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HomeMy WebLinkAboutRS_Secondary_Review_Letter_180809.pdf 1101 South Fawcett Avenue, Suite 200 Tacoma, Washington 98402 253.383.4940 August 7, 2018 City of Renton Community & Economic Development 1055 South Grady Way Renton, Washington 98057-3232 Attention: Matt Herrera Subject: Geotechnical Engineering Review Forest Terrace Renton, Washington File No. 0693-082-00 INTRODUCTION AND PROJECT UNDERSTANDING This report presents comments from our review of geotechnical engineering analyses of the proposed Forest Terrace development located at 2611 Union Avenue NE 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 July 11, 2018 and executed August 2, 2018. The proposed development will include construction of a roadway that crosses through Sensitive Slopes and a Protected Slope as defined by City of Renton Municipal Code 4-5-050 “Critical Areas Regulations.” The Protected Slope, located at the proposed east entrance of the site, will be regraded, and rockeries will be constructed as part of the regrading. The site developer is requesting a Critical Areas Variance for the construction of the road in the Protected Slope area. We understand that the road will be transferred to the City at the completion of the project. Our review of the project is limited to this Protected Slope area. DOCUMENTS REVIEWED We reviewed the document “Protected Slope Analysis, Forest Terrace, 2611 Union Avenue NE, Renton, Washington” prepared by Terra Associates, Inc., dated July 14, 2018 and revised June 18, 2018. We also reviewed the geotechnical report prepared for the site, “Geotechnical Report, Vandermay, 2611 Union Avenue NE, Renton, Washington, Project No. T-7677”, prepared by Terra Associates, Inc., dated July 19, 2017 and the Forest Terrace Preliminary Plat Plans prepared by Barghausen Consulting Engineers, Inc. dated June 18, 2018. We only reviewed the portions of these documents that related to the Protected Slope. City of Renton | August 7, 2018 Page 2 File No. 0693-082-00 GEOTECHNICAL REVIEW COMMENTS Stability Analysis Review The following presents our review comments on the Protected Slope Analysis.: 1. The report states that the purpose of the analysis is to determine if the proposed grading increases the stability of the slope. It is our opinion that if the slope is modified, the stability of the proposed condition should be appropriate for its intended use and not just an increase from the previous condition. In this case, the risk of slope movement or failure is to a roadway. It is our opinion that guidelines provided in the Washington State Department of Transportation (WSDOT) Geotechnical Design Manual (GDM) Chapter 7 are appropriate to evaluate stability of slopes above roadways. These guidelines state that for general slope stability (i.e., slopes not impacting walls, bridges, or other structures) a minimum factor of safety of 1.25 for static (non-seismic) conditions should be used. Seismic slope stability is typically not considered if a slope failure will not affect the stability of a structure, and if the proposed modifications increase the overall stability of the slope. If these conditions are met, the seismic analysis, in our opinion, can be omitted. If seismic conditions are considered, the minimum factor of safety should be 1.05. 2. The report indicates that the rockeries will be 4 feet tall, but there is no indication of the minimum width or batter. The width and batter of the rockeries will affect the overall slope inclination, the stability of the rockery, and the overall stability of the slope. In our opinion, the minimum width and batter of the rockeries should be stated in the analysis so that it can be checked against the design plans for consistency. 3. The report states that a horizontal acceleration of 0.2g was used in the pseudostatic analysis. It’s not clear what seismic design level (i.e., return period) that this is based on or if a reduction was included. A seismic event with a 7 percent probability of exceedance in 75 years (about 1,000-year return period) is typically used for roadway projects. For a pseudostatic analysis, one half of the Peak Ground Acceleration (PGA) is typically used based on the assumption that some slope movement is acceptable. However, as noted above, a seismic stability evaluation might not be required. 4. In our opinion the soil parameters selected for analysis are appropriate based on the geologic conditions identified in the report (advance outwash). However, the closest exploration to the Protected Slope, TP-18, is about 150 feet away and does not extend to the full depth of the proposed cut. As a result, there is some additional uncertainty of the soil conditions in that specific area. The geotechnical report states in Section 6 that Terra Associates, Inc. “should also provide geotechnical service during construction to observe compliance with our design concepts, specifications, and recommendations. This will allow for design changes if subsurface conditions differ from those anticipated prior to the start of construction.” In our opinion, the geotechnical engineer of record should confirm that the soil and groundwater conditions in the Protected Slope area are consistent with those assumed in the analysis and should provide appropriate documentation to the City at the completion of the project. 5. It is our opinion that the analysis presented might not capture all potential failure surface with the lowest factor of safety. The critical failure analysis presented appears to limit the failure surface exit point to an area about 20 feet from the toe of the slope and also limits the analysis to a point located right at the crest of the slope. In our opinion, the analysis should include more of the upslope area and should include a search for critical failure surfaces exiting at the toe of the slope and through the face of the rockeries. Additionally, local instability should also be evaluated and should specifically include City of Renton | August 7, 2018 Page 3 File No. 0693-082-00 potential failure surfaces that begin at the top of the slope above the upper rockery and exit in the area between the two rockeries. Additional Considerations The following presents our comments on the Preliminary Plat Plans and general comments related to design detailing. We understand that the current plans are preliminary and that these details could be included as the design develops. 6. Soils within the advance outwash deposit, as documented in test pits 16, 17, and 18 of the geotechnical report, consists of sand with few fines. This type of soil can have a high erosion potential. In our opinion additional erosion control measures could be required to allow enough time for vegetation to be established. 7. There is no embedment detailed for the rockeries. This can make the foundation of the rockeries susceptible to undercutting by erosion or other activities. In our opinion, an embedment of at least 1 foot should be provided. This is especially important for rockeries founded above a slope. 8. The rockeries appear to be shown with a vertical face. In our opinion, typically a batter of 1H:6V (horizontal:vertical) is appropriate for rockery construction. 9. The advance outwash could be susceptible to internal erosion where the soil from the cut face migrates into the voids of the rockery overtime. In our opinion, filter fabric or filter graded backfill should be used to reduce the potential for this type of erosion. CONCLUSIONS It is our opinion that the Protected Slope can be modified generally as envisioned to allow for construction of the proposed roadway without causing undue risk to the public provided that an appropriately engineered system is constructed to the appropriate standards. In order to confirm that the proposed modification meets these standards, we recommend that the City require the analysis be resubmitted incorporating or otherwise addressing all of our above comments. 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.