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HomeMy WebLinkAboutC_GeoEngineers_Review Letter 2.pdf 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 July 8, 2020 City of Renton Community & Economic Development 1055 South Grady Way Renton, Washington 98057-3232 Attention: Matt Herrera Subject: Geotechnical Engineering Review Services Cedar River Apartments Renton, Washington File No. 0693-084-00 INTRODUCTION This report presents comments from our review of geotechnical engineering, river scour, and river analyses of the proposed Cedar River Apartments located at 1915 Maple Valley Highway in Renton, Washington. The site is located south of Cedar River Park, southwest of Maple Valley Road, and directly north of the Cedar River. Our services are being provided to the City of Renton Department of Community and Economic Development in accordance with our agreement dated and executed September 11, 2019 with an amendment executed June 25, 2020. The proposed project will require development in an area currently designated as Channel Migration Zone (CMZ). We understand that the owner is proposing to proceed with the development based on the presence of an existing concrete wall located at the riverbank. An analysis of the stability of the wall by Terracon and DCI Engineers has been provided to the City. We reviewed these documents and provided comments in a report dated October 28, 2019. Based in part on our review comments, the three additional documents were provided addressing river channel migration, scour potential, and revising the stability analysis to include scour potential. A summary of our review of these reports is provided in this report. City of Renton | July 8, 2020 Page 2 File No. 0693-084-00 TECHNICAL REVIEW We reviewed the documents “Channel Migration Risk Assessment Memo” prepared by The Watershed Company, dated April 8, 2020; “Cedar River Apartments Scour Analysis Technical Report” prepared by The Watershed Company, dated March 31, 2020; and “Bulkhead Wall Stability Addendum No. 2” prepared by Terracon Consultants, Inc., dated May 21, 2020. The purpose of our review is to determine if the analyses required to establish the expected risk of channel migration have been performed with current relevant design standards. We have not evaluated if the structure meets the criteria in the Washington Administrative Code or other criteria. The following presents our review comments of the analyses: 1. The scour report indicates that an 8.5-foot scour hole was measured at the upstream end of the project (Location A). The measured flows in the Lower Cedar River have approached, but never met or exceeded, the 100-year design flood event during the period of record. Therefore, measured scour was produced by flows that are less than the design flow. This implies that scour associated with design flood event must be greater than the existing condition. It is our opinion that a greater, more conservative, scour design depth is consistent with the observed data. For clarity, we also suggest that the recommended scour design depth be referenced to elevation and stated explicitly in the scour report. 2. The scour report provides engineering analysis and should, therefore, include a professional engineers stamp or seal. 3. In the stability report addendum, the concrete was modeled as a cohesive material. This assumes some tension capacity in the material and, therefore, must assume that there are no cracks in the concrete. The concrete mass is unreinforced and, therefore, has likely cracked due to shrinking while curing. The fact that no perched groundwater was noted on top of the concrete is, in our opinion, evidence that the concrete, while massive and intact at a local scale, is likely cracked on a larger scale. We believe that a cohesive soil model can be an appropriate and conservative assumption over short distances such as when considering potential lateral earth pressures on the bulkhead wall and locally at the toe of the wall. However, over larger areas, such as were analyzed in the global stability analysis, cracks in the concrete are likely to control the strength of the mass must be considered. We suggest modeling the concrete as a high friction material in global stability analyses. A similar comment was also provided in our October 28, 2019 review. 4. In the stability report addendum, some portions of the analysis figures appear to be missing. Some legends appear incomplete and don’t include all soil units used in the analysis, specifically the green unit shown in Figure 3. CONCLUSIONS It is our opinion that the conclusion presented in the Bulkhead Wall Stability Addendum is appropriate and justified by the analysis provided, specifically the conclusion “Given that the building setbacks for the riparian areas is a minimum of 100 feet, the proposed development does not appear to be at risk from potential failure of the bulkhead wall.” We expect that using more conservative analyses and assumptions (as noted in comments 1 and 3 above) could result in a risk of impacts from wall failure greater than the City of Renton | July 8, 2020 Page 3 File No. 0693-084-00 10 to 27 feet indicated in the report addendum. However, it is unlikely, in our opinion, that this more conservative analysis would indicate a risk of impacts beyond the minimum riparian buffer as concluded. LIMITATIONS Within the limitations of scope, schedule, and budget, our services have been executed in accordance with generally accepted practices for geotechnical engineering in this area at the time this report was prepared. Our services were provided to assist the City of Renton evaluate a geotechnical analysis submitted as part of a permit application. GeoEngineers cannot attest to the accuracy or completeness of the materials provided. The conclusions, recommendations, and opinions presented in this report are based on our professional knowledge, judgment and experience. No warranty, express or implied, applies to the services or this report. If there are any questions or wish to discuss any of our review comments, please contact us. Thank you for the opportunity to be of service to the City of Renton. Sincerely, GeoEngineers, Inc. Lyle J. Stone, PE Associate Geotechnical Engineer 7/8/2020 LJS:tt Disclaimer: Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official document of record.