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HomeMy WebLinkAboutExh.24_GeoEngineers_Review 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 October 28, 2019 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 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 July 11, 2018 and executed August 2, 2018. The proposed development will require development in an area currently designated as Channel Migration Zone (CMZ). We understand that the owner is proposing to have the site reclassified as Disconnected Migration Area (DMA) 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. Our review of the project is limited to the concrete wall and how it relates to the regulated CMZ. DOCUMENTS REVIEWED AND REVIEW APPROACH We reviewed the documents “Bulkhead Wall Stability Addendum” prepared by Terracon Consultants, Inc., dated June 10, 2019 and “Bulkhead Wall Stability Addendum” prepared by DCI Engineers, dated June 12, 2019. We provide comments on these documents below. We also reviewed “Revised Geotechncial Engineering Report, Cedar River Apartments, Renton, Washington” by Terracon Consultants, Inc. dated October 31, 2018 and preliminary plan sheets prepared by KPFF Consulting Engineers and Runberg Architecture Group. These were reviewed for background information only. EXHIBIT 24 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 City of Renton | October 28, 2019 Page 2 File No. 0693-084-00 “Cedar River Channel Migration Study” prepared by King County Department of Natural Resources, dated April 2015 was also reviewed to provide context as to standards used to establish the regulated CMZs and DMAs within the system. This document states that: “A Disconnected Migration Area (DMA) is the area located landward of an artificial structure that is likely to restrain channel migration and that meets criteria in Washington Administrative Code 173-26-221(3)(b) and King County (2014).” The document further states that: “An artificial structure was considered likely to restrain channel migration if its construction, condition, and configuration are consistent with current relevant design and construction standards and if the present channel is unlikely to migrate landward of the structure (King County 2014).” The purpose of our review is to determine if the analyses required to establish if the existing concrete bulkhead is likely to restrain channel migration have been performed with current relevant design standards. We have not not evaluated if the structure meets the criteria in the Washington Administrative Code or other criteria. GEOTECHNICAL REVIEW The following presents our review comments with regard to the geotehchnical and structural analysis: 1. Analysis for the bulkhead and its ability to restrain channel migration should be based on relevant design guides and methods specific to riverine structures. In our opinion United States Army Corps of Engineers (USACE) design guides provide appropriate design standards. Specific design documents could include EM 1110-2-1418 “Channel Stability Assessment for Flood Control Projects” and EM 1110-2-2502 “Retaining and Floodwalls”. Not all elements of these documents will be applicable. Additionally, there might be other appropriate design guides or design methods that could also be used. 2. Analysis of the bulkhead should include an evaluation of hydraulic conditions. This evaluation should include, at a minimum, predicted scour at the toe of the bulkhead and the effects of long-term bed degradation or aggredation and localized bend scour. The report states that scour was not considered and that scour is being evaluated by others. The hydraulic and scour analysis should be provided for review and the results should be incorporated in the stability analyses as appropriate. This hydraulic analysis should also address the length of the bulkhead and if this is sufficient to manage risk of channel migration at the upstream and downstream ends; specifically, the potential for the river to flank the bulkhead on the upstream end. 3. We agree with the decision to use a 100-year return period seismic event for the evaluation of the bulkhead wall for evaluating resiliency and function as a flood or hydraulic control structure. If slope stability or lateral spread issues will impact inhabited site structures, the appropriate International Building Code (IBC) seismic loading must be used. 4. 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 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 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. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 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 DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 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. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 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. PLANNING DIVISION 10/12/2020 MHerrera RECEIVED DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 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. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1 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. DocuSign Envelope ID: 99BA427F-FE67-49CC-B142-9BE0760232C1