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HomeMy WebLinkAboutC_On Hold Letter UPDATE_200512_v1 May 14, 2020 Phil Becker Abbey Road Group 2102 E Main Ave, Suite 109 Puyallup, WA 98372 SUBJECT: "On Hold" Notice #2 – Secondary Review Complete Mei Lin Short Plat / LUA20-000060 Dear Mr. Becker, The Planning Division of the City of Renton accepted the above master application for review on March 20, 2020 During our review. The project was placed “on-hold” on April 17, 2020, pending the completion of an independent secondary review of the Geotechnical Report prepared by Bergquist Engineering (dated October 17, 2017). The secondary review was completed by GeoEngineerings, Inc. The consultant’s findings are contained in the attached three-page memo (dated May 11, 2020). An updated report should be submitted to the City that addresses all comments/concerns in the secondary review memo, in addition to the four staff comments contained in the original “on-hold” letter. The updated report should be submitted before August 14, 2020 so that we may continue the review of the above subject application: At this time, your project will continue to be “on hold” pending receipt of the requested information. The maximum time for resubmittal shall be within ninety (90) days of this notice. Please contact me at (425) 430-7219 if you have any questions. Sincerely, Alex Morganroth Senior Planner Attachment: Geotechnical Engineering Review prepared by GeoEngineering, Inc (dated May 11, 2020) cc: Maggie Wang/ Owner(s) Gil Hulsmann/Applicant M. Fattorre, M. Koontz, B. Chapman / Party(ies) of Record 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 May 11, 2020 City of Renton Community & Economic Development 1055 South Grady Way Renton, Washington 98057-3232 Attention: Alex Morganroth Subject: Geotechnical Engineering Review Mei Lin View Short Plat Renton, Washington File No. 0693-088-00 INTRODUCTION This report presents comments from our review of geotechnical engineering analyses of the proposed Mei Lin View development located at 1833 NE 12th Street in Renton, Washington. Our services are being provided to the City of Renton Department of Community and Economic Development in accordance with our agreement dated April 20, 2020 and executed May 4, 2020. The proposed development will include construction near a Geologically Hazardous Area as defined by City of Renton Municipal Code. The City has requested a third-party review of the geotechnical report and grading plan to confirm that the analysis and conclusions provided are appropriate before approving the development. We reviewed a geotechnical report prepared for the project by Bergquist Engineering Services dated October 17, 2017 addressing slope stability. We also reviewed the Mei Lin View Grading Plan dated February 18, 2020. GEOTECHNICAL REVIEW The following presents our review comments on the report and project grading plan. Our review is focused on the proposed development of Lot 3, located adjacent a steep slope. Our review only addresses geotechnical aspects of the proposed project. 1. Analysis should be provided that directly demonstrates that the following review criteria can be met: (1) Per Renton Municipal Code Section 4-3-050F.2.a.ii.(a) “The proposal will not increase the threat of the geological hazard to adjacent or abutting properties beyond pre-development conditions”. In our opinion this can be demonstrated by comparing the calculated stability of the existing conditions to the calculated stability of the proposed conditions with the inclusion of typical or design building loads as City of Renton | May 11, 2020 Page 2 File No. 0693-088-00 appropriate. And, (2) Per Section 4-3-050F.2.a.ii.(c) “The development can be safely accommodated on the site.” In our opinion this can be demonstrated by evaluating slope failure surfaces that would affect structures located within the recommended building setbacks. Typically factors of safety greater than 1.5 for static conditions and greater than 1.05 for seismic conditions are considered appropriate. The licensed geotechnical professional preparing the analysis should have some discretion in determining appropriate factors of safety, but must ultimately provide a statement that, in their professional opinion, the criteria have been met. 2. The report states that a seismic coefficient of 0.2g was applied to the project. It is not clear what seismic design level (i.e., return period) and site class this is based on or if a reduction was included. A seismic event with a 2 percent probability of exceedance in 50 years (about 2,475-year return period in accordance with the International Building Code [IBC]) is typically used to analyze slopes and potential failures that will impact inhabited structures. For a pseudo-static analysis, one-half of the Peak Ground Acceleration (PGA) is typically used based on the assumption that some permanent slope movement is acceptable. 3. It is unclear between the text and the stability analyses provided in Appendix B, which soil parameters are used for which subsurface unit and at what elevation these are applied (some apparent inconsistencies are noted below). We suggest providing a table within the text or appendix stating the elevations and parameters used to model each unit. 4. Some of the analyses are reported to use a cohesion of 100 pounds per square foot (psf) in “the uppermost soils to account for strength due to roots”. The analysis provided in the appendix appears to apply the 100 psf cohesion to units well over 10 feet deep, which is beyond what could be expected for significant root penetration. Inclusion of the effects of vegetation, over any extent, to maintain or develop slope stability could, at a minimum, require a Native Growth Protection Area per Renton Municipal Code. Additionally, inclusion of this strength over the entire surface of the site appears to be in contradiction with the recommendation in Section 6.3 Site Preparation that states, “The site should be stripped of all vegetation prior to construction.” Additional information should be provided justifying the magnitude and extent of the root strength and the apparent contradictions should be clarified or explained. 5. A case was analyzed that assumes the upper 12 feet of loose fill soils were replaced with structural fill with a friction angle of 40 degrees. Structural fill of this strength usually requires specific gradation and high compaction criteria and could also require use of an angular or crushed aggregate. This is inconsistent with a soil that will develop significant root penetration to develop the equivalent of 100 psf cohesion, which was also assumed. There are also some apparent inconsistencies between what is stated in the text and what is in the analyses provided in Appendix B. For example, the fill unit appears to have been analyzed with a friction angel of 38 degrees and there is one unit with a cohesion of 150 psf. Additional information should be provided justifying the strength of the proposed fill and the apparent inconsistencies should be clarified or explained. 6. It is stated in Section 6.1 Residential Foundations of the report that “The uppermost soils at this site are not suitable for support of conventional spread footings that are typically used for residential structures.” and that “deep foundations will likely be required”. The slope stability analysis considers a case where the upper loose fill is removed and replaced with structural fill and this section recommends deep foundations to support structures. It is unclear how the structures are proposed to be supported. If the future structure will be supported on structural fill after the removal of the existing fill, then the grading plan appears insufficient to address this level of earthwork. If the structure is supported on City of Renton | May 11, 2020 Page 3 File No. 0693-088-00 deep foundations or with a combination of deep foundations and slope stabilizing structures as suggested in the report, then these features should be considered when evaluating slope stability and evaluating if the construction of such structures will increase the threat of the geological hazard to adjacent or abutting properties. CONCLUSIONS It is our opinion that additional information is required to determine if the proposed development meets the criteria in Renton Municipal Code. We recommend that the City require the analyses as described in Comment No. 1 be submitted for additional review and that other comments be addressed. Additionally, based on the recommendations in the geotechnical report, it appears that either soil supporting structures or nonconventional residential foundations might also be required. If the viability of the Lot 3 development requires the use of such structures or foundations, we recommend that these plans be submitted for concurrent review. In our opinion, foundation or wall plans developed to a typical 30-percent design level could provide sufficient detail for review of slope stability provided that the details include anticipated construction access and staging if it will require grading on the slope. 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 if you 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 5/11/2020 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.