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REVIEWED RESUB2 BLD2023-1134+Geotechnical_Letter+5.9.2024_5.00.38_PM+4248498E111 SHANNON 6WILSON May 9, 2024 Ms. Rachel Dotson Snohomish County Parks and Recreation 6705 Puget Park Drive Snohomish, WA 98296 RE: RESPONSES TO THE CITY OF EDMONDS APRIL 4, 2024 REVIEW COMMENTS LETTER, 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 our responses to comments in the City of Edmonds (the City) Supplemental Geotechnical Peer Review letter by HWA Geosciences (HWA), dated April 4, 2024, (HWA, 2024) for the proposed retaining wall along the access road near the entrance gate for the Meadowdale Beach Park and Estuary Restoration Project (project) in Edmonds, Washington. We previously provided a geotechnical report (Shannon & Wilson, 2018a) and geologically hazardous areas (critical areas) report (Shannon & Wilson, 2018b) for the project. The proposed retaining wall that is the subject of this letter was added to the project during construction. We provided a geotechnical report (Shannon & Wilson, 2023a) and letter with our opinion regarding geotechnical hazard identification and declaration and mitigation statement of risk (Shannon & Wilson, 2023b) for the proposed retaining wall, which were submitted to the the City for review. We provided a letter describing our geotechnical evaluation and global slope stability results (Shannon & Wilson, 2024) in response to the City's review comments, and it amends the four reports referenced above. The City reviewed and provided comments to our 2024 letter. This letter addresses the City's comments. Details regarding the retaining wall analyses and design are presented in the referenced reports, design drawings, and details that have been submitted, and are not described herein. 400 North 34th Street, Suite 100 1 PO Box 300303 1 Seattle, Washington 98103-8636 1 206-632-8020 www.shannonwilson.com 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 9, 2024 REQUEST FOR APPLICATION OF EDMONDS COMMUNITY DEVELOPMENT CODE CHAPTER 23.40.000(F) Edmonds Community Development Code (ECDC) Chapter 23.40.000 (F) (City of Edmonds, 2023) states: This title is to be administered with flexibility and attention to site -specific characteristics. It is not the intent of this title to make a parcel of property unusable by denying its owner reasonable economic use of the property nor to prevent the provision of public facilities and services necessary to support existing development. With this letter, we formally request construction permit issuance and application of the "flexibility and attention to site -specific characteristics" portion of this code as, in our opinion, the specified seismic global stability factor of safety of 1.2 cannot be reasonably or economically met due to both the steep nature of the slope above and below the access road and the composition of the soil deposits observed in both the nearby project boring and soil exposures observed along the road cuts and elsewhere along the slope. As our analyses indicate, modeling the wall as designed improves both static and seismic global stability cases by 30%. In an attempt to satisfy the code requirements, we completed two additional preliminary analyses: one that modeled lengthening the embedded portion of the soldier piles and one that reflected increased soil shear strength parameters. Neither analysis yielded feasible or defensible results as the soldier piles would need to be embedded an additional 45 feet or the soil shear strength would need to be increased beyond acceptable values for the observed soil types. In our opinion, the natural state of the slope does not satisfy the code requirements, and the physical and/or structural slope modifications necessary to satisfy the specified seismic global stability factor of safety (such as flattening the slope and/or constructing structural slope stability improvement measures) are considered neither reasonable nor economical. CITY OF EDMONDS REVIEW DOCUMENTS, COMMENTS, AND RESPONSES This section addresses specific review comments in the April 4, 2024, City of Edmonds (HWA) Supplemental Geotechnical Peer Review letter that identified specific code requirements that had not been satisfied in our previous reports. The City's responses to all other comments indicated they had been adequately addressed and are not considered herein. Review Comment 6: Per Section 19.10.030(H)6 of the ECDC, the geotechnical report shall include an analysis of the rate of retreat of the bluff, prepared by a geologist and an 21-1-22288-210 21-1-22288-210-1_5_FI NAL.docx/wp/axg Page 2 of 7 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 9, 2024 estimate of the bluff retreat amount and regression rate for periods of 25 and 125 years. To be in accordance with the ECDC, this information should be added to the geotechnical evaluation by Shannon & Wilson. HWA Response: The March 2024 and September 2023 letters by Shannon & Wilson indicate that the probability of "progressive raveling block failure at that location over the next 25 years is 100% if left unmitigated." This statement does not satisfy the requirement for an estimate of bluff retreat amount and regression rate for periods of 25 and 125 years. This information, along with a discussion regarding the effects of bluff retreat on the stability of the proposed structures and/or improvements, should be provided per the requirements of Section 19.10.030(H)6 of the ECDC. - Response: Based on observed localized raveling failure and approximately three feet of cut slope regression that has occurred between approximately 2018 and present, we estimate the bluff retreat rate is approximately 0.5 foot per year, and that this bluff may retreat approximately 13 feet over the next 25 years and 63 feet over the next 125 years if left unmitigated. Based on our assumption that the cut slope was relatively uniform along the access road at the time of construction around 1960, we estimate that the localized area may have retreated approximately 6 feet over the approximately 64 year period that the road has existed, for an average bluff retreat rate of 0.094 foot per year, which would indicate that this bluff may retreat approximately 2.3 feet over the next 25 years and 12 feet over the next 125 years if left unmitigated. Continued bluff retreat could undermine one or more mature evergreen trees, potentially causing them to fall, and may initiate other slope instability. The purpose of the subject wall is to mitigate and arrest raveling failure to address this concern. Review Comment 7: Per Section 23.80.050(F)2.e of the ECDC, the critical areas report shall include an estimate of the bluff retreat rate or the percent risk of the landslide area expansion that accounts for potential catastrophic events such as seismic activity or a 100-year storm event. To be in accordance with the ECDC, this information should be added to the geotechnical evaluation by Shannon & Wilson. (HWA) Response: While the March 2024 and September 2023 letters by Shannon & Wilson indicate that the probability of "progressive raveling block failure at that location over the next 25 years is 100% if left unmitigated," this statement does not satisfy the requirement of Section 23.80.050)F)2.e of the ECDC to provide an estimate of the bluff retreat rate or an estimate of the percent risk of landslide area expansion that recognizes and reflects potential catastrophic events such as seismic activity or a 100-year storm event. In HWA's opinion, this comment has not been adequately addressed. - Response: Refer to previous response regarding estimated bluff retreat rate. Based on the results of our global stability analysis, if the wall were constructed in accordance with the design drawings, and the design seismic event occurs within the design life of the wall, we estimate the risk of global slope failure within the wall 21-1-22288-210 21-1-22288-210-1-5_FI NAL.docx/wp/axg Page 3 of 7 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 9, 2024 limits will be reduced by about 30% compared to the existing condition. Note that the subject retaining wall has been designed to account for wall backfill seismic loading such that local stability is satisfied. The "percent risk of the landslide area expansion" due to seismic activity is uncertain and difficult to rigorously quantify. If the greater slope area including the sections north and south of the subject wall were to fail during a seismic event, the landslide area would be expanded. In our opinion, in the event of seismic global slope failure, the wall would likely be damaged or perhaps entrained in the landslide mass, depending on the failure surface depth and geometry. Review Comment 8: Per Section 23.80.050(F)2.g of the ECDC, the critical areas report shall include a study of slope stability including an analysis of proposed cuts, fills, and other site grading. To be in accordance with the ECDC, a slope stability evaluation should be completed and added to the geotechnical evaluation by Shannon & Wilson. (HWA) Response: In their March 2024 letter, Shannon & Wilson provided the results of global slope stability analyses completed for the proposed retaining wall. This comment has been addressed; however, as described subsequently, factors of safety for the post -construction condition do not meet the minimum factors of safety required by Section 23.80.070(A)4.a of the ECDC. - Response 8: The specified minimum global stability factor of safety for the design seismic event is not satisfied. However, in our opinion, the physical and/or structural slope modifications necessary to satisfy the specified seismic global stability factor of safety are considered neither reasonable nor economical. This outcome has prompted this request for application of the "flexibility and attention to site -specific characteristics" portion of this code to facilitate construction permit issuance. Review Comment 9: Per Section 23.80.050(F)3.d of the ECDC, the geotechnical report shall provide recommendations for the mitigation of adverse site conditions that include slope stabilization and seismically unstable soils. The structural calculations provide internal stability calculation, but the results of global stability analyses have not been provided. To be in accordance with the ECDC, these analyses should be completed and added to the geotechnical evaluation by Shannon & Wilson. (HWA) Response: In their March 2024 letter, Shannon & Wilson provided the results of global slope stability analyses completed for the proposed retaining wall. This comment has been addressed; however, as described subsequently, factors of safety for the post -construction condition do not meet the minimum factors of safety required by Section 23.80.070(A)4.a of the ECDC. - Response 9: See previous response. ■ Review Comment 10: Per Section 23.80.060(A)3 of the ECDC, site development alterations to geologically hazardous areas shall eliminate or mitigate the hazard to a 21-1-22288-210 Page 4 of 7 21-1-22288-210-L5_FI NAL.docx/wp/axg 111 SHANNON WALSO Ms. Rachel Dotson Snohomish County Parks and Recreation May 9, 2024 level equal to or less than predevelopment conditions. To be in accordance with the ECDC, an analysis of the global slope stability should be completed to show the retaining wall will mitigate the geologic hazard. The results of this analysis should be added to the geotechnical evaluation by Shannon & Wilson. (HWA) Response: In their March 2024 letter, Shannon & Wilson provided the results of global slope stability analyses completed for the proposed retaining wall. The analyses indicate that the post -construction condition will have higher factors of safety under static and seismic conditions, compared to existing conditions. This comment has been addressed; however, as described subsequently, factors of safety for the post -construction condition do not meet the minimum factors of safety required by Section 23.80.070(A)4.a of the ECDC. - Response: See Comment 8 response. Review Comment 11: Per Section 23.80.070(A)4.a of the ECDC, development within a landslide hazard area shall be designed to meet a factor of safety of 1.5 for static conditions and 1.2 for dynamic conditions. If stability is below these limits at the site, the proposed development shall provide approaches to reduce human safety risk and improve the factor of safety. In no case shall the factor of safety be reduced for the subject property or adjacent properties as a result of the proposed alterations. To be in accordance with the ECDC, a slope stability analysis should be performed and added to the geotechnical evaluation by Shannon & Wilson. (HWA) Response: In their March 2024 letter, Shannon & Wilson provided the results of global slope stability analyses of the proposed retaining wall under static and seismic conditions. The model results indicate that following construction of the proposed retaining wall, the global minimum factor of safety is 1.49 under static conditions. The model results indicate that following construction of the proposed retaining wall, the global minimum factor of safety is 0.89 under seismic conditions. Per Section 23.80.070(A)4.a of the ECDC, the minimum factors of safety under static and seismic conditions should be 1.5 and 1.2, respectively. In their September 2023 letter, Shannon & Wilson states that the purpose of the wall is to "mitigate shallow, raveling block failures upslope of the retaining wall and within the retaining wall limits. No additional mitigation is assumed by constructing this wall." HWA has interpreted this statement as an indication that the intent of the design is for the wall to not withstand seismic conditions. This design approach is not consistent with the requirements of the ECDC. For example Section 23.80.070(A)4.a of the ECDC states that if the stability at the proposed development site is below the required limits (which Shannon & Wilson's March 2024 letter indicates is the case), the proposed development shall provide practicable approaches to reduce risk to human safety and improve the factor of safety for landsliding and in no case shall the existing factor of safety be reduced for the subject property or adjacent properties. Because Shannon & 21-1-22288-210 Page 5 of 7 21-1-22288-210-L5_FI NAL.docx/wp/axg =III SHANNON WILSON Ms. Rachel Dotson Snohomish County Parks and Recreation May 9, 2024 Wilson has not done this, it is HWA's opinion that this comment has not been adequately addressed. - Response: The subject retaining wall has been designed to withstand seismic loading imparted by the design wall backfill and those structural calculations have been submitted as part of the construction permit application materials. Mitigating the ongoing shallow, raveling block failures upslope of the retaining wall will, in our opinion, reduce human safety risk. As our global stability analyses indicate, constructing the wall per design will increase both static and seismic global factors of safety by 30% above existing conditions. As mentioned in our mitigation statement of risk letter (Shannon & Wilson, 2023b), the proposed retaining wall is intended to mitigate shallow, raveling block failure upslope of the retaining wall and within the retaining wall limits. No additional mitigation is assumed by constructing this wall. Sincerely, SHANNON & WILSON a Srp y.�� °F wasy���•�' Tyler Stephens, PE Vice President TJS:NDM/qs Enc. References Important Information About Your Geoteclulical/Environmental Report c: Rob Marchand, Snohomish County Parks and Recreation 21-1-22288-210 21-1-22288-210-L5FINAL.docx/wp/ox9 Page 6 of 7 / T S, 111 SHANNON WALSO REFERENCES Ms. Rachel Dotson Snohomish County Parks and Recreation May 9, 2024 City of Edmonds Washington, 2023, City code and community development code: Available: http://www.codepublishing.com/WA/Edmonds/, accessed February 2024. HWA Geosciences, Inc. (HWA), 2024, Geotechnical peer review, Meadowdale Beach Park - retaining wall, building permit no, BLD202301134, Edmonds, Washington: Letter prepared by HWA, Bothell, Wash., for City of Edmonds, Edmonds, Wash., January 25, 4 p. Shannon & Wilson, 2018a, 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 16,163 p. Shannon & Wilson, 2018b, Geologically hazardous areas, Meadowdale Beach Park Estuary restoration project, Snohomish County, Washington: Memorandum from Tyler Stephens to Logan Daniels, March 30, 22 p. Shannon & Wilson, 2023a, Geotechnical report, proposed retaining wall along access road cut slope near entrance gate, Meadowdale Beach Park and Estuary restoration project, Edmonds, Washington: Memorandum from Tyler Stephens to Rachel Dotson, job no. 21-1-22288-210, May 8, 12 p. Shannon & Wilson, 2023b, Geotechnical hazard identification/declaration and mitigation statement of risk, proposed retaining wall along access road cut slope near entrance gate, Meadowdale Beach Park and Estuary restoration project, Edmonds, Washington: Memorandum from Tyler Stephens to Rachel Dotson, job no. 21-1- 22288-210, September 27, 8 p. Shannon & Wilson, 2024, City of Edmonds review comment response, proposed retaining wall along access road cut slope near entrance gate, Meadowdale Beach park and estuary restoration project, Edmonds, Washington: Memorandum from Tyler Stephens to Rachel Dotson, March 15, 18 p. 21-1-22288-210 Page 7 of 7 21-1-22288-210-1-5_FI NAL.docx/wp/axg 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 Page 1 of 2 May 9, 2024 (1 /2024) 6.111SHMNION&WLSOM 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 Page 2 of 2 May 9, 2024 (1/2024)