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HomeMy WebLinkAboutRS_250522_Springbrook Terrace Pond Retrofit Geotech Final ReportFINAL GEOTECHNICAL REPORT SPRINGBROOK TERRACE POND RETROFIT PROJECT RENTON, WASHINGTON HWA Project No. 2024-062-21 Prepared for David Evans and Associates, Inc. May 2, 2025 Geotechnical Engineering Pavement Engineering Geoenvironmental Hydrogeology Inspection & Testing 21312 30th Dr. SE, STE. 110, Bothell, WA 98021 | 425.774.0106 | hwageo.com May 2, 2025 HWA Project No. 2024-062-21 David Evans and Associates, Inc. 724 Columbia St NW, Suite 320 Olympia, Washington 98501 Attention: Chad Booth, P.E. Subject:Final Geotechnical Engineering Report Springbrook Terrace Pond Retrofit Project Renton, Washington Dear Mr. Booth: As requested, HWA GeoSciences Inc. (HWA) has performed geotechnical engineering evaluations for the Springbrook Terrace Pond Retrofit project in Renton, Washington. This report presents the results of our geotechnical study and geotechnical engineering recommendations to support the evaluation of design alternatives. We appreciate the opportunity to provide geotechnical engineering services for this project. If you have any questions regarding this report or require additional information or services, please contact us at your convenience. Sincerely, HWA GEOSCIENCES INC. Steven R Wright, P.E. Ahmed Mahmoud, P.E, PMP Geotechnical Engineer, Vice President Geotechnical Engineer Springbrook Terrace Pond Retrofit i HWA GEOSCIENCES INC. TABLE OF CONTENTS Page 1.0 INTRODUCTION..........................................................................................................1 1.1 GENERAL.......................................................................................................1 1.2 PROJECT DESCRIPTION ..................................................................................1 2.0 FIELD INVESTIGATION AND LABORATORY TESTING ..................................................2 2.1 GENERAL.......................................................................................................2 2.2 GEOTECHNICAL BORINGS..............................................................................2 2.3 LABORATORY TESTING .................................................................................3 3.0 SITE CONDITIONS......................................................................................................3 3.1 SITE GEOLOGY..............................................................................................3 3.2 SITE SOIL CONDITIONS..................................................................................3 3.3 GROUNDWATER.............................................................................................4 4.0 CONCLUSIONS AND RECOMMENDATIONS..................................................................4 4.1 GENERAL.......................................................................................................4 4.2 SEISMIC DESIGN CONSIDERATIONS ...............................................................5 4.2.1 Liquefaction Susceptibility..........................................................6 4.3 ALTERNATIVE 1 - GRAVITY BLOCK WALLS ..................................................7 4.3.1 Alternative 1 Design Considerations...........................................7 4.3.2 Wall Design Parameters...............................................................7 4.3.3 Subgrade Preparation...................................................................8 4.3.4 Wall Backfill................................................................................9 4.3.5 Wall Drainage..............................................................................9 4.3.6 Global Stability............................................................................9 4.3.7 Retaining Wall Backfill and Compaction....................................10 4.4 ALTERNATIVE 2 – MODIFIED POND SIDE SLOPES..........................................10 4.4.1 Alternative 2 Design Considerations...........................................10 4.4.2 Divider Wall Within Pond...........................................................10 4.5 GENERAL EARTHWORK.................................................................................11 4.5.1 Temporary Excavations...............................................................11 4.5.2 Subgrade Preparation...................................................................12 4.5.3 Structural Fill...............................................................................12 4.5.4 Compaction..................................................................................13 4.5.5 Wet Weather Earthwork ..............................................................13 5.0 CONDITIONS AND LIMITATIONS .................................................................................14 6.0 REFERENCES..............................................................................................................16 Springbrook Terrace Pond Retrofit ii HWA GEOSCIENCES INC. LIST OF FIGURES (FOLLOWING TEXT) Figure 1 Site and Vicinity Map Figure 2 Site and Exploration Plan Figure 3 BH-1 Water Level Data APPENDIX A: LOGS OF HWA EXPLORATIONS Figure A-1 Legend of Terms and Symbols Used on Exploration Logs Figures A-2 and A-3 Boring Logs APPENDIX B: LABORATORY TEST RESULTS Figures B-1 Summary of Material Properties Figures B-2 to B-5 Particle-Size Analysis of Soils Springbrook Terrace Pond Retrofit 1 HWA GEOSCIENCES INC. FINAL GEOTECHNICAL ENGINEERING REPORT SPRINGBROOK TERRACE POND RETROFIT PROJECT RENTON, WASHINGTON 1.0 INTRODUCTION 1.1 GENERAL This report summarizes the results of the geotechnical engineering study completed by HWA GeoSciences Inc. (HWA) for the Springbrook Terrace Pond Retrofit Project in Renton, Washington. The approximate location of the project site is shown on the Site and Vicinity Map, Figure 1 and on the Site and Exploration Plan, Figure 2. Services provided for this project by HWA included a site reconnaissance, a subsurface investigation, a laboratory testing program, geotechnical engineering analyses, the development of geotechnical recommendations to support the evaluation of design alternatives, and preparation of this report. 1.2 PROJECT DESCRIPTION It is our understanding that the City of Renton plans to evaluate and design modifications to the existing Springbrook Terrace Pond. The purpose of the alternative design modifications being considered is to improve the water quality treatment performance of the existing pond. The following pond retrofit alternatives are currently being considered: Alternative 1: Lower the base of the existing Springbrook Terrace Pond by about 5 ft and use relatively short (up to about 5 ft in exposed height) gravity block walls along portions of the side slopes of the pond to allow the current top of pond slope to remain unchanged. Where needed, the proposed retaining walls will be located approximately midway between the top and bottom of the pond side slopes. Alternative 2: Lower the base of the existing pond by about 5 ft and regrade the pond side slopes (starting at the current top of pond slope) at an inclination of either 3H:1V or 2H:1V. This alternative does not include retaining walls within the pond. Alternative 1 (gravity block walls) was not selected for construction; however, the geotechnical recommendations for this alternative are retained in this report for documentation purposes. In addition to the grading modifications described above, the final design also includes construction of a divider wall within the pond. This wall will increase the stormwater flow path length, thereby enhancing sedimentation and improving overall treatment performance. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 2 HWA GEOSCIENCES INC. 2.0 FIELD INVESTIGATION AND LABORATORY TESTING 2.1 GENERAL HWA conducted a field investigation in October 2024 that included two (2) geotechnical borings. Approximate boring locations are shown on the Site and Exploration Plan, Figure 2. Laboratory testing was conducted on select soil samples obtained from the borings at HWA’s geotechnical and materials testing laboratory in Bothell, Washington. These tasks are summarized in the following sections. 2.2 GEOTECHNICAL BORINGS HWA advanced two (2) geotechnical borings, designated BH-1 and BH-2, at the site on October 4, 2024. The borings were advanced to depths ranging between approximately 39.5 and 41.5 ft below the existing ground surface (bgs) using a limited access, track-mounted drill rig and hollow stem auger drilling methods. A groundwater monitoring well was installed in boring BH-1 in accordance with Department of Ecology (DOE) requirements to monitor fluctuations in groundwater levels over time. Drilling was performed by Holocene Drilling, Inc., whose services were provided under subcontract to HWA. An HWA representative was on-site full-time during drilling to oversee boring operations and to observe and document subsurface soil and groundwater conditions. Soil samples were collected at 2½-foot intervals to a depth of 10 feet followed by 5-foot intervals to the termination depth of the borings. Split-spoon soil samples were obtained using Standard Penetration Test (SPT) sampling methods in general accordance with ASTM International (ASTM) D 1586. Soil samples were placed in sealed containers and transported to HWA’s geotechnical laboratory for further classification and testing. Borehole BH-2 was backfilled with bentonite chips upon completion of the exploration per DOE decommissioning requirements. A legend explaining the terms and symbols displayed on the logs as well as logs of the borings are presented in Appendix A, Figures A-1 through A-3. The attached boring logs include pertinent information related to the explorations, including subsurface stratigraphy, soil classification, sample depths, SPT blow counts, and select laboratory test results. The stratigraphic contacts shown on the logs represent the approximate boundaries between soil types; actual transitions may be more gradual. Groundwater levels indicated on the logs are valid only for the specific date and locations reported and, therefore, are not necessarily representative of the conditions at other locations and times. The latitude/longitude coordinates shown on the logs were obtained in the field using handheld GPS equipment and should be considered approximate. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 3 HWA GEOSCIENCES INC. 2.3 LABORATORY TESTING HWA conducted laboratory testing on select soil samples to characterize relevant physical and engineering properties of the soils encountered during the field investigation. Testing included visual classification and determination of natural moisture content and grain size distribution. The tests were conducted in general accordance with appropriate ASTM standards and are discussed in further detail in Appendix B. The test results are presented in Appendix B, and/or displayed on the exploration logs in Appendix A, as appropriate. 3.0 SITE CONDITIONS 3.1 SITE GEOLOGY The project site is located in the Puget Lowland, a region shaped by repeated glacial advances and retreats during the Quaternary period. This glaciation created a complex sequence of depositional and erosional features, including glacial till, outwash, and lacustrine deposits. Each glacial cycle contributed to the layering of sediments, with the most recent advance of the Puget lobe of the Cordilleran ice sheet depositing substantial materials during the Vashon Stade. According to the Geologic Map of the Renton Quadrangle, King County, Washington (D.R. Mullineaux, 1965), the site is underlain primarily by recessional outwash deposits, overlying advance outwash deposits. Recessional outwash consists of stratified sands and gravels deposited by meltwater streams during the retreat of the ice sheet. These materials are typically well-sorted and permeable, reflecting deposition in high-energy environments. The advance outwash beneath the recessional deposits comprises dense to very dense sands with varying amounts of gravel and occasional silt. These deposits formed during the initial advance of the ice sheet as meltwater streams deposited materials ahead of the glacier. The advance outwash was subsequently compacted by the overlying glacier, resulting in high density and reduced permeability. 3.2 SITE SOIL CONDITIONS Our interpretation of the subsurface conditions at the site is based on the results of the geotechnical borings conducted during our field investigation and review of available geologic and geotechnical information for the area in the project vicinity. The soil observed in our borings generally consisted of fill soils overlying native recessional and advanced outwash deposits. A brief description of the soil units observed at the site is provided below in order of deposition, beginning with the most recently deposited unit. Fill/Reworked Native: The uppermost unit, ranging from approximately 7.5 to 10 ft in thickness, consists of loose to dense silty sand with variable gravel content. Organic material, including roots, was noted throughout the unit. This fill likely represents a mix of reworked native soils and imported materials associated with prior construction activities at the site of Springbrook Terrace Pond. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 4 HWA GEOSCIENCES INC. Recessional Outwash: Underlying the fill, recessional outwash deposits were encountered extending to a depth of approximately 15 ft bgs. These deposits generally consist of loose to medium dense, silty sand with varying gravel content. Mottling observed within this unit suggests periodic saturation, potentially related to seasonal fluctuations in groundwater levels. Advance Outwash: Below the recessional outwash, advance outwash deposits were observed, extending to the borehole termination depths. This unit is characterized by dense to very dense, silty sand with gravel that is occasionally interbedded with lenses of sandy gravel. 3.3 GROUNDWATER At the time of drilling in early October 2024, groundwater was not observed in borings BH-1 and BH-2. However, rust mottling observed in samples of the recessional outwash soils in both borings suggests that groundwater levels may elevate seasonally to depths of 7.5 to 10 ft bgs. Alternatively, water that infiltrates through the bottom of the pond could be the cause of the observed mottling. We installed a 2-inch diameter PVC standpipe well in boring BH-1 to monitor fluctuations in groundwater level over time. The monitoring well was screened from approximately 15 to 35 ft bgs. A pressure transducer was installed to monitor groundwater fluctuations in the well and is set to record the hydrostatic pressure every hour for up to one year. Groundwater elevations from October 2024 through March 18, 2025, obtained from the pressure transducer are included in Figure 3. During this time period, the highest groundwater level has generally been observed to be within about 33 ft of the ground surface (about elevation 162 ft). 4.0 CONCLUSIONS AND RECOMMENDATIONS 4.1 GENERAL The soil conditions and site topography are such that design and construction of the proposed improvements is feasible provided the recommendations presented in this geotechnical report are followed. A general geotechnical summary of our recommendations is provided below, and additional details are provided in the following sections. The proposed pond retrofit will involve either lowering the base of the existing pond by about 5 ft and using gravity block walls along portions of the side slopes of the pond to allow the current top of pond slope to remain unchanged (Alternative 1) or lowering the base of the existing pond by about 5 ft and regrading the pond side slopes at an inclination of either 3H:1V or 2H:1V (Alternative 2). May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 5 HWA GEOSCIENCES INC. For Alternative 2, we evaluated the stability of regraded pond side slopes at inclinations of 3H:1V and 2H:1V. The results of the global stability analysis indicated that 2H:1V pond side slopes are not feasible; however, slopes constructed at 3H:1V are suitable for the proposed pond retrofit. Detailed recommendations for Alternatives 1 and 2 are provided in the following report sections. While both alternatives were considered, the gravity block wall option (Alternative 1) will not be implemented. The geotechnical recommendations for this option are retained in this report for documentation purposes. The final design also includes construction of a divider wall within the pond to increase the stormwater flow path length and enhance treatment efficiency. Geotechnical recommendations for the divider wall are provided in Section 4.5 of this report. 4.2 SEISMIC DESIGN CONSIDERATIONS Earthquake loading for the project site was developed in accordance with the General Procedure provided in Section 3.4 of the AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Edition, 2011 with 2022 Interim Revisions, and the Washington State Department of Transportation (WSDOT) amendments to the AASHTO Guide Specifications provided in the Bridge Design Manual (LRFD) (WSDOT, 2023). For seismic analysis, the Site Class is required to be established and is determined based on the average soil properties in the upper 100 ft below the ground surface. Based on our subsurface explorations and understanding of site geology, it is our opinion that the site is underlain by soils that are consistent with Site Class D. The design parameters for the design level event with a 7 percent probability of exceedance in 75 years (equal to a return period of 1,033 years) were obtained from the United States Geological Survey (USGS) Unified Hazard Tool website using the U.S. 2014 Dynamic Conterminous edition (v4.2.0), which provides the probabilistic seismic hazard parameters from the 2014 Updates to the National Hazard Maps (Peterson, et al., 2014). Site coefficients were developed following the WSDOT BDM that adopts the site coefficients provided in American Society of Civil Engineers 7-16 (ASCE, 2016). The recommended seismic coefficients for the design event at the project site are provided in Table 1. The spectral acceleration coefficient at 1-second period (SD1) is between 0.3 and 0.5; therefore, Seismic Design Category D, as given by AASHTO Table 3.5-1 (AASHTO, 2011), should be used. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 6 HWA GEOSCIENCES INC. Table 1. Seismic Coefficients Using AASHTO Guide Specifications Calculated by USGS Seismic Unified Hazard Tool Location: Lat. 47.429499; Long. -122.210551 Site CoefficientsSite Class Peak Horizontal Bedrock AccelerationPBA, (g) Spectral Bedrock Acceleration at 0.2 sec Ss, (g) Spectral Bedrock Acceleration at 1.0 sec S1, (g)Fpga Fa Fv Peak Horizontal Acceleration PGA (As), (g) D 0.4465 1.032 0.272 1.154 2.057 1.857 0.515 4.2.1 Liquefaction Susceptibility Liquefaction is a temporary loss of soil shear strength that occurs in response to ground motions during a strong earthquake. Loose, saturated cohesionless soils are most susceptible to earthquake-induced liquefaction; however, research has shown that certain silts and low- plasticity clays are also susceptible. Primary factors controlling the development of liquefaction include the intensity and duration of ground motions, the characteristics of subsurface soils, in-situ stress conditions, and the depth to groundwater. Liquefaction susceptibility of the soils at the project site was determined using the simplified procedure originally developed by Seed and Idriss (1971) and updated by Youd et al. (2001) and Idriss and Boulanger (2004, 2006). The procedure utilizes a semi-empirical approach that compares the cyclic resistance ratio (CRR) required to initiate liquefaction of the material to the cyclic shear stress ratio (CSR) induced by the design earthquake. The factor of safety relative to liquefaction is the ratio of the CRR to the CSR; where this ratio is computed to be less than one, the analysis would indicate that liquefaction is likely to occur during the design earthquake. The CRR is primarily dependent on soil density, with the current practice being based on the Standard Penetration Test (SPT) N-value corrected for hammer efficiency, fines content, and earthquake magnitude. CSR is generally determined by the formulation developed by Seed and Idriss (1971) and relates equivalent shear stress caused in the soil at any depth to the effective stress at that depth and the peak ground acceleration at the surface. The soils encountered at each of the two (2) geotechnical borings completed for this study were analyzed to evaluate liquefaction susceptibility using the methodology described above. The results of our analysis indicate that the soil profiles at BH-1 and BH-2 are not susceptible to liquefaction. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 7 HWA GEOSCIENCES INC. 4.3 ALTERNATIVE 1 - GRAVITY BLOCK WALLS Alternative 1 includes the use of gravity block walls to accommodate grade changes associated with the proposed pond retrofit. Recommendations for gravity block walls are provided below. 4.3.1 Alternative 1 Design Considerations The following key points should be reviewed when evaluating Alternative 1: To satisfy global wall stability requirements for gravity block walls constructed on the pond slopes, we recommend that walls be embedded at least 2 ft below existing grade but in no case should the embedment depth be less than the exposed height of the gravity block wall (see Section 4.3.2 for additional details). Temporary excavation slopes should be sloped no steeper than 1.5H:1V (see Section 4.5.1 for additional details). This inclination should be considered when evaluating if there is adequate room on the site to achieve the recommended minimum depth of wall embedment and accommodate both the width of the gravity block wall (which could be up to 5 ft for the taller portions of the walls) and the recommended wall drainage system behind the wall (see Section 4.3.5). On the west side of the pond, existing soil that currently prevents water from the pond flowing downslope to the west will need to be removed in order to construct a gravity block wall. Because gravity block walls are not watertight structures, water from the pond could leak through the wall and result in downslope erosion and slope instability. As a result, if Alternative 1 is selected as the preferred alternative, consideration should be given to placing a geosynthetic liner in front of (not under) the western gravity block wall. 4.3.2 Wall Design Parameters We assume that gravity block walls will consist of a proprietary wall system that the wall supplier will design for internal stability. The walls should be designed in accordance with the most current version of the AASHTO LRFD Bridge Design Manual and Section 6.13 of the WSDOT Standard Specifications (WSDOT, 2024). We recommend that the walls be designed using the parameters presented in Table 2. If the design of gravity walls is performed using the Load and Resistance Factor Design (LRFD) method, appropriate resistance factors should be used. For the Extreme Event I Limit State, the walls should be designed for a horizontal seismic acceleration coefficient (Kh) of one-half the peak ground acceleration, or 0.258g, and vertical seismic coefficient (Kv) of 0.0g (assuming the wall is free to move during a seismic event). Extreme Event I Limit State is defined in the AASHTO Standard Specifications as a May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 8 HWA GEOSCIENCES INC. safety check involving an extreme load event resulting from an earthquake in combination with the dead load and a fraction of the live loads. Table 2. Recommended Gravity Block Wall Design Parameters Soil Properties Wall Backfill Retained Soil Foundation Soil Unit Weight (pcf) 125 125 135 Friction Angle (deg) 34 34 38 Cohesion (psf) 10 10 0 Strength Limit State (EP+LL) Extreme Limit State (EP+EQ) Ultimate Bearing Resistance (ksf) 3.5 3.5 Horizontal Seismic Acceleration Coefficient (kh) (g) N/A 0.258 Notes: EP = Earth Pressure LL = Live Load EQ = Earthquake An unfactored coefficient of friction of 0.5 times the effective stress at the base of the wall can be used to estimate sliding resistance. To satisfy global wall stability requirements for gravity block walls constructed on the pond slopes, we recommend that walls be embedded at least 2 ft below existing grade but in no case should the embedment depth be less than the exposed height of the gravity block wall. 4.3.3 Subgrade Preparation Subgrade preparation is important to limit differential settlement of the walls and maintain global stability. Subgrade preparation should begin with the removal of all topsoil, deleterious material, and vegetation to expose competent native soils or adequately compacted fill. A smooth bucket should be used to limit disturbance, and the soils should be thoroughly compacted prior to placement of structural fill for wall bases. We recommend that an HWA geotechnical engineer, or their representative, evaluate the exposed subgrade to identify areas of loose/soft, pumping or otherwise unsuitable soils. If such soils are encountered, they should be over-excavated as directed by the geotechnical engineer or their representative and replaced with properly compacted material. We recommend the retaining walls be constructed on a 1-foot-thick leveling pad consisting of crushed surfacing base course (CSBC) compacted to at least 95 percent of the maximum dry density, as determined by ASTM D 1557 (Modified Proctor). The leveling pad should be graded to establish the proper wall batter. Additional subgrade preparation recommendations are provided in Section 4.5.2 of this report. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 9 HWA GEOSCIENCES INC. 4.3.4 Wall Backfill Wall backfill materials should consist of Gravel Backfill for Walls, as described in Section 9-03.12(2) of the WSDOT Standard Specifications (WSDOT, 2024) and should be compacted to at least 95 percent of the maximum dry density as determined by ASTM D 1557 (Modified Proctor). The wall backfill should be placed and compacted in layers as each row of blocks is placed. The contractor should consider the weight of construction equipment operating within the fill zone behind the wall. For compaction, materials within about 3 ft of the wall face should be compacted with lighter equipment to limit the loading on the back of the wall. 4.3.5 Wall Drainage Drainage should be provided behind all walls and should consist of a 4- to 6-inch diameter, perforated, rigid, plastic pipe, bedded and backfilled with Gravel Backfill for Drains, as specified in Section 9-03.12(4) of the WSDOT Standard Specifications (WSDOT, 2024). The drain rock should surround the drainpipe by at least 12 inches. The pipes should slope to drain to a suitable outlet. 4.3.6 Global Stability HWA performed global stability analyses of the proposed gravity block walls using limit equilibrium methods and the software SLIDE 8.032 (Rocscience, 2020). Limit equilibrium methods consider force (or moment) equilibrium along potential failure surfaces. The analysis computes results in terms of a factor of safety, which is computed as the ratio of the summation of the resisting forces to the summation of the driving forces. Global stability for gravity block walls was evaluated for static and pseudo-static conditions. For static conditions, the WSDOT Geotechnical Design Manual (WSDOT, 2022) requires a minimum factor of safety of 1.3 for walls not directly supporting structures to be considered sufficiently stable. Our analyses yield a factor of safety of 1.3 or greater for walls embedded as recommended herein, indicating that a gravity wall that is properly designed for internal stability will be sufficiently stable to support static loading. Seismic global stability was performed to evaluate pseudo-static (where applicable) conditions for the proposed gravity walls. For seismic conditions, the WSDOT Geotechnical Design Manual (WSDOT, 2024) requires a minimum factor of safety of 1.1 for a wall to be considered stable. The results of our analysis yield a factor of safety of 1.1 for gravity walls embedded as recommended herein, which indicates sufficient stability during a design level earthquake. In addition, our analysis demonstrates that the proposed retaining walls will provide a slight increase in the overall stability of the existing pond slopes for both static and seismic cases. As a result, we conclude that the proposed retaining walls would not have an adverse impact on nearby private property. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 10 HWA GEOSCIENCES INC. 4.3.7 Retaining Wall Backfill and Compaction Backfill materials for retaining walls should meet the specifications for Gravel Borrow in Section 9-03.14(1) of the Standard Specifications (WSDOT, 2024) or an approved alternative. Backfill should be compacted to no less than 95 percent of the Maximum Dry Density as determined by ASTM D 1557 (Modified Proctor). Achieving proper density of compacted fills depends on the size and type of compaction equipment, the number of passes, lift thickness, subgrade conditions, and soil moisture-density properties. We recommend a maximum lift thickness of 12 inches in a loose condition to facilitate compaction. In areas where limited space and/or loading considerations restrict the use of heavy equipment, smaller equipment can be used, but the soil must be placed in sufficiently thin lifts to achieve the required density. 4.4 ALTERNATIVE 2 – MODIFIED POND SIDE SLOPES Alternative 2 includes re-grading the pond slopes (starting at the current top of pond slope) at an inclination of either 3H:1V or 2H:1V. This alternative does not include retaining walls within the pond, but it does include construction of a divider wall within the pond. 4.4.1 Alternative 2 Design Considerations The following key points should be reviewed when evaluating Alternative 2: We evaluated the stability of the proposed 3H:1V and 2H:1V pond slopes under both static and seismic cases. Based on the results of this evaluation, we do not recommend re-grading the slopes at an inclination of 2H:1V. Instead, we recommend that permanent pond and adjacent slopes be constructed no steeper than 3H:1V. Furthermore, for pond slopes inclined at 3H:1V or flatter, we anticipate that adequate factors of safety against global failure will be maintained under both static and seismic cases. If the re-grading of pond slopes under Alternative 2 involves the need to place fill to establish all or a portion of the pond side slopes, measures should be taken to prevent surficial instability and/or erosion of the slopes. This could be accomplished by conscientious compaction of the fill material in level lifts, benched cuts into the slope face from undisturbed existing soil, maintaining adequate drainage, and planting the re-graded slope face with vegetation as soon as possible after construction. To achieve the specified compaction at the slope face when placing fill to establish the pond side slopes, it may be necessary to overbuild the slopes several feet, and then trim back to finish grade. 4.4.2 Divider Wall Within Pond A divider wall is planned within the pond to enhance stormwater treatment performance. The wall will act as a physical partition that increases the flow path length of the influent stormwater, May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 11 HWA GEOSCIENCES INC. thereby improving overall treatment efficiency. Recommended geotechnical parameters for the structural design of the divider wall are presented below in Table 3. Table 3. Recommended Divider Wall Design Parameters Parameter Recommended Value Saturated Unit Weight 120 pcf Effective Unit Weight (Submerged)57.6 pcf Soil Internal Friction Angle (φ)32° Ultimate Bearing Capacity 4 ksf Minimum Embedment Depth 18 inches Horizontal Seismic Acceleration Coefficient 0.258g Ultimate Equivalent Fluid Pressure (Passive Resistance) 187 psf/ft Ultimate Friction Coefficient – Soil to Precast Concrete¹ 0.40 Ultimate Friction Coefficient – Soil to Cast-in-Place Concrete¹ 0.50 ¹ Assumes concrete is underlain by at least 6 inches of crushed surfacing base course. The structural engineer should evaluate the stability of the wall under the critical loading condition in which water is present on one side and dry conditions exist on the other. The actual embedment depth required to maintain equilibrium may exceed the minimum value noted above, depending on final design loads. While WSDOT permits waiving seismic design for non-essential, non-life-safety structures, AASHTO recommends including seismic loading for retaining-type structures unless the seismic hazard is negligible. We defer the final seismic design decision to the City and the structural engineer of record. 4.5 GENERAL EARTHWORK The following sections of this report provide general earthwork recommendations. 4.5.1 Temporary Excavations Maintenance of safe working conditions, including temporary excavation stability, is the responsibility of the contractor. In accordance with Part N of Washington Administrative Code (WAC) 296-155, latest revisions, all temporary cuts more than 4 ft in height must be either sloped or shored. May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 12 HWA GEOSCIENCES INC. Based on the WAC guidelines and the results of our geotechnical explorations, the on-site existing fill/disturbed native and native soils classify as Type C soils. These materials should be sloped no steeper than 1.5H:1V. It is possible that seepage could be encountered at shallow depths particularly during periods of wet weather. The recommended maximum slope presented above is applicable to temporary excavations above the water table only; flatter side slopes would be required for excavations where groundwater seepage is encountered. The contractor should monitor the stability of temporary cut slopes and adjust the construction schedule and slope inclination accordingly. The contractor should be responsible for control of groundwater and surface water and should employ sloping, slope protection, ditching, sumps, dewatering, and other measures, as necessary, to prevent sloughing of soils. 4.5.2 Subgrade Preparation Proper subgrade preparation is critical to the performance of new walls and new fill placed to modify the geometry of the pond side slopes. Based on the planned elevations of the proposed improvements, we anticipate that subgrade soils will consist of existing structural fill soils comprised primarily of sand and gravel with variable amounts of silt. In general, these soils are considered suitable subgrade material. Any organic, soft, disturbed, or otherwise unsuitable subgrade material should be removed from the footprint of all walls and new fills. A geotechnical engineer or their representative should inspect subgrade conditions below the footprint of all walls and new fills. Suitability of subgrade soils underlying retaining walls and new fill material is typically determined by visual inspection and T-probe assessment. For project improvements that occupy large areas, subgrade inspection is typically accomplished by performing a proof roll over the footprint of the improvements using heavy construction equipment, such as a fully loaded dump truck or large roller. Areas of limited access can be evaluated with a steel T-probe. If unsuitable subgrade soils are present, the soils should be removed and replaced with structural fill as directed on-site by a geotechnical engineer. Depending on nature of the subgrade materials, HWA may recommend scarifying and recompacting the subgrade in lieu of removal and replacement. The depth and extent of subgrade repair will be directed by the on-site geotechnical engineer. 4.5.3 Structural Fill All materials used as backfill for retaining walls and to construct new pond slopes are to be considered structural fill. The on-site soils are highly variable in composition and will have varying degrees of moisture sensitivity. The existing fill soils may potentially be reused as structural fill provided the material is granular, free-draining, and devoid of organic matter or other deleterious materials. Structural fill should have a maximum particle size of 4 inches in any dimension and should contain less than 10 percent fines (portion passing the U.S. Standard No. 200 sieve) by weight. If any on-site soil is proposed for reuse as structural fill, a May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 13 HWA GEOSCIENCES INC. representative sample should be provided to HWA so that appropriate laboratory testing can be performed to determine the suitability of the material. Imported structural fill should meet the criteria provided in the Standard Specifications for Road, Bridge, and Municipal Construction (WSDOT, 2024) for one of the following materials: Gravel Borrow, Section 9-03.14(1) Crushed Surfacing Base Course (CSBC), Section 9-03.9(3) Crushed Surfacing Top Course (CSTC), Section 9-03.9(3) Aggregate for Gravel Base, Section 9-03.10 4.5.4 Compaction Structural fill should be moisture conditioned and compacted to the requirements specified in Section 2-03.3(14), Method C, of the WSDOT Standard Specifications (WSDOT, 2023), except that maximum dry densities should be obtained using ASTM D 1557 (Modified Proctor). All fills should be placed in lifts and compacted to at least 95 percent of its maximum dry density, as determined using test method ASTM D 1557 (Modified Proctor). In general, the thickness of loose lifts should not exceed 12 inches for heavy duty compactors and 4 inches for hand-operated equipment (such as jumping jacks and small plate compactors). The procedure to achieve proper density of compacted fill depends on the size and type of compaction equipment, the number of passes, thickness of the layer being compacted, and soil moisture-density properties. Achievement of proper density of a compacted fill depends on the size and type of compaction equipment, the number of passes, thickness of the layer being compacted, and soil moisture- density properties. In areas where limited space restricts the use of heavy equipment, smaller equipment can be used, but the soil must be placed in thin enough layers to achieve the required compaction. 4.5.5 Wet Weather Earthwork During periods of wet weather, even the most permeable soil can become difficult to work and compact. We anticipate considerable variability in the fines content of the in-situ soils. Soils with higher fines contents will be difficult to work and compact when wet. If fill is to be placed or earthwork is to be performed in wet weather or under wet conditions, the following recommendations apply: Earthwork should be performed in small areas to minimize exposure to wet weather. Excavation of unsuitable and/or softened soil should be followed promptly by placement and compaction of clean structural fill. The size and type of construction May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 14 HWA GEOSCIENCES INC. equipment used may need to be limited to prevent soil disturbance. Under some circumstances, it may be necessary to excavate soils with a backhoe to minimize subgrade disturbance caused by equipment traffic. Material used as excavation backfill in wet weather should consist of clean granular soil with less than 5 percent passing the U.S. Standard No. 200 sieve, based on wet sieving the fraction passing the ¾-inch sieve. The fines should be non-plastic. It should be noted this is an additional restriction on the structural fill materials specified herein. The ground surface within the construction area should be graded to promote surface water run-off and to prevent ponding. Within the construction area, the ground surface should be sealed on completion of each shift by a smooth drum vibratory roller, or equivalent, and under no circumstances should soil be left uncompacted and exposed to moisture infiltration. Excavation and placement of backfill materials should be monitored by a geotechnical engineer experienced in wet weather earthwork to determine that the work is being accomplished in accordance with the project specifications and the recommendations contained herein. 5.0 CONDITIONS AND LIMITATIONS We have prepared this geotechnical report solely for the City of Renton and David Evans and Associates, Inc. to support the design and construction of the proposed Springbrook Terrace Pond Retrofit project in Renton, Washington. The conclusions and interpretations presented in this report should not be construed as our warranty of subsurface conditions at the site. Experience has shown that soil and groundwater conditions can vary significantly over small distances and with time. Inconsistent conditions can occur between explorations that may not be detected by a geotechnical study of this scope and nature. If, during future site operations, subsurface conditions are encountered which vary appreciably from those described herein, HWA should be notified for review of the recommendations of this report, and revision of such if necessary. Within the limitations of scope, schedule and budget, HWA provided these services in accordance with generally accepted professional principles and practices in the fields of geotechnical engineering and engineering geology in the area at the time the report was prepared. No warranty, express or implied, is made. HWA does not practice or consult in the field of safety engineering. We do not direct the contractor’s operations and cannot be responsible for the safety of personnel other than our own on the site. As such, the safety of others is the responsibility of the contractor. The contractor May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 16 HWA GEOSCIENCES INC. 6.0 REFERENCES American Association of State Highway and Transportation Officials, 2020, LRFD Bridge Design Specifications, 9th Edition, Washington D.C. American Society of Civil Engineers (ASCE) 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. D.R. Mullineaux, 1965. Geologic Map of the Renton Quadrangle, King County, Washington, Department of the Interior United States Geological Survey, Washington Geologic Quadrangle Map GQ-405, Scale 1:24,000. Idriss, I.M, and Boulanger, RW, 2004, Semi-Empirical Procedures for Evaluating Liquefaction Potential During Earthquakes, presented at the Joint 11th ISCDEE & 3rd ICEGE, January 2004. Idriss, I.M., and Boulanger, R.W., 2006, “Semi-empirical procedures for evaluating liquefaction potential during earthquakes”, Soil Dynamics and Earthquake Engineering, 11th International Conference on Soil Dynamics and Earthquake Engineering (ICSDEE): Part II, Volume 26, Issues 2–4, February–April 2006, Pages 115–130. Petersen, M.D., Moschetti, M.P., Powers, P.M., Mueller, C.S., Haller, K.M., Frankel, A.D., Zeng, Yuehua, Rezaeian, Sanaz, Harmsen, S.C., Boyd, O.S., Field, Ned, Chen, Rui, Rukstales, K.S., Luco, Nico, Wheeler, R.L., Williams, R.A., and Olsen, A.H., 2014, Documentation for the 2014 update of the United States national seismic hazard maps, U.S. Geological Survey Open-File Report 2014–1091, 243 p. Seed, H.B. and Idriss, I.M., 1971, Simplified Procedure for Evaluating Soil Liquefaction Potential. Journal of Soil Mechanics Foundation Division, ASCE, Vol. 97, No. SM9, pp. 1249-1273. WSDOT, 2024, Geotechnical Design Manual, Washington State Department of Transportation. WSDOT, 2024, Bridge Design Manual (LRFD), Washington State Department of Transportation. WSDOT, 2024, Standard Specifications for Road, Bridge, and Municipal Construction, Washington State Department of Transportation. Youd, T.L., et al., 2001, Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of May 2, 2025 HWA Project No. 2024-062-21 Springbrook Terrace Pond Retrofit 17 HWA GEOSCIENCES INC. Soils, Journal of Geotechnical and Geoenvironmental Engineering, Geo-Institute of the American Society of Civil Engineers (ASCE), Vol. 127, No. 10, October 2001. © 2024 Microsoft Corporation © 2024 Maxar ©CNES (2024) Distribution Airbus DS © 2024 TomTom © 2024 Microsoft Corporation © 2024 TomTom CF 2024-062-21 SITE AND VICINITY MAP SPRINGBROOK TERRACE POND RETROFIT RENTON, WASHINGTON 0 200'400'600'800' SCALE: 1" = 400' VICINITY MAP SITE MAP 0 1000'2000'3000'4000' SCALE: 1" = 2000' SITE SC 1 DRAWN BY: PROJECT # C:\USERS\CFRY\DESKTOP\2024-062-21 SPRINGBROOK TERRACE POND RETROFIT\2024-062-21 SPRINGBROOK TERRACE POND RETROFIT.DWG <1> Plotted: 10/23/2024 7:30 AM CHECK BY: FIGURE NO.: DBE/MWBE © 2024 Microsoft Corporation © 2024 Maxar ©CNES (2024) Distribution Airbus DS SITE AND EXPLORATION PLAN 2024-062-21 SPRINGBROOK TERRACE POND RETROFIT RENTON, WASHINGTON BASE MAP PROVIDED BY: BING 0 10 20 30 40 SCALE: 1" = 20' SPRINGBROOK TERRACE Scale: 1" = 20'-0"EXPLORATION LEGEND BH-1 BOREHOLE DESIGNATION AND APPROXIMATE LOCATION (HWA, 2024) SC/JLG CF FIGURE NO.: PROJECT NO.: DRAWN BY: CHECK BY: C:\USERS\CFRY\DESKTOP\2024-062-21 SPRINGBROOK TERRACE POND RETROFIT\2024-062-21 SPRINGBROOK TERRACE POND RETROFIT.DWG <2> Plotted: 10/23/2024 7:30 AM 2 DBE/MWBE AERIAL IMAGERY REFERENCE IS APPROXIMATE AND MAY APPEAR OFFSET FROM SURVEYED DATA AND BASEMAPS. BH-1 BH-2 150 152 154 156 158 160 162 164 166 168 170 Gr o u n d W a t e r E l e v a t i o n ( f t ) Date and Time BH-1 Ground Water Elevation BH-1 WATER LEVEL DATA 2024-062-21 FIGURE NO. PROJECT NO.SPRINGBROOK TERRACE POND RETROFIT PROJECT Renton, Washington 3 APPENDIX A LOGS OF HWA EXPLORATIONS A-12024-062-21 Springbrook Terrace Pond Retrofit Renton, Washington SYMBOLS USED ON EXPLORATION LOGS LEGEND OF TERMS AND Clean Gravel (little or no fines) More than 50% of Coarse Fraction Retained on No. 4 Sieve Gravel with SM SC ML MH CH OH RELATIVE DENSITY OR CONSISTENCY VERSUS SPT N-VALUE Very Loose Loose Medium Dense Very Dense Dense N (blows/ft) 0 to 4 4 to 10 10 to 30 30 to 50 over 50 Approximate Relative Density(%) 0 - 15 15 - 35 35 - 65 65 - 85 85 - 100 COHESIVE SOILS Consistency Very Soft Soft Medium Stiff Stiff Very Stiff Hard N (blows/ft) 0 to 2 2 to 4 4 to 8 8 to 15 15 to 30 over 30 Approximate Undrained Shear Strength (psf) <250 250 - No. 4 Sieve Sand with Fines (appreciable amount of fines) amount of fines) More than 50% Retained on No. 200 Sieve Size Sand and Sandy Soils Clean Sand (little or no fines) 50% or More of Coarse Fraction Passing Fine Grained Soils Silt and Clay Liquid Limit Less than 50% 50% or More Passing No. 200 Sieve Size Silt and Clay Liquid Limit 50% or More 500 500 - 1000 1000 - 2000 2000 - 4000 >4000 DensityDensity USCS SOIL CLASSIFICATION SYSTEM Coarse Grained Soils Gravel and Gravelly Soils Highly Organic Soils GROUP DESCRIPTIONS Well-graded GRAVEL Poorly-graded GRAVEL Silty GRAVEL Clayey GRAVEL Well-graded SAND Poorly-graded SAND Silty SAND Clayey SAND SILT Lean CLAY Organic SILT/Organic CLAY Elastic SILT Fat CLAY Organic SILT/Organic CLAY PEAT MAJOR DIVISIONS GW SP CL OL PT GP GM GC SW COHESIONLESS SOILS Fines (appreciable LEGEND 2024-062.GPJ 10/8/24 PROJECT NO.:FIGURE: Coarse sand Medium sand SIZE RANGE Larger than 12 in Smaller than No. 200 (0.074mm) Gravel 3 in to 12 in 3 in to No 4 (4.5mm) No. 4 (4.5 mm) to No. 200 (0.074 mm) COMPONENT DRY Absence of moisture, dusty, dry to the touch. MOIST COMPONENT DEFINITIONS time of drilling) Groundwater Level (measured in well or open hole after water level stabilized) Groundwater Level (measured at TEST SYMBOLS GROUNDWATER SYMBOLS AL Atterberg Limits: California Bearing Ratio CN Consolidation DD OC Organic Content pH pH of Soils 12 - 30% Clayey, Silty, Sandy, Gravelly 3 in to 3/4 in 3/4 in to No 4 (4.5mm) No. 4 (4.5 mm) to No. 10 (2.0 mm) No. 10 (2.0 mm) to No. 40 (0.42 mm) No. 40 (0.42 mm) to No. 200 (0.074 mm) NOTES: Soil classifications presented on exploration logs are based on visual and laboratory observation. Density/consistency, color, modifier (if any) GROUP NAME, additions to group name (if any), moisture content. Proportion, gradation, and angularity of constituents, additional comments. (GEOLOGIC INTERPRETATION) Please refer to the discussion in the report text as well as the exploration logs for a more complete description of subsurface conditions. Soil descriptions are presented in the following general order: < 5% Damp but no visible water. WET Visible free water, usually soil is below water table. Boulders Cobbles Coarse gravel Fine gravel Sand MOISTURE CONTENT COMPONENT PROPORTIONS Fine sand Silt and Clay 5 - 12% PROPORTION RANGE DESCRIPTIVE TERMS Clean Slightly (Clayey, Silty, Sandy) 30 - 50% Components are arranged in order of increasing quantities. Very (Clayey, Silty, Sandy, Gravelly) PID PP CBR DS Direct Shear GS Grain Size Distribution K Permeability Moisture/Density Relationship (Proctor) Resilient Modulus Photoionization Device Reading Res. Resistivity SG Percent Fines%F MD MR Specific Gravity CD Consolidated Drained Triaxial Torvane (Approx. Shear Strength, tsf) Dry Density (pcf) CU Consolidated Undrained Triaxial TV UU Unconsolidated Undrained Triaxial UC Unconfined Compression SAMPLE TYPE SYMBOLS Non-standard Penetration Test(3.0" OD Split Spoon with Brass Rings) (140 lb. hammer with 30 in. drop) Shelby Tube Small Bag Sample Large Bag (Bulk) Sample Core Run 2.0" OD Split Spoon (SPT) PL = Plastic Limit, LL = Liquid Limit Pocket Penetrometer (Approx. Comp. Strength, tsf) 3-1/4" OD Split Spoon S-1 S-2 S-3 S-4 S-5A S-5B S-6 S-7 S-8 S-9 GS GS GS GS %F GS %F SM SM GM SM 7-8-6 6-11-8 4-3-5 6-7-11 17-50/4" 12-36-50/5" 18-50/6" 13-45-47 50/5" Medium dense, light brown, slightly gravelly, silty, fine to coarse SAND, moist. Wood/roots observed. (FILL) Becomes brown-gray and very silty. Wood/roots observed. Loose, brown, gravelly, silty, fine to medium SAND, moist. Roots and trace charcoal observed. Medium dense, brown, slightly gravelly, silty, fine to medium SAND, moist. Roots and trace rust mottling observed. (RECESSIONAL OUTWASH) Becomes very dense. Wood/roots and trace gravel observed. Very dense, brown with rust-mottling, silty, sandy GRAVEL, moist. Low sample recovery, broken gravel. (ADVANCE OUTWASH) Very dense, brown-gray, slightly gravelly, very silty, fine SAND, moist. Wood chips observed at top of sample. 1 inch of mottling observed at 20.3 ft. Becomes dark brown-gray. Roots observed. Rust-mottling and roots observed. Becomes moist to wet. 0 20 40 60 80 100 Water Content (%) Plastic Limit (140 lb. weight, 30" drop) Blows per foot (b l o w s / 6 i n c h e s ) US C S S O I L C L A S S DESCRIPTION SA M P L E T Y P E PE N . R E S I S T A N C E OT H E R T E S T S PI E Z O M E T E R Standard Penetration Test SY M B O L SC H E M A T I C 0 10 20 30 40 50 Liquid Limit BORING: BH-1 Water Content (%) Natural Water ContentNOTE: This log of subsurface conditions applies only at the specified location and on the date indicatedand therefore may not necessarily be indicative of other times and/or locations. PAGE: 1 of 2 SA M P L E N U M B E R A-2FIGURE:PROJECT NO.:2024-062-21 Renton, Washington Springbrook Terrace Pond Retrofit PZO-DSM 2024-062.GPJ 12/1/24Library: Q:\LIBRARY\LIBRARY - BOTHELL BACKUP BACKUP.GLB DE P T H (f e e t ) 0 5 10 15 20 25 30 35 40 190 185 180 175 170 165 160 155 EL E V A T I O N (f e e t ) DATE COMPLETED: 10/4/2024 DRILLING COMPANY: Holocene Drilling DRILLING METHOD: HSA w/ 4.24" ID, Diedrich D-50 Tracked Rig LOCATION: Lat: 47.429558 Long: -122.210406, Datum: WGS 84 DATE STARTED: 10/4/2024 SAMPLING METHOD: SPT, Autohammer LOGGED BY: S. CORTEZ >> >> SURFACE ELEVATION: 193.6 feet Borehole terminated at 39.5 feet below ground surface (bgs) due to broken sample tube. Bottom sample could not be recovered. Groundwater not observed during the exploration. 2-inch PVC standpipe piezometer installed in a 35 foot well with 20 feet of screen. DOE well tag: BQB 720. 0 20 40 60 80 100 Water Content (%) Plastic Limit (140 lb. weight, 30" drop) Blows per foot (b l o w s / 6 i n c h e s ) US C S S O I L C L A S S DESCRIPTION SA M P L E T Y P E PE N . R E S I S T A N C E OT H E R T E S T S PI E Z O M E T E R Standard Penetration Test SY M B O L SC H E M A T I C 0 10 20 30 40 50 Liquid Limit BORING: BH-1 Water Content (%) Natural Water ContentNOTE: This log of subsurface conditions applies only at the specified location and on the date indicatedand therefore may not necessarily be indicative of other times and/or locations. PAGE: 2 of 2 SA M P L E N U M B E R A-2FIGURE:PROJECT NO.:2024-062-21 Renton, Washington Springbrook Terrace Pond Retrofit PZO-DSM 2024-062.GPJ 12/1/24Library: Q:\LIBRARY\LIBRARY - BOTHELL BACKUP BACKUP.GLB DE P T H (f e e t ) 40 45 50 55 60 65 70 75 80 150 145 140 135 130 125 120 115 EL E V A T I O N (f e e t ) DATE COMPLETED: 10/4/2024 DRILLING COMPANY: Holocene Drilling DRILLING METHOD: HSA w/ 4.24" ID, Diedrich D-50 Tracked Rig LOCATION: Lat: 47.429558 Long: -122.210406, Datum: WGS 84 DATE STARTED: 10/4/2024 SAMPLING METHOD: SPT, Autohammer LOGGED BY: S. CORTEZ SURFACE ELEVATION: 193.6 feet GS GS GS %F S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9A S-9B Dense, brown, slightly gravelly, silty, fine to medium SAND, moist. Wood/roots observed. (FILL/REWORKED NATIVE) Becomes medium dense, dark gray, very gravelly. Minor mottling. Loose, brown-gray with mottling, slightly gravelly, silty, fine to medium SAND, moist. Roots observed. (RECESSIONAL OUTWASH) Becomes medium dense and gravelly. Very dense, brown-gray with mottling, silty, fine to coarse SAND, moist. Low sample recovery. (ADVANCE OUTWASH) Becomes dense, gravelly, and very silty. Becomes very dense and slightly gravelly. Blowcounts overstated due to gravel in sample tip. Becomes silty with minor mottling. Blowcounts overstated due to gravel in sample tip. Becomes dark brown-gray and without mottling. Roots observed. Very dense, brown-gray, slightly silty, gravelly, poorly graded SAND, moist. 11-19-14 3-4-6 1-3-3 2-8-11 10-50/6" 3-13-22 11-50/5" 9-28-50/6" 24-50/3" SM SM SM SP SM FIGURE:PROJECT NO.:2024-062-21 Renton, Washington Springbrook Terrace Pond Retrofit BORING-DSM 2024-062.GPJ 12/1/24Library: Q:\LIBRARY\LIBRARY - BOTHELL BACKUP BACKUP.GLB Natural Water Content US C S S O I L C L A S S Water Content (%) NOTE: This log of subsurface conditions applies only at the specified location and on the date indicated DESCRIPTION BH-2 PAGE: 1 of 2 (b l o w s / 6 i n c h e s ) GR O U N D W A T E R PE N . R E S I S T A N C E Liquid Limit SY M B O L 0 10 20 30 40 50 0 20 40 60 80 100 SA M P L E T Y P E SA M P L E N U M B E R OT H E R T E S T S Plastic Limit BORING: and therefore may not necessarily be indicative of other times and/or locations. (140 lb. weight, 30" drop) Blows per foot Standard Penetration Test A-3 DE P T H (f e e t ) 0 5 10 15 20 25 30 35 40 185 180 175 170 165 160 155 EL E V A T I O N (f e e t ) DATE COMPLETED: 10/4/2024 DRILLING COMPANY: Holocene Drilling DRILLING METHOD: HSA w/ 4.24" ID, Diedrich D-50 Tracked Rig LOCATION: Lat: 47.429503 Long: -122.210550, Datum: WGS 84 DATE STARTED: 10/4/2024 SAMPLING METHOD: SPT, Autohammer LOGGED BY: S. CORTEZ >> >> SURFACE ELEVATION: 189.9 feet S-10Very dense, grayish-brown, slightly gravelly, silty SAND, moist. Borehole terminated at 41.5 feet below ground surface (bgs). Groundwater not observed during the exploration. Boring abandoned with 3/8" bentonite chips. 26-44-50/5"SM FIGURE:PROJECT NO.:2024-062-21 Renton, Washington Springbrook Terrace Pond Retrofit BORING-DSM 2024-062.GPJ 12/1/24Library: Q:\LIBRARY\LIBRARY - BOTHELL BACKUP BACKUP.GLB Natural Water Content US C S S O I L C L A S S Water Content (%) NOTE: This log of subsurface conditions applies only at the specified location and on the date indicated DESCRIPTION BH-2 PAGE: 2 of 2 (b l o w s / 6 i n c h e s ) GR O U N D W A T E R PE N . R E S I S T A N C E Liquid Limit SY M B O L 0 10 20 30 40 50 0 20 40 60 80 100 SA M P L E T Y P E SA M P L E N U M B E R OT H E R T E S T S Plastic Limit BORING: and therefore may not necessarily be indicative of other times and/or locations. (140 lb. weight, 30" drop) Blows per foot Standard Penetration Test A-3 DE P T H (f e e t ) 40 45 50 55 60 65 70 75 80 145 140 135 130 125 120 115 EL E V A T I O N (f e e t ) DATE COMPLETED: 10/4/2024 DRILLING COMPANY: Holocene Drilling DRILLING METHOD: HSA w/ 4.24" ID, Diedrich D-50 Tracked Rig LOCATION: Lat: 47.429503 Long: -122.210550, Datum: WGS 84 DATE STARTED: 10/4/2024 SAMPLING METHOD: SPT, Autohammer LOGGED BY: S. CORTEZ SURFACE ELEVATION: 189.9 feet APPENDIX B LABORATORY TEST RESULTS Representative soil samples obtained from the explorations were placed in moisture sealed containers and transported to our Bothell, Washington laboratory for further examination and testing. Laboratory tests were conducted on selected soil samples to characterize relevant engineering and index properties of the site soils. The laboratory testing program was conducted in general accordance with the following test methods: MOISTURE CONTENT OF SOIL: Laboratory tests were conducted to determine the natural moisture content of selected soil samples, in general accordance with ASTM D 2216. Test results are indicated on Figure B-1, the Summary of Material Properties, and at the sampled intervals on the appropriate exploration logs in Appendix A. PARTICLE SIZE ANALYSIS OF SOILS: Selected samples were tested to determine the particle size distribution of material in general accordance with ASTM D 6913. The results are summarized on the attached Grain Size Distribution reports, Figures B-2 through B-5, which provide information regarding the classification of the sample. PERCENTAGE FINER THAN #200 SIEVE: The percentage of material finer than the #200 sieve was determined for select samples in general accordance with ASTM D 1140. The soil was oven dried and washed over a #200 sieve to determine the percentage of fines. The results are shown on that attached Grain Size Distribution reports, Figures B-2 through B-5. BH-1,S-1 2.5 4.0 7.4 SM Yellowish-brown, silty SAND with organics BH-1,S-2 5.0 6.5 22.5 0.3 60.1 39.6 SM Dark brown, silty SAND with organics BH-1,S-3 7.5 9.0 12.6 13.5 64.0 22.5 SM Brown, silty SAND with organics BH-1,S-4 10.0 11.5 12.9 8.5 67.0 24.4 SM Brown, silty SAND BH-1,S-5A 15.0 16.5 16.1 SM Dark brown, silty SAND with organics BH-1,S-6 20.0 21.5 11.7 5.3 54.9 39.8 SM Olive-brown, silty SAND BH-1,S-7 25.0 26.5 18.5 33.9 SM Dark olive-brown, silty SAND with organics BH-1,S-8 30.0 31.5 12.9 5.1 49.2 45.7 SM Brown, silty SAND with organics BH-1,S-9 35.0 36.5 19.9 39.8 SM Grayish-brown, silty SAND BH-2,S-1 2.5 4.0 8.6 SM Yellowish-brown, silty SAND BH-2,S-2 5.0 6.5 11.7 30.6 49.3 20.1 SM Very dark grayish-brown, silty SAND with gravel BH-2,S-3 7.5 9.0 15.8 11.1 59.2 29.8 SM Dark yellowish-brown, silty SAND with organics BH-2,S-4 10.0 11.5 13.2 SM Brown, silty SAND with gravel BH-2,S-6 20.0 21.5 11.1 19.3 42.5 38.2 SM Brown, silty SAND with gravel BH-2,S-7 25.0 26.5 13.1 37.3 SM Brown, silty SAND BH-2,S-8 30.0 31.5 11.9 SM Brown, silty SAND BH-2,S-9A 35.0 35.4 13.2 SM Brown, silty SAND with organics BH-2,S-9B 35.4 36.5 6.2 SP-SM Yellowish-brown, poorly graded SAND with silt and gravel BH-2,S-10 40.0 41.5 11.6 SM Yellowish-brown, silty SAND (f e e t ) SUMMARY OF LIMITS (%) ATTERBERG BO T T O M D E P T H CO N T E N T ( % ) LL PL PI(f e e t ) Notes: TO P D E P T H MO I S T U R E CO N T E N T ( % ) OR G A N I C AS T M S O I L % F I N E S SP E C I F I C G R A V I T Y EX P L O R A T I O N DE S I G N A T I O N 1. This table summarizes information presented elsewhere in the report and should be used in conjunction with the report test, other graphs and tables, and the exploration logs. 2. The soil classifications in this table are based on ASTM D2487 and D2488 as applicable. MATERIAL PROPERTIES B-1 PAGE: 1 of 1 % S A N D % G R A V E L CL A S S I F I C A T I O N SAMPLE DESCRIPTION 2024-062-21PROJECT NO.: INDEX MATSUM 3 (LONG DESCRIPTIONS) 2024-062.GPJ 11/5/24 FIGURE: Springbrook Terrace Pond Retrofit Renton, Washington 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.1110 B-2 Coarse #60#40#20 Fine Coarse SYMBOL Gravel% 3"1-1/2" PE R C E N T F I N E R B Y W E I G H T #4 #200 0.3 13.5 8.5 Sand% (SM) Dark brown, silty SAND with organics (SM) Brown, silty SAND with organics (SM) Brown, silty SAND Fines% 0.00050.005 GRAIN SIZE IN MILLIMETERS 50 SAMPLE S-2 S-3 S-4 5.0 - 6.5 7.5 - 9.0 10.0 - 11.5 #10 60.1 64.0 67.0 30 CLASSIFICATION OF SOIL- ASTM D2487 Group Symbol and Name U.S. STANDARD SIEVE SIZES SAND CLAY BH-1 BH-1 BH-1 SILT 3/4" GRAVEL 0.05 5/8" 70 #100 0.5 23 13 13 50 Medium Fine 3/8" 5 PI 90 10 % MC LL PLDEPTH ( ft.) 39.6 22.5 24.4 PARTICLE-SIZE ANALYSIS OF SOILS METHODS ASTM D6913/D1140 2024-062-21PROJECT NO.: HWAGRSZ WITH D1140 2024-062.GPJ 11/5/24 FIGURE: Springbrook Terrace Pond Retrofit Renton, Washington 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.1110 B-3 Coarse #60#40#20 Fine Coarse SYMBOL Gravel% 3"1-1/2" PE R C E N T F I N E R B Y W E I G H T #4 #200 5.3 5.1 Sand% (SM) Olive-brown, silty SAND (SM) Dark olive-brown, silty SAND with organics (SM) Brown, silty SAND with organics Fines% 0.00050.005 GRAIN SIZE IN MILLIMETERS 50 SAMPLE S-6 S-7 S-8 20.0 - 21.5 25.0 - 26.5 30.0 - 31.5 #10 54.9 49.2 30 CLASSIFICATION OF SOIL- ASTM D2487 Group Symbol and Name U.S. STANDARD SIEVE SIZES SAND CLAY BH-1 BH-1 BH-1 SILT 3/4" GRAVEL 0.05 5/8" 70 #100 0.5 12 18 13 50 Medium Fine 3/8" 5 PI 90 10 % MC LL PLDEPTH ( ft.) 39.8 33.9 45.7 PARTICLE-SIZE ANALYSIS OF SOILS METHODS ASTM D6913/D1140 2024-062-21PROJECT NO.: HWAGRSZ WITH D1140 2024-062.GPJ 11/5/24 FIGURE: Springbrook Terrace Pond Retrofit Renton, Washington 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.1110 B-4 Coarse #60#40#20 Fine Coarse SYMBOL Gravel% 3"1-1/2" PE R C E N T F I N E R B Y W E I G H T #4 #200 30.6 11.1 Sand% (SM) Grayish-brown, silty SAND (SM) Very dark grayish-brown, silty SAND with gravel (SM) Dark yellowish-brown, silty SAND with organics Fines% 0.00050.005 GRAIN SIZE IN MILLIMETERS 50 SAMPLE S-9 S-2 S-3 35.0 - 36.5 5.0 - 6.5 7.5 - 9.0 #10 49.3 59.2 30 CLASSIFICATION OF SOIL- ASTM D2487 Group Symbol and Name U.S. STANDARD SIEVE SIZES SAND CLAY BH-1 BH-2 BH-2 SILT 3/4" GRAVEL 0.05 5/8" 70 #100 0.5 20 12 16 50 Medium Fine 3/8" 5 PI 90 10 % MC LL PLDEPTH ( ft.) 39.8 20.1 29.8 PARTICLE-SIZE ANALYSIS OF SOILS METHODS ASTM D6913/D1140 2024-062-21PROJECT NO.: HWAGRSZ WITH D1140 2024-062.GPJ 11/5/24 FIGURE: Springbrook Terrace Pond Retrofit Renton, Washington 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.1110 B-5 Coarse #60#40#20 Fine Coarse SYMBOL Gravel% 3"1-1/2" PE R C E N T F I N E R B Y W E I G H T #4 #200 19.3 Sand% (SM) Brown, silty SAND with gravel (SM) Brown, silty SAND Fines% 0.00050.005 GRAIN SIZE IN MILLIMETERS 50 SAMPLE S-6 S-7 20.0 - 21.5 25.0 - 26.5 #10 42.5 30 CLASSIFICATION OF SOIL- ASTM D2487 Group Symbol and Name U.S. STANDARD SIEVE SIZES SAND CLAY BH-2 BH-2 SILT 3/4" GRAVEL 0.05 5/8" 70 #100 0.5 11 13 50 Medium Fine 3/8" 5 PI 90 10 % MC LL PLDEPTH ( ft.) 38.2 37.3 PARTICLE-SIZE ANALYSIS OF SOILS METHODS ASTM D6913/D1140 2024-062-21PROJECT NO.: HWAGRSZ WITH D1140 2024-062.GPJ 11/5/24 FIGURE: Springbrook Terrace Pond Retrofit Renton, Washington