, the topsoil horizon consisted of brown silty sand with gravel, and contained fine to medium roots throughout. In some locations roots were observed to extend below the organic-rich topsoil horizon. Glacial Till The topsoil was underlain by glacial till that typically consisted of medium dense to very dense silty gravelly sand with occasional cobbles up to approximately 12 inches in diameter. Although not observed at the test pit locations, boulders are commonly present within the till. The upper 1 to 3 feet of the glacial till consisted of a medium dense to dense, light orange to brown weathered zone. Below the weathered till we generally observed dense to very dense, gray, silty gravelly sand with some sandy silt horizons with scattered cobbles. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 4 Groundwater Conditions Groundwater was not observed in any of the twelve test pits at the time of excavation. However, in July 2021 we measured groundwater approximately 22 feet below grade (approximately elevation 220 feet) in an old well located in the eastern portion of the site and northeast of the test pit TP-2 location as shown on Figure 1. We also observed approximately 1 foot of standing water in the northwest detention facility cell in July 2021; the water surface was at approximately elevation 222 feet. We have interpreted the water observed in the pond as likely a reflection of the local groundwater table aquifer. It should be noted that groundwater conditions may fluctuate seasonally due to variations in precipitation, land use, irrigation, or other factors. CONCLUSIONS AND RECOMMENDATIONS General The proposed mixed-use development will include the construction of a multi-story mixed-use building (Podium) with two levels of below-grade parking in the northwest portion of the site. A single-story slab- on-grade retail building (Retail) is proposed for construction in the northeast portion of the site, and eight three-story wood-frame apartment buildings (Quads 1 through 8) will be located throughout the site as shown on Figure 1. An excavation with a maximum depth of approximately 36 feet will be necessary to construct the podium building, while moderate grading will be associated with construction of the balance of the site improvements. Plans prepared by Navix, project civil engineers, indicated that there will be approximately 69,000 and 9,000 cubic yards of cut and fill, respectively. Based on the results of the subsurface exploration program and our analysis, we have concluded that the proposed development is feasible from the geotechnical perspective, contingent on proper design and construction practices and implementation of the recommendations presented in this report. Geotechnical engineering recommendations for foundation systems and other earthwork related phases of the project are outlined below. The recommendations contained in this report are based upon the results of the field exploration and laboratory testing, engineering analyses, and our current understanding of the proposed project. ASTM and Washington State Department of Transportation (WSDOT) specification codes cited herein respectively refer to the current manual published by the American Society for Testing & Materials and the current edition of the Standard Specifications for Road, Bridge, and Municipal Construction, (Publication M41-10). Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 5 Regulated Geologic Hazard Environmentally Critical Areas The City of Federal Way regulates certain geologic hazards including areas susceptible to erosion, landsliding, seismic, or other geological events, as well as defined steep slopes. A summary of the regulated geologically hazardous critical areas is presented below. Erosion Hazard The FWRC defines erosion hazard areas as having a severe to very severe erosion hazard due to natural agents such as wind, rain, splash, frost action, or stream flow. Mapping obtained from the USDA Natural Resources Conservation Service (NRCS) indicates that the site has been characterized by the Everett- Alderwood gravelly sand loam soils, 6 to 15 percent (EwC). These soils are formed in glacial till paren t material and are described as presenting a slight to moderate erosion hazard. Consequently, the site does not meet the SMP criteria as an erosion hazard. Provided that site grading and construction occur in accordance with a Temporary Erosion and Sedimentation Control (TESC) plan approved by the City of Federal Way, and provided that TESC BMPs are adequately maintained during construction, it is our opinion that the risk of significant sediment generation and off-site sediment transport is low. The site is not designated as an erosion hazard area on the City’s May 2016 Critical Areas Map. Steep Slope Hazard A regulated steep slope is one with 10 or more feet of relief and an inclination of 40 percent or steeper. Based upon review of the topographic survey of the site provide to us, and our site observations, a slope segment meeting this definition is limited to a small section of the graded slope along the west side of the site bordering 1st Avenue South. The cut slopes along the west and north sides of the site are inclined at about 45 to 50 percent, and a segment of the west slope with approximately 10 to 12 feet of relief meets the criteria for a regulated steep slope. The southern end of this slope segment, which is approximately 120 feet long, is approximately 80 feet north of the site access drive . The approximate location of the steep slope is highlighted on Figure 2. Regrading of this slope is feasible from the geotechnical perspective. Landslide Hazard The FWRC defines landslide hazard areas as those areas potentially subject to episodic downslope movement of a mass of soil or rock with including a combination of slopes greater than 15 percent, permeable sediment, or with springs or groundwater seepage. The site is characterized by dense to very dense glacial till, and within the depths of the test pits or along the existing graded slopes along South 348th Street and 1st Avenue South and the detention ponds in the southeastern portion of the site, we did not observe permeable granular soils and groundwater seepage. The site does not meet the Code definition of a landslide hazard and it is not designated as containing landslide hazards on the City’s May 2016 Critical Areas Map. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 6 Seismic Hazard The FWRC defines a seismic hazard as an area subject to severe risk of earthquake damage as a 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. The site is underlain by glacial till, a glacially consolidated soil characterized by a high density, and shallow groundwater is not present. As such, the risk of damage due to a seismic event is low, in our opinion. The site does not meet the Code definition of a seismic hazard. The site is not designated as a seismic hazard area on the City’s May 2016 Critical Areas Map. Seismic Design Considerations The 2018 IBC indicates that the seismic site classification is based on the average soil and bedrock properties to a depth of 100 feet. Our authorized scope of services did not include a 100-foot depth soil profile determination. The seismic site class criteria recommended in the following table considers that soils encountered at depth in our borings continue below the termination depth. IBC Seismic Design Criteria Parameter Value 2018 International Building Code Site Classification (IBC) Site Class C Site Latitude/Longitude 47.289074/-122.333254 Spectral Short-Period Acceleration, Ss 1.322g (Site Class B) Spectral 1-Second Acceleration, S1 0.454g (Site Class B) Site Coefficient for a Short Period, Fa 1.2 Site Coefficient for a 1-Second Period, Fv 1.5 Spectral Acceleration for a 0.2-Second Period, SMS 1.586g (Site Class C) Spectral Acceleration for a 1-Second Period, SM1 0.681g (Site Class C) Design Short-Period Spectral Acceleration, SDS 1.057g (Site Class C) Design 1-Second Spectral Acceleration, SD1 0.454g (Site Class C) 1. IBC Site Class is based on the average characteristics of the upper 100 feet of the subsurface profile. 2. The test pits completed for this study were advanced to depths as great as 15 feet and at this depth were terminated in glacially consolidated soils. Based on this and geologic mapping, it is assumed that glacially consolidated soil encountered below the deeper test pit termination depth extends to 100 feet as suggested by published geologic maps for the project area. 3. If exceptions presented in Section 11.4.8 of ASCE 7-16 do not apply, a ground motion hazard analysis may be required. 4. If exception presented in Section 20.3.1 of ASCE 7-16 does not apply, a ground motion hazard analysis may be required for Site Class F soils. Site Preparation Erosion Control Measures: The site has variable topography and significant grading is proposed. Consequently, the potential for construction phase erosion may be considered relatively high unless City- approved TESC BMPs are adequately designed, installed, and maintained. We recommend that silt fences, berms, and/or swales be installed around stripped areas and stockpiles in order to capture runoff water Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 7 and sediment. If earthwork occurs during wet weather, we recommend covering stripped surfaces with straw and protecting soil stockpiles with anchored plastic sheeting when areas are not being worked for long periods. Temporary Drainage: Stripping, excavation, grading, and subgrade preparation should be performed in a manner and sequence that will provide drainage at all times and provide proper control of erosion. The site should be graded to prevent water from ponding in construction areas and/or flowing into excavations. Exposed grades should be crowned, sloped, and smooth-drum rolled at the end of each day to facilitate drainage if inclement weather is forecasted. Accumulated water must be removed from subgrades and work areas immediately and prior to performing further work in the area. The site soils have a relatively high fines content and should be considered highly moisture-sensitive. As such, equipment access may be limited and the amount of soil rendered unfit for use as structural fill may be greatly increased if drainage efforts are not accomplished in a timely manner. Stripping: In preparation for grading, we recommend removal of all existing vegetation, root grubbing, and removal of existing fill material containing organic or deleterious material. This would include the stockpile located northwest of the detention ponds. Organic-rich topsoil (soils containing more than 4 percent organic material by weight) will need to be stripped from structure and pavement locations, as well as those areas to receive structural fill. The thickness of organic duff and topsoil observed at the test pit locations ranged from about 6 to 12 inches, and roots extended to depths of about 18 inches. However, variation in the organic material thickness should be expected; deeper accumulations of organics may be encountered in depressions and around root masses. Duff and topsoil should be removed and should not be reused as structural fill. Organic materials may be used in landscaping. Stripping is recommended to include removal of undocumented fill material and any relic organic topsoil below the fill due to the risk of future settlement if these materials are left in place. The undocumented fill we observed at the test pit locations was typically in a loose condition and in some locations contained organic material as well as debris. The depth of the fill ranged from approximately 1.5 to 2 feet (not including the stockpile of strippings at the test pit TP-7 location) and relic topsoil was observed to about a foot below the fill at the locations of test pits TP-3 and TP-12. Variation in the fill depth and composition, and the depth of organics below the fill, should be expected. These materials should be removed under the observation of a ZGA representative. Our representative will identify unsuitable materials that should be removed and those that may be re-used as structural fill. The resultant excavations should be backfilled in accordance with the subsequent recommendations for structural fill placement and compaction. We recommend that site preparation activities take place in the drier summer months. Operating wheeled and tracked equipment when the weathered glacial till soils are wet will result in significant disturbance of the non-organic weathered glacial till soils and likely requiring its removal. This will increase construction costs. Completion of logging and stripping under dry site and weather conditions will reduce the potential for disturbance of the weathered till soils and reduce the likelihood of subgrade disturbance and the need to replace disturbed soils with imported granular fill. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 8 Subgrade Preparation: Once stripping has been completed, all areas that are at design subgrade elevation or areas that will receive new structural fill should be compacted to a firm and unyielding condition and to a compaction level of at least 95 percent of the maximum laboratory density (per ASTM D 1557) within the upper 12 inches. Some moisture conditioning of site soils may be required to achieve an appropriate moisture content for compaction within ±2 percent of the soils laboratory optimum moisture content, particularly during the warmer summer months when the soils will tend to dry relatively quickly when exposed to sun and wind. Subgrades should be evaluated through density testing and proof rolling with a loaded dump truck or heavy rubber-tired construction equipment in order to detect soft and/or yielding soils. In the event that soft or yielding areas are detected during proof rolling, the upper 12 inches of subgrade should be scarified, moisture conditioned and re-compacted as necessary to obtain at least 95 percent of the maximum laboratory density (per ASTM D 1557) and to a firm, non-yielding condition. Those soils which are soft/loose, yielding, or unable to be compacted to the specified criteria should be over-excavated and replaced with suitable material as recommended in the Structural Fill section of this report. If subgrade compaction during wet site conditions or wet weather cannot be achieved, a minimum of 12 inches of subgrade should be over-excavated and backfilled with compacted imported structural fill consisting of free-draining Gravel Borrow or crushed rock. A stabilization geotextile could be used in unstable areas to reduce the depth of over-excavation. We recommend completing earthwork during drier periods of the year when the soil moisture content can be controlled by aeration and drying, if necessary. If earthwork or construction activities take place during extended periods of wet weather, the site-characteristic glacial till may become unstable or not be compactable. In the event the exposed subgrade becomes unstable, yielding, or unable to be compacted due to high moisture conditions, we recommend that the affected material be removed to a sufficient depth in order to develop a stable subgrade that can be compacted to the minimum recommended levels. The severity of construction problems will be dependent, in part, on the precautions that are taken by the contractor to protect the subgrade soils. Once subgrades are compacted, it may be desirable to protect prepared subgrades such as building pads or haul roads. To protect stable subgrades, we recommend using crushed rock. The thickness of the protective layer should be determined at the time of construction and be based on the moisture condition of the soil and the amount of anticipated traffic. Freezing Conditions: If earthwork takes place during freezing conditions, all exposed subgrades should be allowed to thaw and then be compacted prior to placing subsequent lifts of structural fill. Alternatively, the frozen material could be stripped from the subgrade to expose unfrozen soil prior to placing subsequent lifts of fill or foundation components. The frozen soil should not be reused as structural fill until allowed to thaw and adjusted to the proper moisture content, which may not be possible during winter months. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 9 Structural Fill Structural fill includes any material placed below foundations and pavement sections, within utility trenches, and behind retaining walls. Prior to the placement of structural fill, all surfaces to receive fill should be prepared as previously recommended in the Site Preparation section of this report. Laboratory Testing: Representative samples of imported soil to be used as structural fill should be submitted for laboratory testing at least four days in advance of its intended use in order to complete the necessary Proctor tests. Re-use of Site Soils as Structural Fill: The non-organic native soil encountered on the site is adequate for use as general structural fill from a compositional standpoint provided the soil is placed and compacted in accordance with the compaction recommendations presented in this report. Soil will need to be near the optimum moisture content in order to compact it to the recommended density. Drying of over- optimum moisture soils may be achieved by scarifying or windrowing surficial materials during extended periods of dry weather. If encountered, soils which are dry of optimum may be moistened through the application of water and thorough blending to facilitate a uniform moisture distribution in the soil prior to compaction. Simply moistening the upper surface of a loose lift of the site soils will not allow adequate distribution of the water in the lift; blending will be necessary to achieve an adequate moisture distribution in the lift. We recommend that site soils used as structural fill have less than 4 percent organics by weight as determined by the ASTM D 2974 and have no woody debris greater than ½ inch in diameter. We recommend that all pieces of organic material greater than ½ inch in diameter be picked out of the fill before it is compacted. Any organic-rich soil derived from earthwork activities should be utilized in landscape areas or wasted from the site. Imported Structural Fill: In the event that imported structural fill is required, the appropriate type of imported structural fill will depend on weather conditions. During extended periods of dry weather, we recommend that imported fill, at a minimum, meet the requirements of Common Borrow, Option 1 or Option 2, as specified in Section 9-03.14(3) of the 2020 Washington State Department of Transportation, Standard Specifications for Road, Bridge, and Municipal Construction (Publication M41-10). During wet weather, higher-quality structural fill might be required, as Common Borrow may contain sufficient fines to be moisture-sensitive. During wet weather we recommend that imported structural fill meet the requirements of Gravel Borrow as specified in Section 9-03.14(1) of the WSDOT Standard Specifications. Retaining Wall Backfill: Retaining walls should include a drainage fill zone extending at least 2 feet back from the back face of wall for the entire wall height. The drainage fill should meet the requirements of Gravel Backfill for Walls as specified in Section 9-03.12(2) of the WSDOT Standard Specifications. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 10 Pavement Subgrades: Any structural fill used within the upper one foot of pavement subgrades should have a minimum California Bearing Ratio (CBR) of at least 15 when compacted to 95 percent of the modified Proctor maximum dry density. Based on our experience and correlations between soil type and CBR values, we have considered that a CBR value of 15 is representative of the native soil and has been used to develop our pavement section recommendations. Our design recommendations assume that imported fill types as recommended above (Common Borrow or Gravel Borrow) will meet the minimum CBR requirement. However, samples of proposed imported fill should be submitted for laboratory testing and approval prior to use. Moisture Content: The suitability of soil for use as structural fill will depend on the time of year, the moisture content of the soil, and the fines content (that portion passing the US No. 200 sieve) of the soil. As the amount of fines increases, the soil becomes increasingly sensitive to small changes in moisture content. Soils containing more than about 5 percent fines cannot be consistently compacted to the appropriate levels when the moisture content is more than approximately 2 percent above or below the optimum moisture content (per ASTM D1557). The optimum moisture content is that moisture content which results in the greatest compacted dry density with a specified compactive effort. The fines content of the samples tested in our laboratory ranged from approximately 18 to 24 percent. Consequently, the soils should be considered highly moisture-sensitive. The moisture content of the samples of native soil that we tested indicated that the soils were generally in a moist to wet condition relative to an anticipated modified Proctor maximum dry density. However, soil moisture contents at the time of construction should be expected to vary from our test results. Fill Placement: Structural fill should be placed in horizontal lifts not exceeding 12 inches in loose thickness. Thinner lifts may be required, depending on the soil conditions and the type of compaction equipment in use. Each lift of fill should be compacted using compaction equipment suitable for the soil type and lift thickness. Each lift of fill should be compacted to the minimum levels recommended below based on the maximum laboratory dry density as determined by the ASTM D 1557 testing procedure (modified Proctor). The moisture content of fill at the time of placement should be within plus or minus 2 percent of optimum moisture content for compaction as determined by the ASTM D 1557 test method. Compaction Criteria: Our recommendations for soil compaction are summarized in the following table. Structural fill for roadways and utility trenches in municipal rights-of-way should be placed and compacted in accordance with the City of Federal Way standards. We recommend that a ZGA representative be present during grading so that an adequate number of density tests may be conducted as structural fill placement occurs. In this way, the adequacy of the earthwork may be evaluated as it proceeds. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 11 RECOMMENDED SOIL COMPACTION LEVELS Location Minimum Percent Compaction* Stripped native subgrade soils, prior to fill placement (upper 12 inches) 95 All fill below building floor slabs and foundations 95 Upper 2 feet of fill below pavements 95 Conventional pavement fill below 2 feet 90 Retaining wall backfill less than 3 feet from wall 90 Retaining wall backfill more than 3 feet from wall 95 Utility trench backfill 95 Landscape Areas 88 - 90 * ASTM D 1557 Modified Proctor Maximum Dry Density Utility Trenches We recommend that utility trenching conform to all applicable regulations, such as OSHA, for open excavations. Trench excavation safety guidelines are presented in 29 CFR 1926.650, 1926.651, and 1926.652. Trench Dewatering: Groundwater was not observed at the test pit locations, and the observed soil conditions did not suggest the presence of groundwater within the anticipated excavation depths from the upper portion of the site near adjacent street grade. Consequently, we do not anticipate that trench dewatering will be required for excavations associated with the quad and retail buildings and upper level utilities. However, based on our observations of groundwater in the 220 to 2022 foot elevation range made in July 2021 in the old well in the eastern portion of the site and in the northwest detention pond cell, it would not be unusual for groundwater to be encountered in utility excavations made in the foundation excavation for the podium building. Utility Subgrade Preparation: We recommend that all utility subgrades be firm and non-yielding and free of all soils that are loose, disturbed, or pumping. Such soils should be removed and replaced, if necessary. All structural fill used to replace over-excavated soils should be compacted as recommended in the Structural Fill section of this report. If utility foundation soils are soft, we recommend that they be over- excavated 12 inches and replaced with compacted crushed rock. Bedding: We recommend that a minimum of 4 inches of bedding material be placed above and below all utilities or in general accordance with the utility manufacturer’s recommendations and local requirements. We recommend that pipe bedding consist of Gravel Backfill for Pipe Zone Bedding as described in Section 9-03.12(3) of the WSDOT Standard Specifications. All trenches should be wide enough to allow for compaction around the haunches of the pipe, or material such as pea gravel should be used below the spring line of the pipes to eliminate the need for mechanical compaction in this portion of the trenches. If water is encountered in the excavations, it should be removed prior to fill placement. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 12 Trench Backfill: Materials, placement and compaction of utility trench backfill should be in accordance with the recommendations presented in the Structural Fill section of this report. We recommend that the initial lift thickness not exceed one foot unless recommended by the manufacturer to protect utilities from damage by compacting equipment. Light hand operated compaction equipment may be utilized directly above utilities if damage resulting from heavier compaction equipment is of concern. Temporary and Permanent Slopes General Temporary excavation slope stability is a function of many factors, including: • The presence and abundance of groundwater; • The type and density of the various soil strata; • The depth of cut; • Surcharge loadings adjacent to the excavation; and • The length of time the excavation remains open. As a cut is deepened, or as the length of time an excavation is open, the likelihood of bank failure increases; therefore, maintenance of safe slopes and worker safety should remain the responsibility of the contractor, who is present at the site, able to observe changes in the soil conditions, and monitor the performance of the excavation. It is exceedingly difficult under the variable circumstances to pre-establish a safe and “maintenance-free” temporary cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe temporary slope configurations since the contractor is continuously at the job site, able to observe the nature and condition of the cut slopes, and able to monitor the subsurface materials and groundwater conditions encountered. Unsupported vertical slopes or cuts deeper than 4 feet are not recommended if worker access is necessary. The cuts should be adequately sloped, shored, or supported to prevent injury to personnel from local sloughing and spalling. The excavation should conform to applicable Federal, State, and Local regulations. According to OSHA regulations, the contractor should make a determination of excavation side slopes based on classification of soils encountered at the time of excavation. Temporary cuts may need to be constructed at flatter angles based upon the soil moisture and groundwater conditions at the time of construction. Adjustments to the slope angles should be determined by the contractor at that time. It should be noted that much of the native soil encountered in excavations is expected to consist of relatively clean sand and gravel and may be susceptible to rapid collapse in unsupported conditions. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 13 We recommend that all permanent cut or fill slopes constructed in native soils or with imported structural fill be designed at a 2H:1V inclination or flatter. All permanent cut and fill slopes should be adequately protected from erosion both temporarily and permanently. If the slopes are exposed to prolonged rainfall before vegetation becomes established, the surficial soils will be prone to erosion and possible shallow sloughing. We recommend covering permanent slopes with a rolled erosion protection product, such as coir matting or Curlex II, if vegetation has not been established by the wet season (typically November through May). Podium Building Architectural plans provided for our review in July 2021 indicate that the finished floor elevation of the lower parking level in the podium building will be 228 feet. Based on this, we anticipate that foundation excavations may extend to about elevation 226 feet. The adjacent street grade along South 348 th Street along the north side of the building ranges from about elevation 252 to 254 feet, and this grade decreases to the south along the west side of the building. Sections provided for our review indicate that the below- grade portion of the building will be about 13 feet from the adjacent property line. Based on subsurface conditions observed at the test pit locations, we expect that weathered glacial till over dense to very dense unweathered glacial till will be exposed in much of the required excavation. However, it should be noted that the test pits to extended to a maximum 15-foot depth, and as such, did not extend to the anticipated podium building foundation excavation depth. Per WAC 296-155-66403, excavations in the unweathered glacial till, a Type A soil, may be excavated to inclinations as steep as 0.75H:1V to a maximum depth of 20 feet. In consideration of the existing and proposed grades, temporary excavation shoring will be required for a portion of the podium building. On a preliminary basis, we anticipate that both cantilever and tied back soldier pile shoring may be employed for the temporary excavation shoring. Detailed geotechnical recommendations for temporary excavation shoring will be provided under separate cover following the completion of additional boring explorations at the podium building location and additional analysis. Shallow Foundations Based on our analyses, conventional spread footings will provide adequate support for the proposed buildings provided that the foundation subgrades are properly prepared. We anticipate that foundation subgrade soils will generally consist of native glacial till or compacted structural fill. Allowable Bearing Pressure: Continuous and isolated column footings bearing on undisturbed, dense to very dense native glacial till may be designed for a maximum allowable net bearing capacity of 5,000 psf. Foundations bearing on structural fill placed and compacted in accordance with the recommendations presented herein may be designed for a maximum allowable net bearing capacity of 2,500 psf. A one- Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 14 third increase of these bearing pressures may be used for short-term transient loads such as wind and seismic forces. The above-recommended allowable bearing pressures include a factor of safety of 3. Shallow Foundation Depth and Width: For frost protection, the bottom of all exterior footings should bear at least 18 inches below the lowest adjacent outside grade, whereas the bottoms of interior footings should bear at least 12 inches below the surrounding slab surface level. We recommend that all continuous wall and isolated column footings be at least 12 and 24 inches wide, respectively. Lateral Resistance: Resistance to lateral loads can be calculated assuming an ultimate passive resistance of 540 pcf equivalent fluid pressure (triangular distribution) and an ultimate base friction coefficient of 0.5. An appropriate safety factor (or load/resistance factors) should be included for calculating resistance to lateral loads. For allowable stress design, we recommend a minimum 1.5 safety factor. We recommend neglecting passive resistance in the upper 18 inches of embedment. Estimated Static Settlement: Assuming the foundation subgrade soils are prepared in accordance with recommendations presented herein, we estimate that total static settlement may approach 0.75 inches and differential static settlement may approach half the total settlement over a distance of about 40 feet. Backfilled Permanent Retaining Walls The project is expected to include some backfilled cast-in-place (cip) concrete retaining walls. For recommended bearing capacities and lateral resistance parameters, refer to the Shallow Foundations section above. Additional recommendations for these structures are provided below. Lateral Earth Pressures: The lateral soil pressures acting on backfilled retaining walls will depend on the nature and density of the soil behind the wall, and the ability of the wall to yield in response to the earth loads. Yielding walls (i.e., walls that are free to translate or rotate) that are able to displace laterally at least 0.001H, where H is the height of the wall, may be designed for active earth pressures. Non-yielding walls (i.e., walls that are not free to translate or rotate) should be designed for at-rest earth pressures. Non-yielding walls include walls that are braced to another wall or structure, and wall corners. Assuming that walls are backfilled and drained as described in the following paragraphs, we recommend that yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf (active earth pressure). Non-yielding walls should be designed using an equivalent fluid density of 50 pcf (at-rest earth pressure). Design of permanent retaining walls should consider additional earth pressure resulting from the design seismic event. For the seismic case, yielding walls should be designed for a uniform (rectangular), total earth pressure distribution of 10H and non-yielding walls should be designed for a uniform, total earth pressure distribution of 18H. The recommended total earth pressure distributions for the seismic case Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 15 include both the seismic and static components of earth pressures (i.e., the active or at-rest static components of 35 pcf or 50 pcf should not be added to the total uniform pressure distribution). For cantilever cast-in-place walls, the total earth pressure distributions for the seismic case should be applied from finished grade at the bottom of the wall to the top of wall. The above-recommended lateral earth pressures do not include the effects of sloping backfill surfaces, surcharges such as traffic loads, other surface loading, or hydrostatic pressures. If such conditions exist, we should be consulted to provide revised earth pressure recommendations. Tiered Wall Considerations: Preliminary civil engineering plans provided for our review indicated that some tiered retaining walls with a maximum height of 6 feet are planned for the southwest entry/exit drive lane, and along the west and north sides of Quad 1. We recommend configuring the walls such that the foundations of walls above a lower wall not intrude past a 1H:1V slope extended upward from the base of the lower wall. Otherwise, it will be necessary to design successive lower walls to accommodate the additional lateral earth pressures resultant from the loading of the upper wall(s). Wall Drainage: Adequate drainage measures must be installed to collect and direct subsurface water away from retaining walls. All backfilled walls should include a drainage aggregate zone extending at least 2 feet from the back of wall for the full height of the wall. The drainage aggregate should consist of material meeting the requirements of WSDOT 9-03.12(2) Gravel Backfill for Walls. We did not observe at the test pit locations any soils that would meet the gradational requirements for wall backfill, so it will be necessary to import the wall drainage aggregate. A minimum 4-inch diameter perforated rigid thermoplastic drainpipe should be provided at the base of backfilled walls to collect and direct subsurface water to an appropriate discharge point. Drainpipe perforations should be protected using a non-woven geotextile fabric such as Mirafi 140N in order to prevent soil particles from entering the pipe. Wall drainage systems should be independent of other drainage systems such as roof drains. We recommend incorporating cleanouts in wall drainage systems. Rockeries The plans available at the time this report was reviewed suggest that some grade transitions will be made with rockeries rather than retaining walls. Our recommendations for rockeries are presented in the following sections. We recommend constructing cut slope and fill slope rockeries in general conformance with City of Federal Way Drawing No. 3-22 and Drawing No. 3-23, respectively, from the Public Works Standards, included herein. It should be recognized that rockeries function to protect an otherwise stable slope from erosion and sloughing; they are not true retaining walls. Also, rockeries may require periodic maintenance. Rockery Subgrades: We recommend founding the rockeries on a native soil subgrade consisting of at least medium dense undisturbed native soils or engineered fill compacted to at least 95 percent of the modified Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 16 Proctor maximum dry density as determined by the ASTM D 1557 test method. We recommend that a representative from our firm evaluate rockery subgrade conditions prior to placement of the first layer of rocks. The base of the rockery should be embedded at least one-half the thickness of the lowest course of rocks or 12 inches below the adjacent ground surface, whichever is greater. The final rockery face should be constructed with a batter no steeper than 1H:6V, and per City requirements, the exposed height should not exceed 6 feet. Facing Rocks: The rockery rocks should be tabular and rectangular. Rocks should be hard, sound, durable and free of weathered portions, seams, cracks and other defects. The rocks should have a minimum density of 160 pounds per cubic foot per WSDOT Test Method 107 (Bulk Specific Gravity – SSD basis) and exhibit less than 15 percent breakdown per US Army Corps of Engineers Test Method CRD-C-148 (Method of Testing Stone for Expansive Breakdown on Soaking in Ethylene Glycol). Typically, rocks used for rock wall construction are sized as follows in the table below. Rockery Facing Rock Sizing Criteria Rock Size Rock Weight (pounds) Average Dimensions (inches) Two Man 200 – 700 18 - 28 Three Man 700 – 2,000 28 - 36 Four Man 2,000 – 4,000 36 - 48 Five Man 4,000 – 6,000 48 - 54 Rock selection and placement should be accomplished to reduce the number and size of voids. In the exposed face of the rockery, no openings greater than 4 inches in dimension in any direction should be permitted. Rock courses should be gradational in size from bottom to top with the largest rocks of uniform size being placed for the lowest courses. The contact between rocks should slope downward to the backside of the rockery. Each course of rocks should be seated tightly and evenly on the course beneath. Rock placement should be such that each rock above the base course will be supported on two rocks in the next lower row. After seating each course of rock, voids between the rocks should be chinked on the back with quarry spalls to eliminate passage of backfill material. Rockery Backfill: Backfill immediately behind the rockery should consist of quarry spalls. The spalls should consist of well-graded 2 to 4-inch crushed rock and should be durable, uncontaminated by soil or other debris, and not readily susceptible to weathering. The quarry spall fill should be placed to a width of not less than 18 inches between the rockery and the face of the cut. The spalls should be placed in lifts to a level approximately 2 inches below the top of each course of rocks as they are placed, until the uppermost course is placed. Any backfill material falling onto the bearing surface of one rock course must be removed Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 17 before setting the next course. The cut slopes behind several of the rockeries are expected to consist of relatively clean sand and gravel. Depending on site-specific conditions encountered during construction, it may be beneficial to cover the cut slope with a non-woven geotextile, such as Mirafi 140N, or equivalent, to reduce the likelihood of soil particles from the cut slope migrating into the rockery backfill. The need for the geotextile would best be determined during construction. Rockery Drainage: The City’s rockery detail indicates that a minimum 6-inch inside-diameter, perforated drainpipe should be embedded in the backfill at the base of the rockery. This drain should discharge to the site’s storm drain system or other appropriate discharge. Fill Slope Rockeries: It is generally necessary to include geotextile or geogrid reinforcement of the fill material placed behind rockeries that are taller than 4 feet. We would be able to provide design details for fill slope rockeries once final site grades have been established. It may also be feasible to use larger than typical facing rocks or quarry spall backfill as an alternative to reinforcement of soil backfill MSE and Gravity Block Retaining Walls Foundations For fill walls, geogrid-reinforced, segmental block walls commonly referred to as mechanically stabilized earth or MSE walls are a suitable alternative to cast-in-place walls. For cut applications, gravity block walls could be considered. Segmental blocks generally consist of small precast concrete blocks while gravity blocks are much larger precast concrete blocks similar to ecology blocks. Recommendations for specific retaining wall types are provided below. The design of site retaining walls must consider the potential for surcharge loading from adjacent slopes and other possible surcharges, if applicable. ZGA can provide location-specific MSE and gravity wall designs, if requested. MSE and Gravity Block Wall Subgrade We recommend founding MSE walls and gravity block walls on a native soil subgrade consisting of at least medium dense granular soils, or structural fill compacted to at least 95 percent of the modified Proctor maximum dry density as determined by the ASTM D 1557 test method, or above sound bedrock. Prior to placement of crushed rock leveling pads for segmental or gravity block walls, we recommend a representative from ZGA evaluate the subgrade. Segmental and Gravity Block Wall Design and Construction We recommend the design and construction of segmental and gravity block walls be completed in strict accordance with the recommendations presented in the National Concrete Masonry Association’s Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 18 Segmental Retaining Walls Best Practices Guide (current edition). Design of MSE walls should be completed using the following soil design parameters: On-Grade Concrete Slabs The following sections provide recommendations for on-grade floor slabs. Subgrade Preparation: Subgrades for on-grade slabs should be prepared in accordance with the Site Preparation and Structural Fill sections of this report. Capillary Break: We recommend the on-grade slabs be underlain by a minimum 4-inch thick layer of compacted granular fill consisting of coarse sand and fine gravel containing less than 5 percent fines, based on that soil fraction passing the US No. 4 sieve. Alternatively, a clean angular gravel such as No. 7 Aggregate per WSDOT 9-03.1(4) C could be used for this purpose. Alternative capillary break materials should be submitted to ZGA for review and approval before use. Vapor Retarder: The use of a vapor retarder should be considered beneath concrete slabs on grade that will be covered with wood, tile, carpet or other moisture sensitive or impervious coverings, or when the slab will support equipment sensitive to moisture or is otherwise considered moisture-sensitive. When conditions warrant the use of a vapor retarder, the slab designer and contractor should refer to ACI 302 and/or ACI 360 for procedures and cautions regarding the use and placement of a vapor retarder. We generally recommend a minimum 10 mil vapor retarder. Drainage Considerations Surface Drainage: Final site grades should be sloped to carry surface water away from the buildings and other drainage-sensitive areas. Additionally, site grades should be designed such that concentrated runoff toward softscape surfaces is avoided. Any surface runoff directed towards softscape slopes should be collected at the top of the slope and routed to the bottom of the slope and discharged in a manner that prevents erosion. SEGMENTAL AND GRAVITY BLOCK WALL SOIL DESIGN PARAMETERS Soil Properties Reinforced Backfill Retained Soil Foundation Soil Unit Weight (pcf) 130 130 130 Friction Angle (degrees) 34 38 38 Cohesion (psf) 0 0 0 Peak Ground Acceleration (As) NA NA 0.559g Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 19 Building Foundation Drains: Similar to the retaining wall drains, building foundation drains are recommended to consist of a minimum 4-inch diameter, Schedule 40, rigid, perforated thermoplastic pipe placed at the base of the heel of the footing with the perforations facing down. The pipe should be surrounded by a minimum of 6 inches of clean free-draining granular material conforming to WSDOT Standard Specification 9-03.12(4), Gravel Backfill for Drains or similar gradation material. A non-woven geotextile fabric such as Mirafi 140N, or equivalent, should envelope the free-draining granular material. At appropriate intervals such that water backup does not occur, the drainpipe should be connected to a tightline system leading to a suitable discharge. Cleanouts should be provided for future maintenance. The foundation drains should be separate from the roof drain system. Asphalt Pavements Pavement Life and Maintenance: It should be realized that asphaltic pavements are not maintenance- free. The following pavement sections represent our minimum recommendations for an average level of performance during a 20-year design life; therefore, an average level of maintenance will likely be required. A 20-year pavement life typically assumes that an overlay will be placed after about 12 years. Thicker asphalt, base, and subbase courses would offer better long-term performance, but would cost more initially. Conversely, thinner courses would be more susceptible to “alligator” cracking and other failure modes. As such, pavement design can be considered a compromise between a high initial cost and low maintenance costs versus a low initial cost and higher maintenance costs. Please note that we made assumptions regarding traffic type and frequency in the absence of specific traffic count data. Soil Design Values: Pavement subgrade soils are anticipated to consist of the site-characteristic native glacial till (silty gravelly sand). Our analysis assumes the pavement section subgrade will have a CBR value of 15. This value is based upon published correlations between soil type and CBR values and our experience. Recommended Pavement Sections: For light duty pavements (parking stalls), we recommend 2.5 inches of asphalt concrete over either 4 inches of crushed surfacing base course or a full-depth section consisting of 4.5 inches of asphalt concrete. For heavy duty pavements (main access routes, truck delivery routes), we recommend 3 inches of asphalt concrete over either 6 inches of crushed rock surfacing course or a full-depth section consisting of 6 inches of asphalt concrete. Areas subject to heavy surface loading, such as dumpster approach slabs that experience short-term high wheel loading, would benefit from either a thicker asphalt pavement section or the use of concrete pavement. In the event that FNW elects to provide a construction-phased paved surface, we recommend the light duty section described previously. Please note that repeated traffic by heavily-loaded construction vehicles may result in pre-mature degradation of the construction phase pavement and that localized repair during construction and prior to final paving may be required. Please note that using a full-depth asphalt concrete section instead of crushed surfacing base course below the pavement will provide limited opportunity for sub-pavement drainage and may shorten the Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 20 pavement lifespan as the site soils have a relatively low permeability. Placing some clean crushed surfacing base course below the pavement will improve pavement section drainage. Materials and Construction: We recommend the following regarding asphalt pavement materials and pavement construction. • Subgrade Preparation and Compaction: The upper 12 inches of native stripped subgrade should be prepared in accordance with the recommendations presented in the Subgrade Preparation section of this report, and all fill should be compacted in accordance with the recommendations presented in the Structural Fill section of this report. • Asphalt Concrete: We recommend that the asphalt concrete conform to Section 9-02.1(4) for PG 58-22 or PG 64-22 Performance Graded Asphalt Binder as presented in the WSDOT Standard Specifications. We also recommend that the gradation of the asphalt aggregate conform to the aggregate gradation control points for ½-inch mixes as presented in Section 9-03.8(6) HMA Proportions of Materials. • Base Course: We recommend that the crushed aggregate base course conform to Section 9-03.9(3) of the WSDOT Standard Specifications. • Compaction and Paving: All base material should be compacted to at least 95 percent of the maximum dry density determined in accordance with ASTM D 1557. We recommend that asphalt be compacted to a minimum of 92 percent of the Rice (theoretical maximum) density. Placement and compaction of asphalt should conform to requirements of Section 5-04 of the WSDOT Standard Specifications. Stormwater Management Considerations The site contains stormwater detention ponds constructed as part of a previous abandoned development effort. We understand that the City of Federal way will require some additional stormwater management features and that stormwater management improvements will need to comply with the King County Surface Water Design Manual (Manual). Based on the findings of the field exploration, laboratory testing, and our analysis, conventional stormwater infiltration does not appear feasible from the geotechnical perspective given the relatively high fines content and the density of the unweathered glacial till that characterizes the site. The relatively low permeability of the soils is illustrated by the fact that the on-site ponds retain water all year long, based upon our review of historic aerial photographs. We have concluded that limited stormwater infiltration as defined by the Manual may be feasible, but that the long-term infiltration rate that should be applied to the site soils will be relatively low. Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 21 Preliminary Infiltration Rate Determination Our scope of services did not include field infiltration testing as required by the Manual as part of designing an infiltration system. However, conclusions regarding stormwater infiltration can be drawn from subsurface conditions disclosed by the subsurface explorations and laboratory testing completed to date. For reference, per the USDA textural classification method allowed under earlier versions of some stormwater manuals, the glacial till soils at the site are considered sandy loam and would under other circumstances be assigned a short-term infiltration rate on the order of 1 inch per hour and a long-term rate of 0.25 inches per hour using a previously recommended reduction factor of 4. A similar rate is obtained when correlating ASTM grain size distribution test data to observed infiltration system performance. However, these rates would be applicable to normally consolidated soils and do not consider the influence of soil density. The unweathered soils observed at the test pit locations are glacially consolidated, dense to very dense, and would be less permeable than the overlying weathered soils. Based upon our experience with other projects of a similar nature, we would recommend applying a reduction factor of at least 10 to the 1 inch per hour short-term rate to achieve a long-term infiltration rate of no greater than 0.1 inches per hour. Additional reductions of this rate applicable to the type of infiltration system that reflect the potential for lack of future maintenance and soil clogging used should be applied as well. Groundwater Considerations Groundwater was not observed at the test pit locations. However, the possibility exists that a seasonal perched groundwater condition may develop above the unweathered glacial till due to the relatively high density and high fines content of the soils. This condition may adversely affect the performance of systems that rely on infiltration, if on a limited basis. General Stormwater Infiltration Considerations One LID stormwater management technique that may be applicable to the site is the use of permeable pavements/hardscape surfaces. Given the low permeability of the native site soils, little infiltration of water passing through permeable surfaces should be expected. Consequently, it would likely be necessary to include a section of permeable crushed reservoir rock below the pavement, and this would increase the overall pavement section costs. The disposition of water in the reservoir rock section should be considered and measures taken to present water flowing out from the rock section at pavement edges into areas where the water could be detrimental, such as immediately next to the buildings or along property boundaries. Installing perforated collection pipes in the reservoir rock section may be required as well. On sloping sites, it may be necessary to include dams in the rock section in order to achieve the necessary storage capacity. It should be recognized that a conventional permeable pavement section on Proposed Mixed-use Development Project No. 1536.01 22 July 2021 Page 22 the site may not provide the necessary treatment for water originating form pollution generating surfaces and that other means of treatment will be necessary. The use of rain gardens in low permeability soils is increasingly common. Rain gardens typically include a zone of amended soil to provide for treatment of water originating from pollution generating surfaces, as well as a collection zone below consisting of clean washed rock that allows for conveyance of treated water to an appropriate discharge point. We recommend including overflow pipes in raingardens as well. CLOSURE The analysis and recommendations presented in this report are based, in part, on the explorations completed for this study, review of referenced documents, and laboratory testing results. We recommend ZGA be provided an opportunity to review the plans and specifications as the project progresses in order to assess that the recommendations and design considerations presented in this report have been properly interpreted and implemented into the project design. The performance of earthwork, structural fill, foundations, and pavements depend greatly on proper site preparation and construction procedures. We recommend that ZGA be retained to provide geotechnical engineering services during the earthwork-related construction phases of the project. If variations in subsurface conditions are observed at that time, a qualified geotechnical engineer could provide additional geotechnical recommendations to the contractor and design team in a timely manner as the project construction progresses. This report has been prepared for the exclusive use of FNW, Inc., and its agents, for specific application to the project discussed and has been prepared in accordance with generally accepted geotechnical engineering practices. No warranties, express or implied, are intended or made. Site safety, excavation support, and dewatering requirements are the responsibility of others. In the event that changes in the nature, design, or location of the project as outlined in this report are planned, the conclusions and recommendations contained in this report shall not be considered valid unless ZGA reviews the changes and either verifies or modifies the conclusions of this report in writing. TP-9 TP-4 TP-1 TP-2 TP-8 TP-5 TP-10 TP-11 TP-6 TP-7 TP-12 TP-3 APPROXIMATE LIMITS OF REGULATED STEEP SLOPE W-1 BASE DRAWING PROVIDED BY GRAVES + ASSOCIATES, PAGE AS100, PM: BC, DATED 7/01/2021 FIGURE Job No. Zipper Geo Associates, LLC 19019 36th Ave. W.,Suite E Lynnwood, WA, 98036 SHT. of 11 SITE AND EXPLORATION PLAN 1536.01DATE: JULY 2021 1 FEDERAL WAY MIXED-USE 1ST AVENUE SOUTH & SOUTH 348TH STREET FEDERAL WAY, WASHINGTON LEGEND TP-1 TEST PIT NUMBER AND APPROXIMATE LOCATION APPROXIMATE SCALE IN FEET 0100 10050 W-1 EXISTING WELL AND APPROXIMATE LOCATION APPROXIMATE LIMITS OF REGULATED STEEP SLOPE BASE DRAWING PROVIDED BY PRIZM SURVEYING INC., DATED 9/21/2015 FIGURE Job No. Zipper Geo Associates, LLC 19023 36th Ave. W.,Suite D Lynnwood, WA, 98036 SHT. of 11 TOPOGRAPHIC SURVEY 1536.01DATE: JULY 2021 2 FEDERAL WAY MIXED-USE 1ST AVENUE SOUTH & SOUTH 348TH STREET FEDERAL WAY, WASHINGTON APPROXIMATE SCALE IN FEET 0100 10050 APPENDIX A FIELD EXPLORATION PROCEDURES & LOGS APPENDIX A FIELD EXPLORATION PROCEDURES AND LOGS Field Exploration Description The field exploration included excavating 12 test pits (TP-1 through TP-12) at the approximate locations shown on the Site and Exploration Plan, Figure 1. The locations of the test pits were determined by pacing and taping from site features shown on a topographic site map, dated 21 September, prepared by Prizm Surveying, Inc. Ground surface elevations at the test pit locations were interpolated from contours and spot elevations on the referenced site plan. The locations and elevations of the explorations should be considered as accurate as the methods used to determine them. Test Pit Procedures The test pits were excavated with a tracked excavator operated by an FNW, Inc. employee. A ZGA engineering geologist observed the test pit excavations, logged the subsurface conditions, and obtained representative soil samples. The samples were stored in moisture tight containers and transported to our laboratory for further visual classification and testing. The enclosed boring and test pit logs describe the vertical sequence of soils and materials encountered in each exploration, based primarily upon our field classifications. Where a soil contact was observed to be gradational, the logs indicate the average contact depth. Where a soil type changed between sample intervals, the contact depth has been inferred. The logs also graphically indicate the blow count, sample type, sample number, and approximate depth of each soil sample obtained from the boring. If groundwater was encountered in a borehole, the approximate groundwater depth, and date of observation, are depicted on the log. Test Pit TP-1 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 243 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist to wet, dark brown, silty SAND, with gravel, fine to medium roots throughout (Topsoil) Medium dense to dense, moist, light orange to brown, silty SAND, with gravel (Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND (Glacial Till) Grades with scattered cobbles to 10-inch diameter TP-1 completed at approximately 15 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 0.5 ft. 17 2 S-2 @ 2 ft. 17 3 4 S-3 @ 4 ft. 14 5 6 7 8 9 S-4 @ 9 ft. 10 10 11 12 13 14 ` 15 S-5 @ 14.5 ft. 7 Test Pit TP-2 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 244 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist to wet, brown, sandy SILT, with gravel, fine to medium roots throughout (Topsoil) Medium dense to dense, moist, light orange to brown, fine sandy SILT, with gravel (Weathered Glacial Till) Dense to very dense, moist, gray, silty gravelly SAND, trace cobbles to 10-inch diameter (Glacial Till) TP-2 completed at approximately 15 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 0.5 ft. 9 2 S-2 @ 2 ft. 9 3 4 S-3 @ 4 ft. 10 5 6 7 S-4 @ 7 ft. 10 8 9 10 11 12 13 14 ` 15 S-5 @ 14.5 ft. 6 Test Pit TP-3 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 239 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist, brown, silty SAND, with gravel, fine roots throughout (Topsoil) Medium dense, moist, gray, sandy SILT, with gravel, some 10-inch diameter quarry spalls (Fill) Medium dense, moist, black, SILT, with sand, some gravel (Probable Relic Topsoil) Medium dense to dense, moist, light orange to brown, fine sandy SILT, with gravel (Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND, trace cobbles and boulders (Glacial Till) TP-3 completed at approximately 10 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 0.5 ft. S-2 @ 1 ft. 2 3 S-3 @ 2.5 ft. 4 S-4 @ 4 ft. 6 GSA 5 6 7 8 9 10 S-5 @ 9.5 ft. 11 12 Test Pit TP-4 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 254 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist, brown, silty SAND, with gravel, fine roots in upper 10 inches (Possible fill) Medium dense, moist, light orange to brown, sandy SILT, with gravel, trace cobbles (Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND, trace cobbles and boulders to 1-foot diameter (Glacial Till) TP-4 completed at approximately 8 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 1 ft. 2 3 S-2 @ 2 ft. 4 S-3 @ 4 ft. 5 6 7 8 S-4 @ 8 ft. 9 10 11 12 Test Pit TP-5 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 251 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Medium dense, moist, light orange to brown, silty gravelly SAND (Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND, trace cobbles and boulders (Glacial Till) TP-5 completed at approximately 6 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 0.5 ft. S-2 @ 1 ft. 17 GSA 2 3 4 S-3 @ 3.5 ft. 5 6 S-4 @ 6 ft. 7 8 9 10 11 12 13 Test Pit TP-6 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 244 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Medium dense, moist, brown, sandy SILT, with gravel, trace fine roots (Possible Fill) Medium dense, moist, light orange to brown, silty SAND, with gravel (Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND, trace cobbles and boulders (Glacial Till) TP-6 completed at approximately 15 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 1 ft. 7 2 3 S-2 @ 3 ft. 6 4 S-3 @ 4 ft. 6 5 6 7 8 S-4 @ 8 ft. 5 9 10 11 12 13 14 ` 15 S-5 @ 15 ft. 8 Test Pit TP-7 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 257 Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose to medium dense, moist, brown, sandy SILT to silty SAND, with to some gravel (Strippings fill) TP-7 completed at approximately 5.5 feet. No groundwater seepage observed at time of excavation. 1 2 3 4 S-1 @ 4 ft. 5 6 7 8 9 10 11 12 Test Pit TP-8 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 253 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Medium dense, moist, brown, sandy SILT, with gravel (Possible Fill) Medium dense, moist, light orange to brown, silty gravelly SAND, trace cobbles (Weathered Glacial Till) (3-inch diameter tree root at 3 feet) Dense to very dense, moist, gray, silty gravelly SAND, trace cobbles and boulders (Glacial Till) TP-8 completed at approximately 6 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 1 ft. 2 S-2 @ 2 ft. 11 GSA 3 4 S-3 @ 3.5 ft. 5 6 S-4 @ 6 ft. 7 8 9 10 11 12 13 Test Pit TP-9 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 260 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Ferns and grass over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Medium dense, moist, light orange to brown, silty gravelly SAND, trace decaying tree roots (Weathered Glacial Till) Dense to very dense, moist, gray, silty gravelly SAND, trace cobbles (Glacial Till) TP-9 completed at approximately 6 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 0.5 ft. S-2 @ 1 ft. 2 3 S-3 @ 2.5 ft. 4 5 6 S-4 @ 6 ft. 7 8 9 10 11 12 Test Pit TP-10 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 232 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass and brush over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Medium dense, moist, light orange to brown, gravelly sandy SILT, trace decaying tree limb (Possible Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND (Glacial Till) Very dense, moist, gray, silty SAND, with gravel, trace cobbles and boulders (Glacial Till) TP-10 completed at approximately 15 feet. No groundwater seepage observed at time of excavation. 1 S-1 @ 0.5 ft. 12 S-2 @ 1 ft. 15 2 3 S-3 @ 3 ft. 7 GSA 4 5 6 S-4 @ 6 ft. 6 7 8 9 10 11 12 13 14 ` 15 S-5 @ 15 ft. 6 Test Pit TP-11 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 227 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass and brush over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Medium dense, moist, light gray, silty gravelly SAND (Weathered Glacial Till) Very dense, moist, dark gray, silty gravelly SAND, trace cobbles and boulders (Glacial Till) Very dense, moist, gray, silty SAND to sandy SILT, with gravel, trace cobbles and boulders (Glacial Till) TP-11 completed at approximately 15 feet. No groundwater seepage observed at time of excavation. S-1 @ 0.3 ft. 9 1 S-2 @ 1 ft. 7 2 3 4 S-3 @ 3.5 ft. 5 5 6 7 8 9 10 S-4 @ 10 ft. 6 11 12 13 14 ` 15 S-5 @ 15 ft. 7 Test Pit TP-12 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 235 feet Project: Federal Way Mixed-use Project No: 1536.01 Date Excavated: 11/12/15 Depth (ft) Material Description Sample NC %M Testing Grass and brush over loose, moist, brown, sandy SILT, with gravel, fine roots throughout (Topsoil) Loose to medium dense, moist, gray, silty SAND, with gravel, trace roots (Fill) Medium dense, to soft, moist, dark brown to black, SILT, with sand (Probably Relic Topsoil) Medium dense to dense, moist, light orange to brown, sandy SILT, with gravel (Weathered Glacial Till) Very dense, moist, gray, silty gravelly SAND, trace cobbles and boulders (Glacial Till) TP-12 completed at approximately 15 feet. No groundwater seepage observed at time of excavation. S-1 @ 0.3 ft. 1 S-2 @ 1 ft. 2 S-3 @ 1.5 ft. S-4 @ 2 ft. 3 S-5 @ 3 ft. 4 5 6 7 8 9 10 11 12 13 14 ` 15 S-6 @ 15 ft. APPENDIX B LABORATORY TESTING PROCEDURES & RESULTS APPENDIX B LABORATORY TESTING PROCEDURES AND RESULTS Descriptions of the types of tests performed on selected soil samples by TCI are given below. Visual Classification Samples recovered from the exploration locations were visually classified in the field during the exploration program. Representative portions of the samples were carefully packaged in moisture tight containers and transported to our laboratory where the field classifications were verified or modified as required. Visual classification was generally done in accordance with ASTM D 2488. Visual soil classification includes evaluation of color, relative moisture content, soil type based upon grain size, and accessory soil types included in the sample. Soil classifications are presented on the exploration logs in Appendix A. Moisture Content Determinations Moisture content determinations were performed on representative samples obtained from the explorations in order to aid in identification and correlation of soil types. The determinations were made in general accordance with the test procedures described in ASTM D 2216. The results are shown on the exploration logs in Appendix A. Grain Size Analysis A grain size analysis indicates the range in diameter of soil particles included in a particular sample. Grain size analyses were performed on representative samples in general accordance with ASTM D 422. The results of the grain size determinations for the samples were used in classification of the soils, and are presented in this appendix. 0 10 20 30 40 50 60 70 80 90 100 0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS GRAIN SIZE ANALYSIS Comments: 36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200 Coarse Medium Fine Silt ClayFineCoarse COBBLESBOULDERS GRAVEL SAND FINE GRAINED SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER Project No.:PROJECT NAME: Federal Way Mixed-UseDATE OF TESTING: Exploration Sample Depth (feet)Moisture (%)Fines (%)Description TP-3 5.0 6.4 Silty gravelly SANDS-4 23.7 1536.01 11/25/2015 ASTM D 422Test Results Summary Zipper Geo Associates, LLC Geotechnical and Environmental Consultants 0 10 20 30 40 50 60 70 80 90 100 0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS GRAIN SIZE ANALYSIS Comments: 36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200 Coarse Medium Fine Silt ClayFineCoarse COBBLESBOULDERS GRAVEL SAND FINE GRAINED SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER Project No.:PROJECT NAME: Federal Way Mixed-UseDATE OF TESTING: Exploration Sample Depth (feet)Moisture (%)Fines (%)Description TP-5 1.0 16.9 Silty gravelly SANDS-2 22.8 1536.01 11/25/2015 ASTM D 422Test Results Summary Zipper Geo Associates, LLC Geotechnical and Environmental Consultants 0 10 20 30 40 50 60 70 80 90 100 0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS GRAIN SIZE ANALYSIS Comments: 36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200 Coarse Medium Fine Silt ClayFineCoarse COBBLESBOULDERS GRAVEL SAND FINE GRAINED SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER Project No.:PROJECT NAME: Federal Way Mixed-UseDATE OF TESTING: Exploration Sample Depth (feet)Moisture (%)Fines (%)Description TP-8 2.0 10.6 Silty gravelly SAND S-2 18.3 1536.01 11/25/2015 ASTM D 422Test Results Summary Zipper Geo Associates, LLC Geotechnical and Environmental Consultants 0 10 20 30 40 50 60 70 80 90 100 0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS GRAIN SIZE ANALYSIS Comments: 36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200 Coarse Medium Fine Silt ClayFineCoarse COBBLESBOULDERS GRAVEL SAND FINE GRAINED SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER Project No.:PROJECT NAME: Federal Way Mixed-UseDATE OF TESTING: Exploration Sample Depth (feet)Moisture (%)Fines (%)Description TP-10 3.0 7.4 Silty gravelly SAND S-3 20.6 1536.01 11/25/2015 ASTM D 422Test Results Summary Zipper Geo Associates, LLC Geotechnical and Environmental Consultants