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REVIEWED PLN BLD2023-1134+Geotechnical_Letter+9.8.2023_3.08.11_PM+3772977E111 SHANNON 6WILSON May 8, 2023 Ms. Rachel Dotson Snohomish County Parks and Recreation 6705 Puget Park Drive Snohomish, WA 98296 RE: GEOTECHNICAL REPORT, PROPOSED RETAINING WALL ALONG ACCESS ROAD CUT SLOPE NEAR ENTRANCE GATE, MEADOWDALE BEACH PARK AND ESTUARY RESTORATION PROJECT, EDMONDS, WASHINGTON Dear Ms. Dotson: This letter presents the results of geotechnical analyses and recommendations for a proposed retaining wall along the access road near the entrance gate for the Meadowdale Beach Park and Estuary Restoration Project (Meadowdale Project) in Edmonds, Washington (Figure 1). The recommendations in this letter augment the final geotechnical report for the Meadowdale Project (Shannon & Wilson, 2018), as specifically applicable to the proposed retaining wall structure and location and do not supersede the recommendations in the final project geotechnical report that are applicable to other project elements. To the extent practical, this letter references recommendations in the final project geotechnical report and augments others as appropriate for the proposed structure. SITE AND PROJECT DESCRIPTION Meadowdale Beach Park is accessible via a gated access road that extends from the north terminus of 75th Place West in Edmonds, Washington, to the paved parking area within the park, in Snohomish County, Washington. The paved parking area and gated access road provide Americans with Disabilities Act (ADA) access to the park. Just inside the ADA gate, a section of the cut slope along the east side of the access road has exhibited indications of progressive slope instability. The subject portion of the slope is denuded of vegetation such that the slope soils are visible. We observed joints and tension cracks within the slope soils, blocks of intact soil formed by open joints and cracks, and clasts of intact soil that had detached from the upper slope and accumulated along the lower slope and within the ditch at the base of slope. These features indicate ongoing, progressive slope failure. The failure mode is block failure in which blocks and clasts of intact soil detach from the cut slope and ravel downslope into the ditch along the road shoulder. Blocks form along cracks and fissures that form due to weathering, tree root growth, freeze/thaw and wetting/drying cycles, or other causes. There are several mature coniferous trees at the top of the slope, one 400 North 34th Street, Suite 100 I PO Box 300303 I Seattle, Washington 98103-8636 1206-632-8020 www.shannonwilson.com 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 of which has been partially undermined by the slope failure. Continued soil raveling presents a risk of further undermining one or more of the trees, potentially causing them to fall onto the access road and slope below. This project serves to design a soldier pile wall that will retain free -draining gravel backfill between the wall and slope and prevent further slope failure. The cut slope along the east side of the ADA access road is irregularly shaped, containing multiple slope breaks, ranging from about 1 Horizontal to 1 Vertical (1H:1V) to near vertical, with multiple arcuate features indicating previous slope failure and/or irregular slope geometry resulting from cut slope construction. The cut slope is inclined at about 0.6H:1V over a vertical distance of about 24 feet from the drainage ditch invert to the slope break at the upper edge of the failure scarp. Above the slope break, the ground surface is inclined upward to the east at about 1H:1V and is densely vegetated with mature conifers, deciduous trees and brush. Adjacent to the slope failure scarp, the slope is densely vegetated with deciduous trees and brush. Surface water flowing from the slope collects in an asphalt -lined ditch along the east side of the ADA access road and flows to the south toward a storm drain inlet. The asphalt ditch lining extends about 12 to 16 inches up the slope from the ditch invert. SUBSURFACE EXPLORATIONS The final Meadowdale Project geotechnical report (Shannon & Wilson, 2018) included logs of eight borings including five borings located along the ADA access road. Boring MB-1 is located about 65 feet north of the slope failure site (Figure 1). We did not complete any additional subsurface explorations for this scope of services. Details regarding the exploration dates, methods, sampling, and laboratory testing are included in the project geotechnical report (Shannon & Wilson, 2018) and are not repeated herein. SUBSURFACE CONDITIONS Boring MB-1 encountered about 3 inches of asphalt pavement, overlying about 2 inches of crushed gravel base, overlying interbedded, dense to very dense, moist, poorly graded sand with silt, sandy silt, silt, and silty sand consistent with the Whidbey Formation, mapped in the approximate location of the access road (Minard, 1983). Groundwater was not encountered in boring MB-1. 21-1-22288-210 Page 2 of 8 21-1-22288-210-L2.docx/wp/tvv 111 SHANNON WALSO DESIGN ANALYSES Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 Geotechnical Failure Mode Interpretation Shannon & Wilson provided field measurements depicting proposed wall end points based on observed slope conditions and slope failure area extents. The wall end points were intended to provide wall length sufficient to support gravel backfill placement throughout the observed failure area and extending either direction from the failure area as necessary to provide confinement along the adjacent slope areas where similar failures could occur. Based on observed features, in our opinion, the adjacent slope areas exhibit indications that the potential for similar failure modes does not extend as far upslope as the existing failure area, such that the wall height and retained fill height may be reduced either direction from the current failure location. Topographic Survey Duane Hartman Associates (DHA), under subcontract to Shannon & Wilson, provided topographic site survey for the ADA access road, cut slope, and ground surface above the cut slope for use in retaining wall design. DHA completed their survey in September 2022, and provided a topographic surface for use in civil and structural design. Structural and Civil Design Hanson Professional Services, under subcontract to Shannon & Wilson, provided structural and civil design services for this retaining wall, prepared wall construction drawings, details, and special provisions for wall construction. Note that our scope does not include preparation of a complete Snohomish County construction contract package, as we assumed this wall will be constructed by Strider Construction as a change order to their existing construction contract for the Meadowdale project. Civil Design Based on observed slope conditions, Shannon & Wilson recommended wall end point locations and provided guidance in identifying the upslope wall backfill limits. Hanson provided a preliminary wall section drawing on May 31, 2023, showing both 10- and 15-foot-tall walls and both 1.5H:1V and 2H:1V wall backfill backslope angles. Based on their preliminary section, it appears the wall will need to be about 15 feet tall at its tallest point near the slope failure location, with 1.5H:1V wall backfill backslope to tie into the upper cut slope where progressive failure is located. Based on preliminary structural analysis results Hanson provided, structural wall capacity will likely be exceeded by lateral earth pressure 21-1-22288-210 Page 3 of 8 21-1-22288-210-L2.docx/wp/tvv 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 imparted on the wall by gravel backfill sloping as described. We provided engineering properties for foamed glass lightweight backfill material as a means to reduce lateral earth pressure. As of this report date, we understand the use of this lightweight material throughout the lower portion of the wall within the taller wall sections will likely be effective in reducing lateral earth pressure, such that pile spacing can be identical along the full wall length, and structural and civil design analyses are ongoing. ENGINEERING CONCLUSIONS AND RECOMMENDATIONS Seismic Design Parameters The International Building Code (IBC) has been updated since the project geotechnical report was issued (Shannon & Wilson, 2018) such that revised seismic design parameters are needed for the retaining wall design. Effective February 1, 2021, the Washington State Building Code Council adopted the IBC 2018 (with amendments) followed by the Snohomish County adoption of the same code (with amendments), which specifies the use of revised methods to establish seismic hazard and site class for seismic design. Computation of seismic forces using standard procedures of the IBC 2018 is based on soft rock spectral acceleration values and site soil response factors consistent with site class. Computation of forces that meet these criteria is based on seismological input and site soil response factors. The seismological inputs are the risk -targeted maximum considered earthquake (MCER) short -period spectral acceleration (Ss) and spectral acceleration at 1-second period (Si). The geometric mean maximum considered earthquake (MCEc) input is the geometric mean peak ground acceleration (PGA). The MCER ground motion parameters correspond to a target risk of 1% in 50 years of structural collapse (based on a generic structural fragility) and are derived from probabilistic ground motions with a return period of approximately 2,500 years; MCEc motions are the 2,500-year ground motions without adjustment for a target collapse risk. The MCER and MCEc ground motion values are based on U.S. Geological Survey data calculated in the 2014 National Seismic Hazard Model. The mapped MCER values for Ss and S1 and mapped MCEc value for PGA are provided in Table 1. The site soil response factors are based on determination of the site class. These factors were used to adjust soft rock ground motions to include soil response effects in the design ground motions. Using the subsurface soil information data collected for borings MB-1 through MB-5, we estimated the site class as Site Class C. Site Class C is characterized by soils that within 21-1-22288-210 Page 4 of 8 21-1-22288-210-L2.docx/wp/tvv 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 100 feet below the ground surface have a time -averaged shear wave velocity of between 1,200 and 2,500 feet per second. The site soil factors for PGA (FPCA), Ss (Fa), and Si (Fv) for Site Class C are provided in Table 1. The resulting design ground motion parameters, PGAM, SDs, and SDl, are also provided in Table 1. Table 1: Recommended Seismic Design Parameters Site Class Mapped MCEc peak ground acceleration (PGA) C 0.574g Peak ground acceleration site coefficient (FPCA) 1.2 Peak ground acceleration adjusted for site effects (PGAM) 0.689g Mapped MCER short -period spectral acceleration (Ss) 1.334g Mapped MCER 1-second period spectral acceleration (Si) 0.475g Short -period site coefficient (Fa) 1.2 1-second period site coefficient (Fv) 1.5 Short -period design spectral acceleration (SDs) 1.067g 1-second period design spectral acceleration (Sot) 0.475g Long -period transition period in seconds (TL) 6.0 g = standard gravitational acceleration Geotechnical Wall Design Parameters During early discussions with Hanson, their designers requested geotechnical wall design parameters, including angle of internal friction (y), unit weight (y), active earth pressure coefficient (Ka), and passive earth pressure coefficient (Kp) for retaining wall backfill materials and for the soil in which the retaining wall piles will be embedded. We provided the requested design parameters via email on March 27, 2023, and April 13, 2023. Recommended geotechnical wall design parameters are provided in Table 2. Table 2: Recommended Geotechnical Wall Design Parameters Angle of Internal Friction (degrees) 34 44 38 Unit Weight (pounds per cubic foot) 130 15 135 Active Earth Pressure Coefficient 0.47 0.18 0.24 Passive Earth Pressure Coefficient - - 2.3 21-1-22288-210 21-1-22288-210-L2.docx/wp/tvv Page 5 of 8 111 SHANNON bVVILSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 The values recommended in Table 2 are unfactored except for the passive earth pressure coefficient, which has been reduced by a factor of 2. We recommend ignoring passive earth pressure within the uppermost two feet of the embedded portion of the wall. Site Preparation Throughout the proposed retaining wall location, the slope above the access road is vegetated with coniferous and deciduous trees of varying sizes and with low brush. We recommend cutting trees approximately 6 to 12 inches above the adjacent ground elevation and removing the trees from the slope. Stumps and root masses can remain in place. We recommend clearing all remaining vegetation from the slope. Grubbing to remove root mass is not recommended. Mature trees located along the top of the cut slope, above the progressive failure site, should remain intact. Temporary Slopes Consistent with conventional practice, temporary excavation slopes should be made the responsibility of the Contractor, who is continually at the site and able to observe the nature and conditions of the subsurface materials and groundwater and has responsibility for the methods, sequence, and schedule of construction. Backfill Materials Angular Gravel Backfill We recommend specifying that gravel wall backfill consist of angular material meeting the requirements of Washington State Department of Transportation Standard Specification 9-03.12(2), and that the material be crushed or partially crushed to provide angular or subangular particles. Foamed Glass Aggregate Backfill We recommend specifying AeroAggregates UL-FGA G15 backfill material within portions of the wall backfill where lateral earth pressure reduction is necessary for structural wall performance. Retaining Wall Backfill Placement and Compaction We recommend placing retaining wall backfill in horizontal lifts of loose thickness not greater than 8 inches and compacting to a firm and unyielding condition using hand - operated compaction equipment, such as a plate compactor or jumping jack. 21-1-22288-210 Page 6 of 8 21-1-22288-210-L2.docx/wp/tvv 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 The uppermost portions of retaining wall backfill will not be horizontal due to the permanent backslope included in the design. Sloped backfill should be placed in lifts of loose thickness not greater than 8 inches and tamped with an excavator bucket. CONSTRUCTION CONSIDERATIONS Obstructions Although we did not encounter cobbles, boulders, wood debris, or other obstructions during our subsurface explorations, we anticipate obstructions may be present within the subsurface materials through which the soldier pile boreholes will be advanced. The Contractor should be prepared to drill through or otherwise clear such obstructions from the boreholes should they be encountered. Underground Utilities We understand underground utilities are present beneath the road and ditch just west of the proposed wall location. The Contractor should be made responsible for locating all underground utilities prior to wall construction and notifying the owner and designers of any potential utility conflicts. Wet Weather Earthwork If earthwork occurs during the wet season, or if wet conditions are encountered, then we recommend applying the recommendations contained in Section 9.5 of the project geotechnical report. Erosion Control Consistent with the recommendations in Section 9.6 of the project geotechnical report, the Contractor should employ appropriate erosion control measures during construction. LIMITATIONS Within the limitations of the scope, schedule, and budget, the recommendations presented in this letter are based on our understanding of the project as described in this letter and the site conditions as interpreted from our site reconnaissance and field explorations. If during construction, the subsurface conditions differ from those encountered in the field explorations, we should be advised so that we can review these conditions and reconsider our recommendations if necessary. 21-1-22288-210 Page 7 of 8 21-1-22288-210-L2.docx/wp/tvv ON SHANNON 8WILSO( Ms. Rachel Dotson Snohomish County Parks and Recreation May 8, 2023 We have prepared the enclosed "Important Information About Your Geotechnical/ Environmental Report" to assist you and others in understanding the use and limitations of our reports. Sincerely, SHANNON & WILSON 9 MAA 7,O23 Tyler Stephens, PE Senior Associate TJS:NDM/tjs Enc. References Figure 1— Site and Exploration Plan Important Information About Your Geotechnical Report c: Rob Marchand, Snohomish County Parks and Recreation 21-1-22288-210 Page 8 of 8 21-1-22288-21 O-L2. d ocx/wp/tvv l TS• Will SHANNON WILSON REFERENCES Minard, J.P., 1983, Geologic map of the Edmonds East and part of the Edmonds West quadrangles, Washington: U. S. Geological Survey Miscellaneous Field Studies Map MF-1541, scale 1:24,000. Shannon & Wilson, 2018, Geotechnical report, Meadowdale Beach, park and estuary restoration, Snohomish County, Washington: Report prepared by Shannon & Wilson, Seattle, Wash., project no. 21-1-22288-060, for Snohomish County Parks & Recreation, Snohomish, Wash., February, 163 p. 21-1-22288-210 >I i C IO 1 N n V h h V V PROPERTY BOUNDARY GUARDRAIL MB-1 PROPOSED WALL (APPROX.) s 760 L� VA AAA P6 \\\ Q P \\ TRAFFIC LOOP \ GATE CONTROL BOX BM 3137-1 - SW CORNER CONCRETE PAD FOR GATE ACCESS EL 114.73 6" DIAM. PVC - BADGE READER - GATE - EDGE OF GRAVEL O 18" DIAM. PVC N 0 20 40 Scale in Feet LEGEND 0 DATA SURVEY CONTROL POINT -LL SIGN 0 DECIDUOUS TREE CONIFER TREE ® M113-1 BORING ASPHALT EDGE OF ASPHALT PROPOSED WALL (APPROX.) — — — — — — — — CREEKTOE CREEK TOP x FENCE ----SD SD- UNDERGROUND STORM LINE - - - - SS- - - - SS- UNDERGROUND SEWER GRAVITY LINE ----FM - - - - FM- UNDERGROUND SEWER FORCEMAIN LINE - W W- UNDERGROUND WATER LINE - E E- UNDERGROUND POWER LINE - - - - T - - - - T - UNDERGROUND TELEPHONE LINE o GUARDRAIL Meadowdale Beach Park and Estuary Restoration Snohomish County, Washington SITE AND EXPLORATION PLAN May 2023 22288-210 J�3IiMN13NfW!t- I!+IG FIG. 1 Ms. Rachel Dotson E 1 I I SHAN N O N E WI LSO N Snohomish County Parks and Recreation IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVIRONMENTAL REPORT CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS. Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil engineer may not be adequate for a construction contractor or even another civil engineer. Unless indicated otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first conferring with the consultant. No party should apply this report for any purpose other than that originally contemplated without first conferring with the consultant. THE CONSULTANT'S REPORT IS BASED ON PROJECT -SPECIFIC FACTORS. A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project -specific factors. Depending on the project, these may include the general nature of the structure and property involved; its size and configuration; its historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots, and underground utilities; and the additional risk created by scope -of -service limitations imposed by the client. To help avoid costly problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations. Unless your consultant indicates otherwise, your report should not be used (1) when the nature of the proposed project is changed (for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors that were considered in the development of the report have changed. SUBSURFACE CONDITIONS CAN CHANGE. Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical/environmental report. The consultant should be kept apprised of any such events and should be consulted to determine if additional tests are necessary. MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS. Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface between materials may be far more gradual or abrupt than your report indicates. Actual conditions in areas not sampled may differ from those predicted in your report. While nothing can be done to prevent such situations, you and your consultant can work together to help reduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect. 21-1-22288-210 Page 1 of 2 May 8, 2023 (1/2023) 6.111SHMN01146VALSOM Ms. Rachel Dotson Snohomish Countv Parks and Recreation A REPORT'S CONCLUSIONS ARE PRELIMINARY. The conclusions contained in your consultant's report are preliminary, because they must be based on the assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions can be discerned only during earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report's recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The consultant who developed your report cannot assume responsibility or liability for the adequacy of the report's recommendations if another party is retained to observe construction. THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION. Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/environmental report. To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications relative to these issues. BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM THE REPORT. Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site personnel), field test results, and laboratory and/or office evaluation of field samples and data. Only final boring logs and data are customarily included in geotechnical/environmental reports. These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process. To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotechnical engineering/environmental report prepared or authorized for their use. If access is provided only to the report prepared for you, you should advise contractors of the report's limitations, assuming that a contractor was not one of the specific persons for whom the report was prepared, and that developing construction cost estimates was not one of the specific purposes for which it was prepared. While a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with your consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes that aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because geotechnical/environmental engineering is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against consultants. To help prevent this problem, consultants have developed a number of clauses for use in their contracts, reports, and other documents. These responsibility clauses are not exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are definitive clauses that identify where the consultant's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to your questions. The preceding paragraphs are based on information provided by the Geoprofessional Business Association (https://www.geoprofessional.org) 21-1-22288-210 Page 2 of 2 May 8, 2023 (1/2023)