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HomeMy WebLinkAboutC_Response_Letter_TERRA_2003120_v1.pdfFebruary 24, 2020 Project No. T-7886 Mr. Jordan Salisbury Blue Fern Development, LLC 11232 – 120th Avenue NE, Suite 204 Kirkland, Washington 98033 Subject: Response to City of Renton Comments Canopy 4130 Lincoln Avenue NE Renton, Washington References: 1. Geotechnical Report, Canopy, 4130 Lincoln Avenue NE, Renton, Washington, Project No. T-7886, prepared by Terra Associates, Inc., dated February 21, 2020 2. Civil Plans, Canopy, 4196 Lincoln Avenue NE, Renton, Washington, prepared by CORE Design, dated September 11, 2019 3. Geotechnical Engineering Review Services, Canopy Subdivision, Renton, Washington, Project No. 0693-084-00, prepared by GeoEngineers, dated December 19, 2019 Dear Mr. Salisbury: As requested, we have reviewed the referenced comments from the City of Renton letter dated January 28, 2020, prepared by GeoEngineers regarding the project site. The following is our response to the geotechnical comments. Comment #2 The report states that a horizontal acceleration of 0.2g was used in the pseudo-static analysis. It’s not clear what seismic design level (i.e., return period) 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 IBC) is typically used to analyze slopes and potential failures that will impact inhabited structures. A seismic event with a 7 percent probability of exceedance in 75 years (about 1,000- year return period in accordance with American Association of State Highway and Transportation Officials [AASHTO] and Washington State Department of Transportation [WSDOT]) is typically used to analyze slopes and potential failures that will impact structures such as retaining walls surrounding or supporting roadways. 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. Response The horizontal acceleration for the project site has been increased from 0.2g to 0.3g which represents one-half of the Peak Ground Acceleration (PGA) for the project. The analysis has been updated in the revised, referenced geotechnical report. 12220 113th Avenue NE, Ste. 130, Kirkland, Washington 98034  Phone (425) 821‐7777 • Fax (425) 821‐4334  Mr. Jordan Salisbury February 24, 2020 Project No. T-7886 Page No. 2 Comment #3 Groundwater, perched or regional, is not included in the stability analysis. The reported critical failure surfaces do occur at the contact between the upper weathered glacial till or fill and the lower unweathered glacial till. This contact is where perched groundwater is indicated in the report. In our opinion the perched groundwater should be included in the stability analyses at the locations indicated in the logs and as described in the text. If this perched groundwater condition is expected to be transient and is only expected to appear in response to recent rains, it would be appropriate in our opinion, to reduce the extent or omit the perched groundwater from the seismic analysis. Response We have revised the slope stability analysis to include a perched groundwater table at the contact between the weathered and unweathered till or between the unweathered till and silt soil layers. Based on our explorations, this groundwater seepage is transient and is only expected to be present during the wet winter months or following rain events. Therefore, we have not included the water in the seismic analysis. Comment #4 The analysis of section B-B’, at the protected slope, models the rockery as having “infinite strength” and the failure surface search feature limits the failure surface to an area above and below the rockery. Failure surfaces through the rockery were apparently not considered or not reported. In our opinion the analysis should either consider a failure surface through a rockery (modeled with an appropriate friction angle) or a separate calculation should be provided indicating that the internal stability of the rockery or other retaining system is not the critical failure mechanism. Additionally, both analyses might be prudent given the proximity to an inhabited structure. As noted in the report Section 4.9 “a rockery is not intended to function as an engineered structure.” Response The rockeries were placed around the site to support the vertical grade transitions that were necessary for the site grading. These walls were placed as holders to allow the site to be fully developed and then the final wall type would be selected based on the final grading and loading. Our slope stability analyses were focused on the overall stability of the slope and not necessarily the potential failure surfaces through the retaining walls since the wall designs were not completed and additional analysis of the walls would be required. We did evaluate the cross sections without the rockery walls but found the failure surfaces would remain isolated within the cut for the retaining wall. Since this was not the focus of our analysis, we moved the limits of the failure surfaces to more reasonable areas within the slope. With the finalized grading plan, we have been able to further evaluate the wall types at each location. For Cross Section B-B’ with the slope above and buildings below it was determined that a more robust wall system should be used. The rockeries along the toe of this slope have been revised to Gravity Redi-Rock walls. The design for these walls has been completed so that the slope stability analysis can continue to start and stop above and below the wall itself. The wall designs have been completed and are in a separate package. Mr. Jordan Salisbury February 24, 2020 Project No. T-7886 Page No. 3 Comment #5 We generally concur with the conclusion in Section 3.4.3 that slopes “are not at risk of a deep-seated failure in their current state.” However, proposed modifications in the Landslide Hazard Area consist of cutting into the toe of the slope and leaving the above slope unmodified. In our opinion this increases the risk that shallow surficial sloughs or even erosion could impact adjacent structures. We suggest that either the standard minimum setbacks be maintained or that additional details be considered to mitigate risk of run out damage to the adjacent structures. For example, increasing the height of the retaining wall above the ground surface to provide a catchment for surface sloughs and erosion. Response A chain-link fence could be added to the top of the wall to reduce shallow surficial sloughs or erosion from impacting the project structures. The need for these additional measures should be determined in the field during grading when clear views of the slopes will be available for evaluation. Comment #7 In Section 4.7, lateral earth pressures do not include pressures for the condition with slopes above the wall. This condition is shown in multiple locations in the plans. It is also not clear what seismic design level was used to develop the seismic loading. Different seismic loadings might be appropriate based on the location of the retaining walls and risks as described in comment No. 2, above. Response The revised geotechnical report has been updated to include earth pressures for walls with 2:1 (Horizontal:Vertical) slopes. The seismic design of 8H is based on a horizontal ground acceleration of 0.3g which is one-half of the Peak Ground Acceleration (PGA) for the site. Comment #8 The plans indicate many tall retaining walls close to structures. In our opinion, if the collapse of a retaining wall can significantly damage an inhabited structure, the wall must be designed to the same seismic design standards as the adjacent structure. Alternatively, analysis can be provided indicating that significant movement of the wall in the design seismic event will not impact the adjacent structure or that the structure would not be damaged if it is impacted. In our opinion, any retaining wall and structure that are closer horizontally than the vertical exposed height of the retaining wall should be evaluated with these criteria. Response The walls have been designed using a horizontal ground acceleration of 0.3g and an allowable movement of 4 inches. In our experience, these standards will allow the wall to flex during a seismic event but not collapse. This standard is used throughout the Pacific Northwest with success in protecting adjacent structures. Mr. Jordan Salisbury February 24, 2020 Project No. T-7886 Page No. 4 Comment #9 The plans indicate a tiered retaining wall system located at Tract C. A global slope stability analysis should be performed for this condition. Response Cross Section C-C’ in the revised geotechnical has been completed, as requested. Comment #10 In Alley 3, a storm drain and a sanitary sewer are shown behind the proposed MSE retaining wall. Utilities can be placed close to these walls or even incorporated into the reinforced section. However, replacing or connecting to these utilities can require demolishing and reconstructing the wall, in whole or in part. Wall details, once developed, should confirm that utility installation or replacement (including space for trenching) will not encroach on the reinforced soil section of the wall. Alternatively, if utilities must encroach on the reinforced zone, there could be a clear written understanding of how costs will be shared between whomever is responsible for maintaining or replacing the utilities and whomever is responsible for maintaining the retaining wall. Response The wall design has taken into consideration the proposed utilities and is shown on Figure 8 prepared for the wall design package. Comment #12 There are multiple locations, specifically Alley 3 and at the turn around on Alley 1, where a traffic barrier or guardrail will likely be required above a retaining wall. In these cases, the walls must be designed to withstand the design impact load from the guardrail and detailed to accommodate the guardrail. Response Agreed. The walls have not been designed for the traffic loading, instead, reinforcement will be included with the installation of the guard rail to prevent the loading from transferring from the guard rail to the retaining walls. Regarding the FLEX MSE walls. This wall type was proposed because the City of Renton requested that these walls be planted in order to soften the face and we have had great success with the FLEX MSE system. Due to the utility conflicts in the area, we have switched the walls to a gravity Redi-Rock system that will utilize the planting blocks to allow for greenery to be placed on the wall face. This design has been included in the wall design package.