Loading...
HomeMy WebLinkAboutC_GeoEngineers_Final_Secondary_Review_201012.pdf 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 October 12, 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, additional documents were provided addressing river channel migration, scour potential, and revising the stability analysis to include scour potential. We reviewed these documents and provided comments in a report dated July 8, 2020. City of Renton | October 12, 2020 Page 2 File No. 0693-084-00 TECHNICAL REVIEW We reviewed the documents “Cedar River Apartments Scour Analysis Technical Report” prepared by The Watershed Company, dated March 30, 2020, revised July 10, 2020; and “Bulkhead Wall Stability Addendum No. 3” prepared by Terracon Consultants, Inc., dated July 24, 2020. We have no comments on these two documents. 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. CONCLUSIONS It is our opinion that the conclusion presented in the Bulkhead Wall Stability Addendum No. 2 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.” A revised, more conservative analysis is provided in Addendum No. 3. It indicates that under some conditions calculated factors of safety could be as low as 1.1 for the design scour case and 0.8 for the extreme combination of the design scour case combined with the design seismic case. Terracon further concludes that some movement would be expected in this extreme case but could be managed through maintenance. Although this level of stability is not appropriate for areas that would directly impact inhabited structures, it is our opinion that it is appropriate for a riparian buffer and appropriate for the building site when combined with the proposed minimum building setbacks. We recommend that any future development that would limit, reduce, modify, or otherwise change the proposed riparian buffer or building setbacks consider riverbank stability concerns as well as any regulatory requirements. 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. City of Renton | October 12, 2020 Page 3 File No. 0693-084-00 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 10/12/2020 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.