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Exhibit E Geotechnical Engineering Study and Addendum, dated 8-23-21
Geotechnical Engineering Construction Observation/Testing Environmental Services PRELIMINARY GEOTECHNICAL ENGINEERING STUDY TWIN TRAILS RESIDENTIAL SUBDIVISION 1605 SOUTHWEST 341ST PLACE FEDERAL WAY, WASHINGTON ES-7961 PREPARED FOR PROSPECT DEVELOPMENT, LLC August 23, 2021 Updated September 13, 2021 H Scott S. Riegel, L.G., L.E.G. Senior Project Manager Kyle R. Campbell, P.E. Principal Engineer PRELIMINARY GEOTECHNICAL ENGINEERING STUDY TWIN TRAILS RESIDENTIAL SUBDIVISION 1605 SOUTHWEST 341 ST PLACE FEDERAL WAY, WASHINGTON ES-7961 Earth Solutions NW, LLC 15365 Northeast 90th Street, Suite 100 Redmond, Washington 98052 Phone: 425-449-4704 1 Fax: 425-449-4711 www.earthsolutionsnw.com r Geolechnical-Engineeping Report --) The Geoprofessional Business Association (GBA) has prepared this advisory to help you — assumedly a client representative — interpret and apply this geotechnical-engineering report as effectively as possible. In that way, you can benefit from a lowered exposure to problems associated with subsurface conditions at project sites and development of them that, for decades, have been a principal cause of construction delays, cost overruns, claims, and disputes. If you have questions or want more information about any of the issues discussed herein, contact your GBA-member geotechnical engineer. Active engagement in GBA exposes geotechnical engineers to a wide array of risk -confrontation techniques that can be of genuine benefit for everyone involved with a construction project. Understand the Geotechnical-Engineering Services Provided for this Report Geotechnical-engineering services typically in the planning, collection, interpretation, and analysis of exploratory data from widely spaced borings and/or test pits. Field data are combined with results from laboratory tests of soil and rock samples obtained from field exploration (if applicable), observations made during site reconnaissance, and historical information to form one or more models of the expected subsurface conditions beneath the site. Local geology and alterations of the site surface and subsurface by previous and proposed construction are also important considerations. Geotechnical engineers apply their engineering training, experience, and judgment to adapt the requirements of the prospective project to the subsurface model(s). Estimates are made of the subsurface conditions that will likely be exposed during construction as well as the expected performance of foundations and other structures being planned and/or affected by construction activities. The culmination of these geotechnical-engineering services is typically a geotechnical-engineering report providing the data obtained, a discussion of the subsurface model(s), the engineering and geologic engineering assessments and analyses made, and the recommendations developed to satisfy the given requirements of the project. These reports may be titled investigations, explorations, studies, assessments, or evaluations. Regardless of the title used, the geotechnical-engineering report is an engineering interpretation of the subsurface conditions within the context of the project and does not represent a close examination, systematic inquiry, or thorough investigation of all site and subsurface conditions. Geotechnical-Engineering Services are Performed for Specific Purposes, Persons, and Projects, and At Specific Times Geotechnical engineers structure their services to meet the specific needs, goals, and risk management preferences of their clients. A geotechnical-engineering study conducted for a given civil engineer will not likely meet the needs of a civil -works constructor or even a different civil engineer. Because each geotechnical-engineering study is unique, each geotechnical-engineering report is unique, prepared solely for the client. Likewise, geotechnical-engineering services are performed for a specific project and purpose. For example, it is unlikely that a geotechnical- engineering study for a refrigerated warehouse will be the same as one prepared for a parking garage; and a few borings drilled during a preliminary study to evaluate site feasibility will not be adequate to develop geotechnical design recommendations for the project. Do not rely on this report if your geotechnical engineer prepared it: • for a different client; • for a different project or purpose; • for a different site (that may or may not include all or a portion of the original site); or • before important events occurred at the site or adjacent to it; e.g., man-made events like construction or environmental remediation, or natural events like floods, droughts, earthquakes, or groundwater fluctuations. Note, too, the reliability of a geotechnical-engineering report can be affected by the passage of time, because of factors like changed subsurface conditions; new or modified codes, standards, or regulations; or new techniques or tools. If you are the least bit uncertain about the continued reliability of this report, contact your geotechnical engineer before applying the recommendations in it. A minor amount of additional testing or analysis after the passage of time — if any is required at all — could prevent major problems. Read this Report in Full Costly problems have occurred because those relying on a geotechnical- engineering report did not read the report in its entirety. Do not rely on an executive summary. Do not read selective elements only. Read and refer to the report in full. You Need to Inform Your Geotechnical Engineer About Change Your geotechnical engineer considered unique, project -specific factors when developing the scope of study behind this report and developing the confirmation -dependent recommendations the report conveys. Typical changes that could erode the reliability of this report include those that affect: the site's size or shape; • the elevation, configuration, location, orientation, function or weight of the proposed structure and the desired performance criteria; the composition of the design team; or project ownership. As a general rule, always inform your geotechnical engineer of project or site changes — even minor ones — and request an assessment of their impact. The geotechnical engineer who prepared this report cannot accept responsibility or liability for problems that arise because the geotechnical engineer was not informed about developments the engineer otherwise would have considered. Most of the "Findings" Related in This Report Are Professional Opinions Before construction begins, geotechnical engineers explore a site's subsurface using various sampling and testing procedures. Geoteehnical engineers can observe actual subsurface conditions only at those specific locations where sampling and testing is performed. The data derived from that sampling and testing were reviewed by your geotechnical engineer, who then applied professional judgement to form opinions about subsurface conditions throughout the site. Actual sitewide-subsurface conditions may differ — maybe significantly — from those indicated in this report. Confront that risk by retaining your geotechnical engineer to serve on the design team through project completion to obtain informed guidance quickly, whenever needed. This Report's Recommendations Are Confirmation -Dependent The recommendations included in this report — including any options or alternatives — are confirmation -dependent. In other words, they are not final, because the geotechnical engineer who developed them relied heavily on judgement and opinion to do so. Your geotechnical engineer can finalize the recommendations only after observing actual subsurface conditions exposed during construction. If through observation your geotechnical engineer confirms that the conditions assumed to exist actually do exist, the recommendations can be relied upon, assuming no other changes have occurred. The geotechnical engineer who prepared this report cannot assume responsibility or liability for confirmation -dependent recommendations ifyou fail to retain that engineer to perform construction observation. This Report Could -Be Misinterpreted Other design professionals' misinterpretation of geotechnical- engineering reports has resulted in costly problems. Confront that risk by having your geotechnical engineer serve as a continuing member of the design team, to: • confer with other design -team members; help develop specifications; review pertinent elements of other design professionals' plans and specifications; and be available whenever geotechnical-engineering guidance is needed. You should also confront the risk of constructors misinterpreting this report. Do so by retaining your geotechnical engineer to participate in prebid and preconstruction conferences and to perform construction - phase observations. Give Constructors a Complete Report and Guidance Some owners and design professionals mistakenly believe they can shift unanticipated -subsurface -conditions liability to constructors by limiting the information they provide for bid preparation. To help prevent the costly, contentious problems this practice has caused, include the complete geotechnical-engineering report, along with any attachments or appendices, with your contract documents, but be certain to note conspicuously that you've included the material for information purposes only. To avoid misunderstanding, you may also want to note that "informational purposes" means constructors have no right to rely on the interpretations, opinions, conclusions, or recommendations in the report. Be certain that constructors know they may learn about specific project requirements, including options selected from the report, only from the design drawings and specifications. Remind constructors that they may perform their own studies if they want to, and be sure to allow enough time to permit them to do so. Only then might you be in a position to give constructors the information available to you, while requiring them to at least share some of the financial responsibilities stemming from unanticipated conditions. Conducting prebid and preconstruction conferences can also be valuable in this respect. Read Responsibility Provisions Closely Some client representatives, design professionals, and constructors do not realize that geotechnical engineering is far less exact than other engineering disciplines. This happens in part because soil and rock on project sites are typically heterogeneous and not manufactured materials with well-defined engineering properties like steel and concrete. That lack of understanding has nurtured unrealistic expectations that have resulted in disappointments, delays, cost overruns, claims, and disputes. To confront that risk, geotechnical engineers commonly include explanatory provisions in their reports. Sometimes labeled "limitations; many of these provisions indicate where geotechnical engineers' responsibilities begin and end, to help others recognize their own responsibilities and risks. Read these provisions closely. Ask questions. Your geotechnical engineer should respond fully and frankly. Geoenvironmental Concerns Are Not Covered The personnel, equipment, and techniques used to perform an environmental study — e.g., a "phase -one" or "phase -two" environmental site assessment — differ significantly from those used to perform a geotechnical-engineering study. For that reason, a geotechnical-engineering report does not usually provide environmental findings, conclusions, or recommendations; e.g., about the likelihood of encountering underground storage tanks or regulated contaminants. Unanticipated subsurface environmental problems have led to project failures. If you have not obtained your own environmental information about the project site, ask your geotechnical consultant for a recommendation on how to find environmental risk -management guidance. Obtain Professional Assistance to Deal with Moisture Infiltration and Mold While your geotechnical engineer may have addressed groundwater, water infiltration, or similar issues in this report, the engineer's services were not designed, conducted, or intended to prevent migration of moisture — including water vapor — from the soil through building slabs and walls and into the building interior, where it can cause mold growth and material -performance deficiencies. Accordingly, proper implementation of the geotechnical engineer's recommendations will not of itself be sufficient to prevent moisture infiltration. Confront the risk of moisture infiltration by including building -envelope or mold specialists on the design team. Geoteehnical engineers are not building -envelope or mold specialists. GEOPROFESSIONAL BUSINESS SEA ASSOCIATION Telephone: 301 / 565-2733 e-mail: info@geoprofessional.org www.geoprofessional.org Copyright 2019 by Geoprofessional Business Association (GBA). Duplication, reproduction, or copying of this document, in whole or in part, by any means whatsoever, is strictly prohibited, except with GBAs specific written permission. Excerpting, quoting, or otherwise extracting wording from this document is permitted only with the express written permission of GBA, and only for purposes of scholarly research or book review. Only members of GBA may use this document or its wording as a complement to or as an element of a report of any kind. Any other firm, individual, or other entity that so uses this document without being a GBA member could be committing negligent or intentional (fraudulent) misrepresentation. August 23, 2021 Updated September 13, 2021 ES-7961 Prospect Development, LLC 2913 — 5th Avenue Northeast, Suite 201 Puyallup, Washington 98372 Attention: Mr. Justin Holland Dear Mr. Holland: Earth i Solutions NW«C Earth Solutions NW LLC Geotechnical Engineering, Construction Observation/Testing and Environmental Services Earth Solutions NW, LLC (ESNW) is pleased to present this preliminary report that supports the current project. Based on the results of our investigation, construction of the proposed residential subdivision is feasible from a geotechnical standpoint. Our study indicates the site is underlain primarily by glacially consolidated soils with areas of outwash type deposits. This update includes the current site plan. In general, typical residences up to three stories in height may be supported on conventional continuous and spread footing foundations bearing on competent native soil, recompacted native soil, or new structural fill placed directly on competent native soil. In general; competent native soil, suitable for support of the new foundations, will likely be encountered beginning at depths of about two to four feet below the existing ground surface. Where loose or unsuitable soil conditions are exposed at foundation subgrade elevations, compaction of soils to the specifications of structural fill, or overexcavation and replacement with suitable structural fill, will be necessary. Because no design details were available at the time of this report, ESNW should review the project details to confirm the recommendations in this report are applicable. Infiltration is feasible in the northeastern portion of the site (TP-1 through TP-3) where outwash type soils were encountered at depth. Infiltration facilities planned for this site must expose the clean outwash sand and gravel soils across the entirety of the system. We appreciate the opportunity to be of service to you on this project. If you have questions regarding the content of this geotechnical engineering study, please contact us. Sincerely, EARTH SOLUTIONS NW, LLC J-025 --i,- D, Scott S. Riegel, L.G., L.E.G. Senior Project Manager 15365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 • (425) 449-4704 • FAX (425) 449-4711 Table of Contents ES-7961 PAGE INTRODUCTION................................................................................. General..................................................................................... ProjectDescription................................................................. SITE CONDITIONS............................................................................. 2 Surface.................................................................................... 2 Subsurface............................................................................. 2 Topsoil and Fill............................................................. 2 NativeSoil..................................................................... 3 Geologic Setting........................................................... 3 Groundwater................................................................. 3 GEOLOGICALLY HAZARDOUS AREAS ........................................... 3 DISCUSSION AND RECOMMENDATIONS- ..... ............................... 3 General..................................................................................... 3 Site Preparation and Earthwork ............................................. 4 Temporary Erosion Control ......................................... 4 Stripping...................................................................... 5 Excavations and Slopes .............................................. 5 In -situ and Imported Soils ........................................... 5 Wet -Season Grading .................................................... 6 StructuralFill................................................................ 6 Foundations........................................................................... 6 Seismic Design ...................................................................... 7 Slab -on -Grade Floors............................................................. 8 RetainingWalls...................................................................... 8 Landscape Retaining Walls ......................................... 9 Drainage.............................:.................................................... 9 Infiltration Evaluation ................................................... 9 Utility Support and Trench Backfill....................................... 10 Preliminary Pavement Sections ............................................. 10 LIMITATIONS..................................................................................... 11 Additional Services................................................................. 11 Earth Solutions NW, LLC Table of Contents Cont'd ES-7961 GRAPHICS Plate 1 Vicinity Map Plate 2 Test Pit Location Plan Plate 3 Retaining Wall Drainage Detail Plate 4 Footing Drain Detail APPENDICES Appendix A Subsurface Exploration Test Pit Logs Appendix B Laboratory Test Results Earth Solutions NW, LLC PRELIMINARY GEOTECHNICAL ENGINEERING STUDY TWIN TRAILS RESIDENTIAL SUBDIVISION 1605 SOUTHWEST 341 ST PLACE FEDERAL WAY, WASHINGTON ES-7961 INTRODUCTION General This preliminary geotechnical engineering study (study) was prepared for the proposed residential development to be constructed off the south side of Southwest 341 st Place near 16th Avenue Southwest in Federal Way, Washington. The purpose of this study was to develop geotechnical recommendations for the proposed project. The scope of services for completing this study included the following: • Subsurface exploration consisting of test pit excavations; • Laboratory testing of soil samples collected at the test pit locations,- 0 Engineering analyses and recommendations for the proposed development, and; • Preparation of this report. The following documents and maps were reviewed as part of preparing this study: • Preliminary Site Plan, Sheet PP-03 and PP-04, prepared by ESM Consulting Engineers dated September 3, 2021; • Geologic Map of the Poverty Bay 7.5' Quadrangle, King County, Washington, prepared by Tabor, et al, 2014; ■ Federal Way Municipal Code (FWMC) section19.145 Environmentally Critical Areas, and; ■ Web Soil Survey (WSS), provided by the United States Department of Agriculture (USDA), Natural Resources Conservation Service. Project Description Based on review of the referenced site plan, the subject site will be developed with 38 detached residential lots, access roadways, a stormwater detention tract and utility improvements. The stormwater tract will be located in the northern portion of the site. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 ES-7961 Page 2 Based on existing grades, we anticipate mass grading activities will include minor cuts and fills of up to about seven feet. Perimeter footing, loads will likely be 1 to 2 kips per lineal foot. Slab - on -grade loading is anticipated to be approximately 150 pounds per square foot (psf). The referenced plan indicates a stormwater infiltration pond will be constructed in the northern portion of the site where outwash soils were encountered during our fieldwork. The pond base will be excavated at least seven feet below existing grade to expose outwash soils and will use a combination of cut and fill berms. If the above design assumptions are incorrect or change, ESNW should be contacted to review the recommendations provided in this report. ESNW should review final designs to confirm that our geotechnical recommendations have been incorporated into the plans. SITE CONDITIONS Surface The subject site is located off the south side Southwest 341 st Place in Federal Way, Washington. The approximate location of the property is illustrated on Plate 1 (Vicinity Map). The site consists of two adjoining tax parcels (King County Parcel Numbers 2421039051 and the east half of 2421039008). The property at 1605 Southwest 341 st Place is occupied by a residence, outbuildings and landscaping in the northeastern portion of the site, but is otherwise forested. The site topography generally descends gently to the west and north from the southeastern portion of the property with about 30 feet of total elevation change. Subsurface A representative of ESNW observed, logged, and sampled eight test pits excavated across the overall project area, on May 28, 2021 using a trackhoe and operator provided by the client. The test pits were completed for purposes of assessing soil conditions, classifying site soils, and characterizing near -surface groundwater conditions within the overall development area. The approximate locations of the test pits are depicted on Plate 2 (Test Pit Location Plan). Please refer to the test pit logs provided in Appendix A for a more detailed description of subsurface conditions. Representative soil samples collected at the test pit locations were analyzed in general accordance with Unified Soil Classification System (USCS) and USDA methods and procedures. Topsoil and Fill Topsoil (where encountered) generally extended to depths of ranging from about two to eight inches below the existing ground surface (bgs). The topsoil was characterized by the observed dark brown color, the presence of fine organics, and root intrusions extending into the shallow, weathered soils. Fill was not encountered during our exploration; however, fill is likely present near the existing development areas of the site. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 Native Soil ES-7961 Page 3 Underlying topsoil, native soils encountered on the subject site were consisting primarily of silty sand with gravel (USCS: SM) that extended to the maximum exploration depth of about nine and one-half feet except test pit locations TP-1 through TP-3. At test pit locations TP-1 through TP- 3, underlying about five to six feet of silty sand, we encountered medium dense to dense, outwash-type sand and gravel deposits (USCS: SP and GW-GM) that extended to the termination depths of the test pits. Geologic Setting The referenced geologic map resource identifies ice contact (Qvi) deposits as the primary geologic unit underlying the site and surrounding areas. The referenced WSS map resource identifies Alderwood gravelly sandy loam (Map Unit Symbol: AgC) across the property. The Alderwood series soils formed in glacial till plains. Based on our field observations, the majority of the native soils encountered during our fieldwork are consistent with ice contact deposits, and the sandy soils encountered below the glacially consolidated cap within test pit locations TP-1 through TP-3 are consistent with outwash deposits. Groundwater During our subsurface exploration completed on May 2021, groundwater seepage was not encountered at the test pit locations. However, perched seepage should be expected within the weathered zone of soils on this site depending on the time of year grading occurs. In general, groundwater flow rates and elevations are higher during the winter, spring, and early summer months. GEOLOGICALLY HAZARDOUS AREAS Based on our review of the referenced Federal Way municipal code section and site conditions encountered during our fieldwork, there are no geologic hazard areas (erosion, landslide, seismic, or mine hazards) on or within 300 feet of the subject site. Standard development BMPs may be used for this site development plans. DISCUSSION AND RECOMMENDATIONS General Based on the results of our investigation, construction of typical single-family residences on this site is feasible from a geotechnical standpoint. The primary geotechnical considerations associated with the proposed development include site grading, infiltration facility construction, foundation support, slab -on -grade subgrade support, and the suitability of using on -site soils as structural fill. Earth solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 ES-7961 Page 4 Typical single-family residences may be supported on conventional continuous and spread footing foundations bearing on competent native soil, recompacted native soil, or new structural fill placed directly on competent native soil. In general, competent native soil, suitable for support of the new foundations, will likely be encountered beginning at depths of about two to four feet bgs. Where loose or unsuitable soil conditions are exposed at foundation subgrade elevations, compaction of soils to the specifications of structural fill, or overexcavation and replacement with suitable structural fill, will be necessary. ESNW should review the proposed plans to confirm the recommendations in this report remain applicable. The relatively clean sand and gravel deposits encountered at test pit locations TP-1 through TP- 3 are suitable for infiltration but in -situ testing will be required to determine the infiltration rate for formal designs. Due to the low infiltration capacity of the glacially consolidated soils on this site, infiltration is not recommended for the southern portion of the development area. This study has been prepared for the exclusive use of Prospect Development, LLC, and their representatives. A warranty is neither expressed nor implied. This study has been prepared in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area. Site Preparation and Earthwork Initial site preparation activities will consist of installing temporary erosion control measures, establishing grading limits, removing structural improvements, and clearing and stripping the site. Subsequent earthwork activities will involve site grading and related infrastructure improvements. Temporary Erosion Control The following temporary erosion control measures are offered: ■ Temporary construction entrances and drive lanes, consisting of at least six inches of quarry spalls, should be considered to both minimize off -site soil tracking and provide a stable access entrance surface. Placing geotextile fabric underneath the quarry spalls will provide greater stability, if needed. ■ Silt fencing should be placed around the site perimeter. • When not actively graded, soil stockpiles should be covered -or otherwise protected. ■ Temporary measures for controlling surface water runoff, such as interceptor trenches, sumps, or swales, should be installed prior to beginning earthwork activities. • Dry soils disturbed during construction should be wetted to minimize dust and airborne soil erosion. Additional Best Management Practices (BMPs), as specified by the project civil engineer and indicated on the plans, should be incorporated into construction activities. Temporary erosion control measures should be actively managed and may be modified during construction as site conditions require, to ensure proper performance. Earth Solutions NW. LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 Stripping ES-7961 Page 5 Topsoil was generally encountered within the upper approximately four to eight inches at the test pit locations. The organic -rich topsoil should be stripped and segregated into a stockpile for later use on site or to haul off site. The material remaining immediately below the topsoil may have some root zones and will likely be variable. in composition, density, and/or moisture content. The material exposed after initial topsoil stripping will likely not be suitable for direct structural support as is and will likely need to be compacted in place_ or stripped .and stockpiled for reuse as fill; depending on the time of year stripping occurs, the soil exposed below the topsoil may be too wet to compact and may need to be aerated or treated. ESNW should observe initial stripping activities to provide recommendations regarding stripping depths and material suitability. Excavations and Slopes Based on the soil conditions observed at the subsurface exploration locations, the maximum allowable temporary slope inclinations provided below may be used. The applicable Federal Occupation Safety and Health Administration and Washington Industrial Safety and Health Act soil classifications are also provided. Areas exposing groundwater seepage 1.5H:1V (Type C) Loose soil; fill 1.5H:1V (Type C) • Medium dense to dense native soil 1 H:1 V (Type B) Permanent slopes should be planted with vegetation to both enhance stability and minimize erosion. The presence of perched groundwater may cause localized sloughing of temporary slopes. An ESNW representative should observe temporary and permanent slopes to confirm the slope inclinations are suitable for the exposed soil conditions and to provide additional excavation and slope recommendations as necessary. If the recommended temporary slope inclinations cannot be achieved, temporary shoring may be necessary to support excavations. In -situ and Imported Soils The majority of the near -surface soils encountered during our subsurface exploration have a high sensitivity to moisture and were generally in a damp to moist condition at the time of the exploration (May 2021). Exposed soils will degrade rapidly if exposed to wet weather and/or construction traffic. In general, soils encountered during site excavations that are excessively over the optimum moisture content will require aeration or treatment prior to placement and compaction. Conversely, soils that are substantially below the optimum moisture content will require moisture conditioning through the addition of water prior to use as structural fill. A representative of ESNW should determine the suitability of in -situ soils for use as structural fill at the time of construction. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 ES-7961 Page 6 Imported soil intended for use as structural fill should consist of a well -graded, granular soil with a moisture content that is at (or slightly above) the optimum level. During wet weather conditions, imported soil intended for use as structural fill should consist of a well -graded, granular soil with a fines content of 5 percent or less (where the fines content is defined as the percent passing the Number 200 sieve, based on the minus three -quarter -inch fraction). Wet -Season Grading Because the site soils are highly sensitive to moisture, grading during the rainy season will be very difficult. If grading takes place during the winter, spring, or early summer months, a contingency in the project budget should be included to allow for export of native soil and import of structural fill. Structural Fill Structural fill is defined as compacted soil placed in foundation, slab -on -grade, roadway, permanent slope, retaining wall, utility trench, and vault backfill areas. Soils placed in structural areas should consist of a granular material devoid of deleterious debris and organics, placed in loose lifts of 12 inches or less and compacted to a relative compaction of 95 percent, based on the laboratory maximum dry density as determined by the Modified Proctor Method (ASTM D- 1557). Foundations Typical two to three story residential structures may be supported on conventional spread and continuous footings bearing -on competent native soil, recompacted native soil, or new structural fill placed directly on competent native soil. In general, competent native soil suitable for the support of foundations will likely be encountered at depths of about two to four feet bgs. ESNW should evaluate the design subgrade conditions to confirm suitable conditions are exposed and to provide additional preparation recommendations, where necessary. Where loose, organic, or otherwise unsuitable soil conditions are observed at foundation subgrade elevations, compaction of the soils to the specifications of structural fill, or overexcavation and replacement with granular structural fill, will likely be necessary. Provided residential structures will be supported as described above, the following parameters can be used for design of the new foundations: ■ Allowable soil bearing capacity 2,500 psf Passive earth pressure 300 pcf (equivalent fluid) • Coefficient of friction 0.40 The passive earth pressure and coefficient of friction values include a safety factor of 1.5. A one- third increase in the allowable soil bearing capacity may be assumed for short-term wind and seismic loading conditions. With structural loading as expected, total settlement in the range of 1 inch is anticipated, with differential settlement of about 0.5 inch. The majority of settlement should occur during construction, as dead loads are applied. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 Seismic Design ES-7961 Page 7 The 2018 International Building Code (2018 IBC) recognizes the most recent edition of the Minimum Design Loads for Buildings and Other Structures manual (ASCE 7-16) for seismic design, specifically with respect to earthquake loads. Based on the soil conditions encountered at the boring locations, the parameters and values provided below are recommended for seismic design per the 2018 IBC.. Parameter Value Site Class D* Mapped short period spectral response acceleration, Ss (g) 1.336 Mapped 1-second period spectral response acceleration, S1(g) 0.459 Short period site coefficient, Fa 1 Long period site coefficient, Fv 1.86 Adjusted short period spectral response acceleration, SMs (g) 1.336 Adjusted 1-second period spectral response acceleration, SM1 (g) 0.854 Design short period spectral response acceleration, SDS (g) 0.891 Design 1-second period spectral response acceleration, SD1 (g) 0.569 * Assumes medium dense soil conditions, encountered to a maximum depth of nine and one-half feet bgs during the May 2021 field exploration, remain medium dense or better to at least 100 feet bgs. Based on our experience with the project geologic setting (glacial till) across the Puget Sound region, soil conditions are likely consistent with this assumption. Further discussion between the project structural engineer, the project owner (or their representative), and ESNW may be prudent to determine the possible impacts to the structural design due to increased earthquake load requirements under the 2018 IBC. ESNW can provide additional consulting services to aid with design efforts, including supplementary geotechnical and geophysical investigation, upon request. Liquefaction is a phenomenon where saturated or loose soil suddenly loses internal strength and behaves as a fluid. This behavior is in response to increased pore water pressures resulting from an earthquake or another intense ground shaking. In our opinion, site susceptibility to liquefaction may be considered negligible. The absence of a shallow groundwater table and the relative density of the native soil were the primary bases for this opinion. Earth Solutions NW, LLC Prospect Development, LLC ES-7961 August 23, 2021 Page 8 Updated September 13, 2021 Slab -on -Grade Floors Slab -on -grade floors should be supported on a firm and unyielding subgrade consisting of competent native soil or new structural fill. Unstable or yielding areas of the subgrade should be recompacted or overexcavated and replaced with suitable structural fill prior to construction of the slab. A capillary break, consisting of a minimum of four inches of free -draining crushed rock or gravel, should be placed below the slab. The free -draining material should have a fines content of 5 percent or less defined as the percent passing the number 200 sieve based on the minus three-quarters inch fraction. In areas where slab moisture is undesirable, installation of a vapor barrier below the slab should be considered. If used, the vapor barrier should consist of a material specifically designed to function as a vapor barrier and should be installed in accordance with the manufacturer's specifications. Retainina Walls Retaining walls must be designed to resist earth pressures and applicable surcharge loads. The following parameters may be used for design: Active earth pressure (unrestrained condition) At -rest earth pressure (restrained condition) • Traffic surcharge (passenger vehicles) Passive earth pressure ■ Coefficient of friction • Seismic surcharge * Where applicable ** Where H equals the retained height (in feet) 35 pcf (equivalent fluid) 55 pcf 70 psf (rectangular distribution) * 300 pcf (equivalent fluid) 0.40 8H psf** The above design parameters are based on a level backfill condition and level grade at the wall toe. Revised design values will be necessary if sloping grades are to be used above or below retaining walls. Additional surcharge loading from adjacent foundations, sloped backfill, or other relevant loads should be included in the retaining wall design, where applicable. A safety factor of 1.5 is included in the passive earth pressure and coefficient of friction values. Retaining walls should be backfilled with free -draining material or suitable sheet drainage that extends along the height of the wall and a distance of at least 18 inches behind the wall. The upper 12 inches of the wall backfill can consist of a less permeable soil, if desired. A perforated drainpipe should be placed along the base of the wall and connected to an approved discharge location. A typical retaining wall drainage detail is provided on Plate 3. If drainage is not provided, hydrostatic pressures should be included in the wall design. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 Landscape Retaining Walls ES-7961 Page 9 Based on the existing site grades, retaining walls may be used along the portions of the lots to raise grades for new building pads. Final wall heights, alignments and facing materials have not been determined at the time of this report. Walls over four feet in total height, including toe embedment will require building permits supported by an engineered design. ESNW can prepare and engineered retaining wall design, upon request. ESNW should review the final grading plans to confirm the recommendations are incorporated and to provide additional recommendations where appropriate. Drainage Groundwater seepage was not encountered during our exploration; however, groundwater seepage will likely be encountered within site excavations, particularly utility trenches and deeper excavations such as detention vault/pond areas. Temporary measures to control surface water runoff and groundwater during construction would likely involve passive elements, such as interceptor trenches and sumps. ESNW should be consulted during preliminary grading to identify areas of groundwater and to provide recommendations to reduce the potential for instability related to groundwater effects. Depending on the flow volumes encountered during grading, an interceptor trench drain system may be warranted along the up -slope perimeter of the work area to help mitigate or otherwise control shallow perched groundwater flows. Finish grades must be designed to direct surface water away from the new structures and/or slopes for a distance of at least 10 feet or as setbacks allow. Water must not be allowed to pond adjacent to the new structures and/or slopes. A typical foundation drain detail is provided on Plate 4. Infiltration Evaluation Outwash type soils were encountered at depth within test pits TP-1 through TP-3 with corresponding fines content of about 5 percent. The outwash soils may be suitable for infiltration pending in -situ infiltration testing. Based on textural analyses and using the DOE saturated hydraulic conductivity calculation, for feasibility considerations, a preliminary infiltration rate of 10 inches per hour was recommended to the project civil engineer for the relatively clean outwash sand and gravel deposits such as those encountered at test pit locations TP-1 through TP-3. The facility illustrated on the referenced preliminary plans were developed based on information provided by ESNW during the early phase of design. ESNW should conduct targeted infiltration testing to better characterize the infiltration potential for the outwash type soils. Based on the conditions observed during our fieldwork and anecdotal information provided by the project civil engineer, groundwater separation will likely be feasible. We understand projects nearby to the north also use infiltration systems and these are functioning as intended. The native soils in the southern portion of the site consist predominately of silty sand with gravel, glacially consolidated deposits that exhibit fines contents ranging from about 33 to 49 percent (passing the U.S. No. 200 sieve), and are not suitable for infiltration. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 Utility Support and Trench Backfill ES-7961 Page 10 The native soils observed at the test pit locations are generally suitable for support of utilities; however, the native soils may not be suitable for use as structural backfill in the utility trench excavations unless the soil is at or near the optimum moisture content at the time of placement and compaction. Moisture conditioning or cement treatment of the soils may be necessary at some locations prior to use as structural fill. If utility backfill occurs during wet weather, cement treatment of native soils or import of a suitable material will be necessary. Utility trench backfill should be placed and compacted to the specifications of structural fill provided in this report, or to the applicable requirements of presiding jurisdiction. Preliminary Pavement Sections The performance of site pavements is largely related to the condition of the underlying subgrade. To ensure adequate pavement performance, the subgrade should be in a firm and unyielding condition when subjected to proofrolling with a loaded dump truck. Structural fill in pavement areas should be compacted to the specifications detailed in the Site Preparation and Earthwork section of this report. It is possible that soft, wet, or otherwise unsuitable subgrade areas may still exist after base grading activities. Areas of unsuitable or yielding subgrade conditions may require remedial measures such as overexcavation and replacement with structural fill or thicker crushed rock sections prior to pavement. For lightly loaded pavement areas subjected primarily to passenger vehicles such as driveways, the following preliminary pavement sections may be considered: ■ A minimum of two inches of hot mix asphalt (HMA) placed over four inches of crushed rock base (CRB), or; A minimum of two inches of HMA placed over three inches of asphalt treated base (ATB). Heavier traffic areas generally require thicker pavement sections depending on site usage, pavement life expectancy, and site traffic. For preliminary design purposes, the following pavement sections for occasional truck traffic areas may be considered: • Three inches of HMA placed over six inches of crushed rock base (CRB), or; ■ Three inches of HMA placed over four -and -one-half inches of ATB. The HMA, CRB and ATB materials should conform to WSDOT specifications. If pavement areas will include a reverse crown, additional drainage should be used to effectively convey water that may enter the subgrade toward the storm drainage system. ESNW can provide recommendations for enhanced drainage upon request. Earth Solutions NW, LLC Prospect Development, LLC August 23, 2021 Updated September 13, 2021 LIMITATIONS ES-7961 Page 11 The recommendations and conclusions provided in this study are professional opinions consistent with the level of care and skill that is typical of other members in the profession currently practicing under similar conditions in this area. A warranty is neither expressed nor implied. Variations in the soil and groundwater conditions observed at the test locations may exist and may not become evident until construction. ESNW should reevaluate the conclusions provided in this study if variations are encountered. Additional Services ESNW should have an opportunity to review final project plans with respect to the geotechnical recommendations provided in this report. ESNW should also be retained to provide testing and consultation services during construction. Earth Solutions NW, LLC 'riry Flrmenfprr $NIQpi L �.r. 'L. ,Y�_ •_;,.,ten!'• i� ...,,.n„. -. r.. ,r.� rE77 4"L :4��r aT :']rile P'.-• !Q� -. i a �. Qla 1 . � .,� • 4 `� �. �q. T — "!r•Ir�YvF[s'! L - — � —_ ._ .. —.�.. _ 5 _ C . Fyn g a _. +� = u' F - Svgrrvfrr •Avo,rn . Ift oui}iwesi .i4l1iA 5'trC�!^ = � 1•c �"+"'-- _ � ~� . .. _ W �[P ti`. ry - 4l°4n.'Sl� W,.. tl S U a v.f'b 'Ly - • sou[n:,:i x,�• v. �Vyrr.sEor•re Y.: . �9 _ SITE r'�r a ar.jsxri r '„nrkrr i •i p00q - u��• v-�`Yy, u� � � i.O ���� � t `�• C� Seu[nhc:r ?:Sin clxr f.:" nl•• 'J q:a rQl� `SL.:..- ` , - o ,..ca ;aumw.0 ju•m sl.vrr �,. {i Q Y L .e •.� i ,� yr.,e-, M6. I"a[r - S!` - • a` �3 a $eurn[.efi }i1A SUVMy• '...._-:..----- f'T. or . \ r + `- Z�� Og V C 4 rxinrrcsi ii8'Ir :feel y - ., b s ]'¢ r 41 R _•-- E. +COAcb� m� .. n - •' 6Ri-. � •..ps gg Q` 9•�J7 K� �rf'fi'i'rrlf i�I! ., [, .0 ii�l r+sy n+vru�eai[ �• a w - 3fn s�yy" 27ln ifi{'C: N6rrnt•�ys - _ - .�,�. — •�F.�__. - -_.. i ��jp�i I1!�$CilL� s �oltl5 ee— x Noriheilst 6 Me �•` i � r, '? - �^ : E.: it � �' Solutions Reference: NORTH King County, WashingtonAOMN" Im +• Open StreetMap.org VP Vicinity Map Twin Trails Federal Way, Washington NOTE: This plate may contain areas of color. ESNW cannot be Drwn. MRS Date 08/13/2021 Proj. No. 7961 responsible for any subsequent misinterpretation of the information resulting from black & white reproductions of this plate. Checked SSR Date Aug. 2021 Plate 1 uo�6u!yseM '�(eM !e�apad s!!e�l u!nnl ue!d uo!;eoo� �!d 3sal ,n rn � d � Y � � N d z � o N W CZE 29 E = m 1 J c c iG •O-• 7 J Q m `o G UJ m v � Wa� ai o �Ftm -'e mV CZn : x5 n J Q W W co W D_ ~O ❑ Z -Ea ■ — Q 2 — E IL � � o m 5� Lij F g, s w z a �'ne.m w a o Z m h I I l P r 1 \Cti IIIN 1 \\ 11161 j. i I 1 i i 1 1 I 1 I0 r q 0 _ MS BAV H16 L 1 I `l I I Ill— r NOTES: 18" Min. 00 �o V � 00 DpvOo 00 po �P. o 0 OpO C 0 00 0 0 0 0 0 D000o. 00 00 O 000 0 0o � 0 0 0 p 0 0 0 0 0 0 0p 0 0 0 000 0 0 000C 00 0 op 0 O00ooOp o O 0p0 00 O 0 0o Do 0 0�4 Do 0 0 08`o 00. 0 00 8 0 0 00 O p .0.0 0 00 0 0 0 00.0 0 0 0 00 0 00 0 0 0 % 0 0 00 0 0 00 0 8 D 0 0 a0 0 0 0000 0 0 .0 0 0* 0 • Free -draining Backfill should consist of soil having less than 5 percent fines. Percent passing No. 4 sieve should be 25 to 75 percent. • Sheet Drain may be feasible in lieu of Free -draining Backfill, per ESNW recommendations. • Drain Pipe should consist of perforated, rigid PVC Pipe surrounded with 1-inch Drain Rock. LEGEND: 00 a p__00'-a0o Free -draining Structural Backfill 1-inch Drain Rock Structural Fill Perforated Rigid Drain Pipe (Surround in Drain Rock) SCHEMATIC ONLY - NOT TO SCALE NOT A CONSTRUCTION DRAWING A Retaining Wall Drainage Detail Twin Trails Federal Way, Washington Drwn. MRS Date 08/13/2021 Proj. No. 7961 Checked SSR Date Aug. 2021 Plate 3 NOTES: ■ Do NOT tie roof downspouts to Footing Drain. + Surface Seal to consist of 12" of less permeable, suitable soil. Slope away from building. LEGEND: Surface Seal: native soil or other low -permeability material. �•r•r•r•r . 4.ti..•.ti.fi r•r•r•r•: �tirtirti.�t 1-inch Drain Rock ti�ti�4•�• Perforated Rigid Drain Pipe (Surround in Drain Rock) SCHEMATIC ONLY - NOT TO SCALE NOT A CONSTRUCTION DRAWING Footing Drain Detail Twin Trails Federal Way, Washington Drwn. MRS Date 08/13/2021 Proj. No. 7961 Checked SSR Date Aug. 2021 Plate 4 Appendix A Subsurface Exploration Test Pit Logs ES-7961 Subsurface conditions at the subject site were explored on May 28, 2021 by excavating eight test pits using a mini-trackhoe and operator provided by the client. The approximate locations test pits are illustrated on Plate 2 of this study. The test pit logs are provided in this Appendix. The maximum exploration depth was approximately nine and one-half feet bgs and were terminated in firm native soils. The final logs represent the interpretations of the field logs and the results of laboratory analyses. The stratification lines on the logs represent the approximate boundaries between soil types. In actuality, the transitions may be more gradual. Earth Solutions NW, LLC Earth Solutions NWLLC SOIL CLASSIFICATION CHART MAJOR DIVISIONS SYMBOLS TYPICAL DESCRIPTIONS GRAPH I LETTER GRAVEL AND CLEAN GRAVELS .' �� ���ib ■ +* GW WELL -GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES ° f10 o1O0 ❑ b 0 0 Q ❑ n0 GP POORLY -GRADED GRAVELS, GRAVEL- SAND MIXTURES, LITTLE OR NO FINES GRAVELLY SOILS (LITTLE OR NO FINES) COARSE GRAINED SOILS MORE THAN 50% OF COARSE GRAVELS WITH FINES ° a ° °0 ° 0 GM SILTY GRAVELS, GRAVEL - SAND - SILT MIXTURES FRACTION RETAINED ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) G.`. CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES MORE THAN 50% OF MATERIAL IS SAND AND CLEAN SANDS SW WELL -GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES '; SP POORLY -GRADED SANDS, GRAVELLY SAND, LITTLE OR NO FINES LARGERTHAN NO. 200 SIEVE SIZE SANDY SOILS (LITTLE OR NO FINES) SANDS WITH FINES SM SILTY SANDS, SAND - SILT MIXTURES MORE THAN 50% OF COARSE FRACTION PASSING ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) SG. CLAYEY SANDS, SAND - CLAY MIXTURES INORGANIC SILTS AND VERY FINE M L SANDS, ROCK FLOUR, SILTY OR CLAYEY FINE SANDS OR CLAYEY SILTS WITH SLIGHT PLASTICITY CL INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS, LEAN CLAYS FINE GRAINED SOILS SILTS LIQUID LIMIT AND LESS THAN 50 CLAYS — — OL ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY MORE THAN 50% OF MATERIAL IS SMALLER THAN NO. 200 SIEVE M H INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS FINE SAND OR SILTY SOILS slzE SILTS LIQUID LIMIT AND CLAYS GREATER THAN 50 CH INORGANIC CLAYS OF HIGH PLASTICITY OH ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY, ORGANIC SILTS HIGHLY ORGANIC SOILS °'PT „� L-1 `„ PEAT, HUMUS, SWAMP SOILS WITH HIGH ORGANIC CONTENTS DUAL SYMBOLS are used to indicate borderline soil classifications. The discussion in the text of this report is necessary for a proper understanding of the nature of the material presented in the attached logs. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-1 PAGE 1 OF 1 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND ELEVATION EXCAVATION CONTRACTOR Client Provided LATITUDE 47.29579 EXCAVATION METHOD GROUND WATER LEVEL: LOGGED BY SES CHECKED BY SSR SZ AT TIME OF EXCAVATION NOTES Depth of Topsoil_& Sod 6": grass w U _W W_j g TESTS 2 O 0 Cn 0 mz 0 5L SM MC = 13.3% MC = 4.5% Fines = 5.3% GM MC = 4.8°/ LONGITUDE-122.35659 MATERIAL DESCRIPTION Dark brown TOPSOIL, minimal root intrusions Brown silty SAND with gravel, medium dense, moist (Weathered Till) -becomes gray, dense Gray well -graded GRAVEL with silt and sand, dense, damp [USDA Classification: extremely gravelly loamy coarse SAND] Test pit terminated at 9.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. y Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 NOR I Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-2 PAGE 1 OF 1 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND ELEVATION EXCAVATION CONTRACTOR Client Provided LATITUDE 47.29516 LONGITUDE-122.35649 EXCAVATION METHOD GROUND WATER LEVEL: LOGGED BY SES CHECKED BY SSR �Z AT TIME OF EXCAVATION NOTES Depth of Topsoil & Sod 4": grass }w }W U _ I— W g TESTS C 2 Q O ❑ ¢ z 0_ 0 SM MC = 10.1 % 5.5 GW- i GM i MC = 5.3% 8.p MATERIAL DESCRIPTION Dario brown TOPSOIL, minimal root intrusions Brown silty SAND with gravel, medium dense, damp (Weathered Till) -becomes gray, dense Gray well -graded GRAVEL with silt and sand, dense, damp (Unweathered Till) Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. J y Earth Solutions NW, LLC TEST PIT NUMBER TP-3 15365 N.E. 90th Street, Suite 100 PAGE 1 OF 1 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND ELEVATION EXCAVATION CONTRACTOR Client Provided LATITUDE 47.29502 LONGITUDE-122.35591 EXCAVATION METHOD GROUND WATER LEVEL: LOGGED BY SES CHECKED BY SSR Q AT TIME OF EXCAVATION NOTES Depth of Topsoil & Sod 8": grass W U _ o W TESTS 2 O MATERIAL DESCRIPTION 0 Z Q C7 U) a P5L _= '�• p 5 Dark brown TOPSOIL, minimal root intrusions Brown silty SAND with gravel, medium dense, damp -large cobbles 1.5 diameter MC = 7.3% SM 5 16.0 Brown poorly graded SAND with gravel, medium dense, damp SP '- Test pit terminated at 8.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 ► Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-7961 DATE STARTED 6/28/21 COMPLETED 5/28/21 EXCAVATION CONTRACTOR Client Provided EXCAVATION METHOD LOGGED BY SES CHECKED BY SSR NOTES Depth of Topsoil & Sod 2": duff TEST PIT NUMBER TP-4 PAGE 1 OF 1 PROJECT NAME Twin Trails GROUND ELEVATION LATITUDE 47.29368 LONGITUDE-122.35622 GROUND WATER LEVEL: a AT TIME OF EXCAVATION w _ wW �w Im TESTS , _ Q O MATERIAL DESCRIPTION Qz co Gray silty SAND with gravel, medium dense, moist MC = 15.7% I Fines = 49.1 /° j; } •' _ [USDA Classification: gravelly LOAM] III iii I 'I SM -becomes dense, damp -weakly cemented MC = 12.W/._j Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-5 PAGE 1 OF 1 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND ELEVATION EXCAVATION CONTRACTOR Client Provided LATITUDE 47.29382 LONGITUDE-122.35755 EXCAVATION METHOD _ GROUND WATER LEVEL: LOGGED BY SES CHECKED BY SSR SZ AT TIME OF EXCAVATION NOTES Depth of Topsoil & Sod 3": duff w _ �x " U Lu 2 w j 2 TESTS ai O MATERIAL DESCRIPTION o a= j Qz O co 0 Gray silty SAND, medium dense, damp SM MC = 10.5% -increasing sand content MC = 13.2% [USDA Classification: slightly gravelly sandy LOAM] Fines = 33.5% Test pit terminated at 9.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 s Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-6 PAGE 1 OF 1 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND ELEVATION EXCAVATION CONTRACTOR Client Provided LATITUDE 47.2944 LONGITUDE-122.35822 EXCAVATION METHOD GROUND WATER LEVEL: LOGGED BY SES CHECKED BY SSR Q AT TIME OF EXCAVATION NOTES Depth of Topsoil & Sod 3": duff W CL U ww W TESTS = O EL Q z C7 U) 0 MC=14.7% I SM MC = 13.2% inpm a 'Al A91 MATERIAL DESCRIPTION Brown silty SAND, medium dense, moist -becomes gray -becomes dense -iron oxide staining [USDA Classification: gravelly sandy LOAM] Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 • l Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-7 PAGE 1 OF 1 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND ELEVATION EXCAVATION CONTRACTOR Client Provided LATITUDE 47.29438 LONGITUDE-122.3571 EXCAVATION METHOD GROUND WATER LEVEL: LOGGED BY SES CHECKED BY SSR SZ AT TIME OF EXCAVATION NOTES Depth of Topsoil & Sod 6": duff W L W � g TESTS c CL O CL a z c� 0 MC = 14.4% I SM = 14.1 % MATERIAL DESCRIPTION Dark brown TOPSOIL, shp1low root intrusions Brown silty SAND with gravel, medium dense, moist -becomes gray, dense -weakly cemented -becomes gray -iron oxide staining Test pit terminated at 8.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. Earth Solutions NW, LLC IhArp M. 15365 N.E. 90th Street, Suite 100 woRedmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 PROJECT NUMBER ES-7961 DATE STARTED 5/28/21 _ COMPLETED 5/28/21 EXCAVATION CONTRACTOR Client Provided EXCAVATION METHOD LOGGED BY SES CHECKED BY SSR NOTES Depth of Topsoil & Sod 3": duff TEST PIT NUMBER TP-8 PAGE 1 OF 1 PROJECT NAME Twin Trails GROUND ELEVATION LATITUDE 47.29431 GROUND WATER LEVEL: a AT TIME OF EXCAVATION LONGITUDE-122.35624 W CL _ W U W = w� Lu Co TESTS 0_O MATERIAL DESCRIPTION 0 0 z 0 c� co 0 Gray silty SAND with gravel, medium dense, damp -becomes dense -weakly cemented MC = 10.8% Fines = 39.2% SM [USDA Classification: slightly gravelly sandy LOAM] -becomes very dense MC = 10.4% Test pit terminated at 6.5 feet below existing grade due to refusal on very dense soils. No groundwater encountered during excavation. No caving observed. Appendix B Laboratory Test Results ES-7961 Earth Solutions NW, LLC [:11'1h • 1 11 NwIll 1. 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I1111 ■■ lllllll■■I!I IIII■■ 1■■1111111■■IIIIIII■■IIIIIII■■IIIIIII■■IIIIIII■■ 1 ■ ■lllllll■■I II II11■■III I111■■IIII III ■ ■IIIIIII■ ■ ,11■■lllllll■IIIIIII■■IIIIIII■■IIIIIII■■IIIIIII■■ 11■■IIIIIII■■IIIIIII■ ■I II1111■ ■I II1111■■lllllll ■■ ,11■■IIIIIII■�II11I11■■IIIIIIIOIIIIIII■■1III111■■ r Specimen Report Distribution ES-7961 EMAIL ONLY Prospect Development, LLC 2913 — 5th Avenue Northeast, Suite 201 Puyallup, Washington 98372 Attention: Mr. Justin Holland EMAIL ONLY Mr. Clay Loomis Earth Solutions NW, LLC June 30, 2022 ES-7961.02 Earth !Solutions NW«C Earth Solutions NW LLC Geotechnical Engineering, Construction Observation/Testing and Environmental Services RESUBMITTED 9-14-22 CITY OF FEDERAL WAY COMMUNITY DEVELOPMENT 21-10415-S U Prospect Development, LLC 2913 — 5th Avenue Northeast, Suite 201 Puyallup, Washington 98372 Attention: Mr. Justin Holland Subject: Infiltration Evaluation Proposed Twin Trails Residential Development 1605 Southwest 3415t Place Federal Way, Washington Reference: Earth Solutions NW, LLC Geotechnical Engineering Study Project No. ES-7961, dated September 13, 2021 ESM Consulting Engineers, LLC Site Plan Set, Sheets 1 to 7, dated September 30, 2021 Greetings, Mr. Holland: As requested, Earth Solutions NW, LLC (ESNW) has prepared this infiltration evaluation for the proposed residential development. A summary of the subsurface exploration activities, infiltration testing, and recommendations and conclusions about infiltration design are provided in this letter. Proiect Description The subject site is located at 1605 Southwest 341st Place, in Federal Way, Washington. The attached Plate 1 (Vicinity Map) depicts the approximate site location. The project is currently pursuing the construction of a 38-lot residential development and associated infrastructure improvements. We understand that infiltration is being pursued via a stormwater pond located within the northwest site corner (Tract A). 15365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 • (425) 449-4704 • FAX (425) 449-4711 Prospect Development, LLC June 30, 2022 Subsurface ES-7961.02 Page 2 ESNW performed a general subsurface exploration program on May 28, 2021, in association with the referenced geotechnical engineering study. As part of the scope of services associated with the current phase of work, additional test pit explorations and in -situ infiltration testing in the form of Pilot Infiltration Tests (PITs) were performed on May 17, 2022. This most recent work was targeted to the proposed stormwater facility with Tract A. These approximate test pit locations are depicted on the attached Plate 2 (Test Pit Location Plan). Please note that the following sections and recommendations pertain to the conditions observed during the May 2022 exploration and testing program, unless specifically stated otherwise. Approximately six to eight inches of topsoil was encountered at the test pit locations. The topsoil was characterized by a dark brown color, trace organics, and minor root intrusions. Fill was not encountered at the test pit locations. Underlying topsoil, native soils exhibited a fairly uniform soil stratigraphy; consisting primarily of silty sand with or without gravel (USCS: SM) encountered to a depth of about four -and -one-half to seven -and -one-half feet below the ground surface (bgs). Thereafter, soils transitioned into gravelly and sandy outwash deposits (USCS: GP, GP -GM, and SP). These soil conditions extended to the terminus of each test pit exploration which occurred at about 13 to 14 feet bgs. In general, the silty sand was encountered in a loose to dense condition and the outwash sands and gravels were encountered in a dense to very dense condition. All observed native soils were characterized as moist at the time of the May 2022 exploration. Groundwater Groundwater was not encountered at the test pit locations during the May 2022 field exploration. The presence of seepage, flow rates, and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. In general, groundwater flow rates are higher during the winter, spring, and early summer months. Infiltration Feasibility ESNW performed in -situ infiltration testing in the form of PITs during the May 2022 fieldwork. The location and depth of tests correlate with the proposed facility design dimensions provided by the project civil engineer. The following table depicts the infiltration test location, encountered soil type, test depth, measured rate, appropriate safety factors, and recommended design rate. Location Soil Type Test Depth (ft Measured Rate (in/hr) Correction Factors Recommended Design Rate (in/hr) Ftesting Fgeometry Fpliugging bgs) TP-102 GM 6 15.6 0.5 1.0 1.0 7.8 I Earth Solutions NW, LLC Prospect Development, LLC ES-7961.02 June 30, 2022 Page 3 From a geotechnical standpoint, a design rate of 7.8 in/hr is appropriate for infiltration targeted within the outwash sand/gravel deposit. A PIT was also performed at TP-101 during the May 2022 exploration but the recorded field rate was significantly lower than that observed at TP-102 and is attributed to the presence of discrete silty sand lensing within the larger gravel deposit. After additional test pit exploration, we noted that a thin silty layer in TP-101 may have affected the infiltration rate. This appears to be an isolated occurrence; and, as such, the infiltration rate recorded at this location, in our opinion, is not representative of the infiltration capacity of the gravel deposit as a whole. This is confirmed by the laboratory data that shows the sand and gravel generally has 5 percent or less fines content. Soil conditions below the test depth transitioned to lower fines content, which is more consistent with the other soil deposits in this area. While the recommended infiltration rate is based on one test, the additional test pits and associated laboratory testing confirm consistent soil conditions present within and surrounding the proposed pond area. Based on the conditions encountered during our fieldwork, groundwater will not impact the performance of the infiltration facility. In our opinion, based on the results of in -situ infiltration testing and laboratory analysis, using a design infiltration rate of 7.8 in/hr is acceptable from a geotechnical standpoint. Confirmation testing will provide adequate data regarding the design infiltration rate. Further evaluation and delineation of potential discrete lensing within the facility footprint should be performed at the time of construction. This could include supplementary test pits within the pond subgrade area to help delineate silty lensing. If present, silty sand lenses should be removed and areas should be restored with a material that possesses a similar infiltration capacity to that of the native sands/gravels. Limitations & Additional Services This letter has been prepared for the exclusive use of the Prospect Development, LLC, and its representatives. No warranty, express or implied, is made. The recommendations and conclusions provided in this letter are professional opinions consistent with the level of care and skill that is typical of other members in the profession currently practicing under similar conditions in this area. If the design assumptions outlined herein either change or are incorrect, or if construction conditions differ from those encountered during the fieldwork, ESNW should be contacted to review the recommendations and conclusions provided in this letter. ESNW should have an opportunity to review the final project plans concerning the geotechnical recommendations and conclusions provided in this letter. ESNW should also be retained to provide testing, observation, and other consultation services during construction. Earth Solutions NW, LLC Prospect Development, LLC June 30, 2022 ES-7961.02 Page 4 We trust this evaluation, meets your, current needs. If you have any questions regarding the content of this letter report, or if you require additional information, please call. Sincerely, EARTH SOLUTIONS NW, LLC JA/-1- G. L Chase G. Halsen, L.G. Senior Project Geologist Attachments: Plate 1 — Vicinity Map Plate 2 — Test Pit Location Plan Test Pit Logs Grain Size Distribution cc: Mr. Clay Loomis (Email only) QJ W Enpinwrin��l 25d Ge°�°06/30/2022 Scott S. Riegel Scott S. Riegel, L.G., L.E.G. Associate Principal Geologist Earth Solutions NW, LLC -..,� � r�rrr?7rmrmn,r 1 � :: {G . .•`.6.. ��. .r�: 'r9z=:ndn:e �`•. +• 1..er, afe Sn.,ervn.i fl6:R Sf•a.e _:M'[AfL �_, �¢}J! L A So u:n.w r•3i-Erh S[.•I •� f 1}�; B � _ ..�� .,r yi' Lore � n,g n �iv� � •�yhii V � � � { 3v � �M1 �/ •. aeuen..rr 1.31.E v11ee �� 5 � �' ��rr � � .fnr , '�'. ' -- --- nrn'Cvi?S'i -- -. .. _ ^ 1 �•F,.p X%. .f ••j ,q;m .� . .rrn n13Yi11 sitCil "�V i .. �'!Sp 7! 5"rh a u:0Kr st U&ffsi riio soy n....i I*M' MY�� , - 'a � �y._ - �l� .r "`�;, co- .. .'ryirseunr •'� � � .r �o .._ 'y �f � aC�i F'4rrc •w++'� 6 � a Fq d 9 S T E =, Y1q'e M1rr,OrMf 1 :; 11 •• 1 YS y 'td "�•'� .r f1[Alfr rz7; ,u } r+u.: s` Q6 'RO 1 '6 '2' _ L t• C D 4 a• 0 9' Sou ri 1 vm yi..' sos~ j4 wn ?r yn jl�u_ - ..'�. �� I ya an.. •;i ^ar,r. r+¢r. _'L.� �r�.= ,. t', 9 i.- R' y � r t:�•� r ! � i sa,rr r u7m nxe � .. , .: � ��^� • � � :�v "'�F� . _ I`� - - ey ISn.iaRw.�,.u¢ryf•r... .SR•1� q. ederal ay .� ¢,r,.. . '`� 3 1.7 .rH I .... ,.., :B C �� � .. � p1r. _ EnSr+pi •sr' i7 � .A,' ' . rirs�iY _•_ .. � .�� a�P� t ... � �r� nrr EI F 5r. '7 ,.}" s. ,. -.,- �w�nl ,. o+g. ?' i- '+��bw'ye0!+W��'� yn�j�.� Y♦aS+r+ . �>11� .. .� t—. v •$.'i"�o a;:". _ ~._ � 't+Onn Da �. E' w f = rc.r a. � •1r51ieer rm•-nr¢ief __ I. _:•. i ,y k'v: 'I r-- , "`� h — � -+•-ram, � - T saumwesi 35rxiri5lrecs. _ -. ��v+':356sn 5iicci 47 �'. $ h}prlliedil• � �a �� C ry, �S 1t1 ., :; s � ( _ { —� , Y- D ♦ ! 0 ll ��A`� 72rth Solution& NORTH Reference: King County, Washington �r Open StreetMap.org Vicinity Map Twin Trails Federal Way, Washington NOTE: This plate may contain areas of color. ESNW cannot be Drwn. MRS Date 06/28/2022 Proj. No. 7961.02 responsible for any subsequent misinterpretation of the information resulting from black & white reproductions of this plate. Checked SSR Date June 2022 Plate 1 m Q Earth Solutions NWLLC SOIL CLASSIFICATION CHART MAJOR DIVISIONS SYMBOLS TYPICAL DESCRIPTIONS GRAPH I LETTER GRAVEL AND CLEAN GRAVELS*loseb+ ' L� �' �� b GW WELL -GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES GRAVELLY SOILS (LITTLE OR NO FINES) °❑° 01 a o ❑Q0 1� Q o❑ GP POORLY -GRADED GRAVELS, GRAVEL- SAND MIXTURES, LITTLE OR NO FINES COARSE GRAINED SOILS MORE THAN 50% OF COARSE GRAVELS WITH FINES ° d a n �O o 0 GM SILTY GRAVELS, GRAVEL - SAND - SILT MIXTURES FRACTION RETAINED ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) G`+ CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES SAND AND CLEAN SANDS SW WELL -GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES MORE THAN 50% OF MATERIAL IS = SP POORLY -GRADED SANDS, GRAVELLY SAND, LITTLE OR NO FINES LARGER THAN NO. 200 SIEVE SIZE SANDY SOILS (LITTLE OR NO FINES) SANDS WITH FINES 3. SM SILTY SANDS, SAND - SILT MIXTURES MORE THAN 50% OF COARSE FRACTION PASSING ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) S`. CLAYEY SANDS, SAND - CLAY MIXTURES INORGANIC SILTS AND VERY FINE ML SANDS, ROCK FLOUR, SILTY OR CLAYEY FINE SANDS OR CLAYEY SILTS WITH SLIGHT PLASTICITY FINE GRAINED SOILS SILTS LIQUID LIMIT AND LESS THAN 50 CLAYS C�- INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS, LEAN CLAYS — — __ _ — — OL ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY MORE THAN 50% OF MATERIAL IS SMALLER THAN NO. 200 SIEVE MH INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS FINE SAND OR SILTY SOILS SIZE SILTS LIQUID LIMIT AND LAYS GREATER THAN 50 CH INORGANIC CLAYS OF HIGH PLASTICITY OH ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY, ORGANIC SILTS HIGHLY ORGANIC SOILS PT PEAT, HUMUS, SWAMP SOILS WITH HIGH ORGANIC CONTENTS DUAL SYMBOLS are used to indicate borderline soil classifications. The discussion in the text of this report is necessary for a proper understanding of the nature of the material presented in the attached logs. Earth Solutions NW, LLC At 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-101 PAGE 1 OF 1 PROJECT NUMBER ES-7961.02 PROJECT NAME Twin Trails DATE STARTED 5/17/22 COMPLETED 5/17/22 GROUND ELEVATION EXCAVATION CONTRACTOR NW Excavating_ LATITUDE 47.2955 LONGITUDE-122.35623 LOGGED BY CGH CHECKED BY SSR GROUND WATER LEVEL: NOTES Q AT TIME OF EXCAVATION SURFACE CONDITIONS Grass AFTER EXCAVATION w �x U _ wuj W co TESTS "i = Q O EL Z W 2 CD Q 0 7PSL' o.s SM MC = 18.0% MATERIAL DESCRIPTION Dark brown TOPSOIL, roots to 6' Brown silty SAND, loose to medium dense, moist -becomes gray, dense o Gray poorly graded GRAVEL, dense, moist a� D MC = 6.2% Fines = 3.6% GP ° [USDA Classification: extremely gravelly SAND] �D ° ❑v o D 9 5 -becomes very dense MC = 10.1 % Gray poorly graded SAND with gravel, very dense, moist Fines = 2.0% [USDA Classification: very gravelly coarse SAND] SP Test pit terminated at 13.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. LIMITATIONS: Ground elevation (if listed) is approximate; the test location was not surveyed. Coordinates are approximate and based on the WGS84 datum. Do not rely on this test log as a standalone document. Refer to the text of the geotechnical report for a complete understanding of subsurface conditions. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 F + Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-102 PAGE 1 OF 1 PROJECT NUMBER ES-7961.02 PROJECT NAME Twin Trails DATE STARTED _ 5/17/22 COMPLETED 5/17/22 GROUND ELEVATION EXCAVATION CONTRACTOR NW Excavating LATITUDE 47.2957 LOGGED BY CGH CHECKED BY SSR GROUND WATER LEVEL: NOTES SZ AT TIME OF EXCAVATION SURFACE CONDITIONS Grass AFTER EXCAVATION W (- �o U 2 a E Lu TESTS V a 0 MATERIAL DESCRIPTION LU Q z CD U) n i A0.5 SM MC = 10.2% 45 0 MC=7.1% Fines = 5.4% ° GP- o GM MC = 16.2% Sp 9.0 Dark brown TOPSOIL, roots to 7' Brown silty SAND, loose to medium dense, moist -becomes gray, dense LONGITUDE-122.35637 1 Gray poorly graded GRAVEL with silt and sand, dense, moist [USDA Classification: extremely gravelly loamy SAND] Gray poorly graded SAND with gravel, dense, moist MC = 12.2% [USDA Classification: very gravelly coarse SAND] sines = 2.8% T Test pit terminated at 13.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. LIMITATIONS: Ground elevation (if listed) is approximate; the test location was not surveyed. Coordinates are approximate and based on the WGS84 datum. Do not rely on this test log as a standalone document. Refer to the text of the geotechnical report for a complete understanding of subsurface conditions. N 0 m Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 ■ i Redmond, Washington 98052 Telephone: 4254494704 Fax: 425-4494711 TEST PIT NUMBER TP-103 PAGE 1- OF 1 PROJECT NUMBER ES-7961.02 PROJECT NAME Twin Trails DATE STARTED 5/17/22 COMPLETED 5/17/22 GROUND ELEVATION EXCAVATION CONTRACTOR NW Excavating LATITUDE .47.29563 LONGITUDE-122.35656 LOGGED BY CGH CHECKED BY SSR GROUND WATER LEVEL: NOTES SZ AT TIME OF EXCAVATION SURFACE CONDITIONS Grass AFTER EXCAVATION W _ �LU � U W = W g TESTS a o MATERIAL DESCRIPTION ❑ aZ SM MC = 15.5% 5 MC = 4.5% MC = 3.8% MC = 5.3% lnwc = 7 90i SP Dark brown TOPSOIL, roots 0.6 Brown silty SAND, loose to medium dense, moist -becomes gray, medium dense to dense Gray poorly graded SAND with gravel, dense, moist -trace cobbles [USDA Classification: very gravelly coarse SAND] Test pit terminated at 13.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. LIMITATIONS: Ground elevation (if listed) is approximate; the test location was not surveyed. Coordinates are approximate and based on the WGS84 datum. Do not rely on this test log as a standalone document. Refer to the text of the geotechnical report for a complete understanding of subsurface conditions. Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-104 PAGE 1 OF 1 PROJECT NUMBER ES-7961.02 PROJECT NAME Twin Trails DATE STARTED 5/17/22 COMPLETED 5/17/22 GROUND ELEVATION EXCAVATION CONTRACTOR NW Excavating LATITUDE 47.29539 LOGGED BY C_GH CHECKED BY SSR GROUND WATER LEVEL: NOTES AT TIME OF EXCAVATION SURFACE CONDITIONS Grass AFTER EXCAVATION W _ rw � U H W =U, LONGITUDE-122.35666 w j L TESTS q Q O MATERIAL DESCRIPTION Q Z j rn l3 10 MC = 18.5% SM MC = 13.6% MC=5.1% MC = 7.3% inac=971 Dark brown TOPSOIL, roots to 3.5' Brown silty SAND, loose to medium dense, moist -erratic small diameter boulders -becomes gray, dense • r.5 ❑ Gray poorly graded GRAVEL with sand, dense, moist OD QD GP OD ° ❑o n D O °& n D tan [USDA Classification: extremely gravelly coarse SAND] Test pi; terminated at 14.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. LIMITATIONS: Ground elevation (if listed) is approximate; the test location was not surveyed. Coordinates are approximate and based on the WGS84 datum. Do not rely on this test log as a standalone document. 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