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Geotech 3rd party review - creekwood - NV5 - 5-1-23 19201 120TH AVENUE NE, SUITE 201 | BOTHELL, WA 98011 | WWW.NV5.COM | OFFICE 206.838.9900 May 1, 2023 City of Federal Way Department of Community and Economic Development 33325 8th Avenue South Federal Way, WA 98003 Attention: Evan Lewis, Associate Planner Geotechnical Third-Party Review of 2020 Creekwood Development Submittal Creekwood Development 31000 Block of 22nd Avenue SW and 21st Avenue SW Federal Way, Washington File #22- 105973-SU (Plat), 22-105974-SE (SEPA) Project: FederalWay-1-05 INTRODUCTION This letter presents a summary of our geotechnical review of geotechnical-related documents submitted to support a new application for the proposed Creekwood Residential Plat Development. We previously provided a review of the 2014, 2017, and 2019 development submittals, which have either been withdrawn or expired. The information provided with the new submittal and provided for our review includes the following:  GeoResources, LLC, 2022. Updated Geotechnical Engineering Report; Proposed Creekwood Residential Plat; Federal Way, Washington, dated December 20, 2022  Previous geotechnical reports provided for earlier submittals include the following:  GeoResources, LLC, 2020. Geotechnical Engineering Report Addendum; Revised Ravine Crossing and Comment Response Letter; Creekwood Residential Plat; 31000 Block of 22nd Avenue SW; Federal Way, Washington, dated October 23, 2020  GeoResources, LLC, 2014. Geotechnical Engineering Report; Proposed Creekwood Residential Plat; XXX 22nd Avenue SW/21st Avenue SW; Federal Way, Washington, dated February 24, 2014  GeoResources, LLC, 2014. Updated Geotechnical Engineering Report; Proposed Creekwood Residential Plat; XXX 22nd Avenue SW/21st Avenue SW; Federal Way, Washington, dated December 1, 2014 2 FederalWay-1-05:050123  GeoResources, LLC, 2017. Updated Geotechnical Engineering Report; Creekwood Residential Plat; Federal Way, Washington, dated April 28, 2017  Associated Earth Sciences Inc., 2017. Geotechnical Evaluation; Creekwood Plat; Federal Way, Washington, dated May 12, 2017  Barghausen Consulting Engineers, January 3, 2023. Creekwood, Preliminary Plat Plan Sheets 1 through 12  Sheet 1 – Cover Sheet  Sheet 2 – Existing Conditions Map  Sheet 3 – Preliminary Early Clear and Grade Map  Sheet 4 – Preliminary Plat Map  Sheet 5 – Preliminary Open Space Plan  Sheet 6 – Preliminary Slope Analysis Exhibit  Sheet 7 – Preliminary Grading and Drainage Plan  Sheet 8 – Preliminary Vault Plan  Sheet 9 – Preliminary Sewer and Water Plan  Sheet 10 – Preliminary Road Profile  Sheet 11 – Preliminary Ravine Crossing Detail  Sheet 12 – Preliminary Landscape Plan (Details)  Barghausen Consulting Engineers, 2023. Preliminary Plat of Creekwood, Preliminary Tree Retention Plan, Sheet T-1  Barghausen Consulting Engineers, 2023. Preliminary Plat of Creekwood, Landscaping Plan Sheets L1 through L4 The proposed development now includes the following elements:  The proposed road crossing of the northern tributary to Lakota Creek, identified as Drainage “Y”, will include the following:  Re-grading of the ravine slopes, which will include cuts to decrease the slope inclination on both sides of the ravine. The area of the proposed grading is within an existing Landslide Hazard Area.  Placement of a fill embankment along the base of the ravine north of the creek emergence to support the new retaining wall.  Construction of a cantilever cast-in-place concrete retaining wall and continued fill placement behind the new wall to support the new road. The proposed retaining wall will be up to approximately 16 feet in height and plans indicate it will be designed in accordance to City Federal Way General Way Public Works and Washington State Department of Transportation standards.  The previously proposed open detention pond on the north portion of the site has been replaced with a non-infiltrating underground detention vault.  The grading plan includes the following:  The north property line of Tract “A” includes a slope cut along the north property line of up to 8 feet high that will be supported by a proposed rockery. The area is outside of and not adjacent to any Landslide Hazardous Areas. 3 FederalWay-1-05:050123  The excavation for the vault will extend to an elevation of 213 feet. We anticipate this will require a cut slope at of least 1H:1V to remain within the north property line. After installation of the vault, the slope will be reconstructed to 2H:1V to establish the surface grade of the drainage facility open space. The area is outside of and not adjacent to any Landslide Hazardous Areas.  Cuts along the north property line of Tracts 6 through 12 are proposed to establish a level lot area. Cuts of up to 6 feet are planned and proposed. A rockery is proposed to support the cut on Lots 7 through 10 and a set of tiered rockeries along Lots 11 and 12. The area is outside of and not adjacent to any Landslide Hazardous Areas.  A proposed sidehill fill embankment of up to approximately 6 feet along the west side of Lots 12, 13, and 14 sloped at 2H:1V. The area is outside of and not adjacent to any Landslide Hazardous Areas.  A proposed sidehill fill embankment on the south side of Lots 15 and 16. A set of three tiered retaining walls, each up to 6 feet high, are proposed to support a sidehill fill embankment. The fill embankment will raise the exiting site grade to establish level building areas. The area is adjacent to the 25-foot Landslide Hazard Area buffer area.  A retaining wall, less than 4 feet in height, is planned to support grading on the south side of Lot 14 and Tract “F”. The area is adjacent to the proposed 25-foot Landslide Hazard Area buffer.  A buffer width of 25 feet is proposed between the lots and the areas identified as Landslide Hazard Areas. The purpose of our geotechnical services was to review the submitted geotechnical engineering information and submitted development plans for conformance with the requirements of Federal Way Revised Code (FWRC) 19.145 with regards to Article II, Geologically Hazardous Areas. As part of our review, we visited the site on March 29, 2023, to observe the existing site conditions. COMPLIANCE REVIEW SUMMARY Our review of the submittal materials is organized similar to the FWRC code. Provided below are our review comments with reference to the applicable codes. FWRC 19.145.080 CRITICAL AREA REPORT The submittal includes plans that identify the areas meeting the FWRC 19.05.070 definition of Landslide Hazard Areas, the proposed buffer, and the proposed site grading. The slopes meeting the FWRC definition of Landslide Hazard Area are identified on Sheet 6 – Preliminary Slope Analysis Exhibit. The “severe” Erosion Hazard Area is identified in the report as including the sloped portions of the site bordering the upland area. The submitted information appears to generally satisfy the requirements for a site-specific Critical Areas Report as required by FWRC 19.145.080, although additional geotechnical information is required to address excavation for the proposed detention vault, the sidehill fill embankment on the south side of Lots 15 and 16, and lot surface water drainage. 4 FederalWay-1-05:050123 FWRC 19.145.080 – Additional Information Requirements Additional geotechnical information is required for the following:  Detention Vault Tract “B”: The proposed detention vault excavation will be approximately 39 feet deep below existing site grades to establish a vault floor elevation of 215 feet. The cut will be up to approximately 39 feet deep below existing site grades and will be adjacent to the north property line. The slope stability impacts associated with the temporary cut on the north side of the detention vault in Tract “B” should be addressed in the geotechnical report.  Tiered Retaining Walls Lots 15 and 16: A fill embankment is proposed on the south side of Lots 15 and 16 adjacent to the 25-foot Landslide Hazard Area buffer. Three tiered retaining walls, each up to 6 feet high with 5 feet of horizontal separation, are proposed to support a sidehill fill embankment. The updated geotechnical report (GeoResources, 2022) did not address the tiered wall or fill placement on Lots 15 and 16. Additional information and recommendations are required and should specifically address wall embedment, tier setbacks, and the slope of the bench between the walls and include an evaluation of the impacts on slope stability.  Individual Lot Drainage: The submitted plans did not include a detailed drainage plan addressing how surface water runoff from each proposed lot will be managed. This information is necessary and should be provided to the geotechnical engineer to review and confirm that it will not impact slope stability of the adjacent Landslide Hazard Areas. FWRC 19.145.130 MITIGATION SEQUENCING The proposed development appears to minimize impacts to the adjacent Landslide Hazard Areas and does not adversely impact the existing slope stability based on the slope stability analyses that addressed most of the site areas, except as identified above. Lot Layout: The lot layout and the lot size have been configured so that they do not extend into the proposed 25-foot Landslide Hazard Area buffer. Drainage “Y” Crossing: The proposed Drainage “Y” crossing includes significant grading of the upper ravine area along the proposed road alignment that will increase slope stability and reduce the erosion hazard associated with the existing ravine slopes. The proposed grading extends into the Landslide Hazard Area and will result in decreased slope inclinations. The road alignment is along the north edge of the property, which helps minimize impacts to the Landslide Hazard Area. The slope stability analyses provided in the geotechnical report indicate that the proposed grading for the crossing of Drainage “Y” and the encroachment into the Landslide Hazard Area will result in improved slope stability of the area and reduce the erosion hazard. The stability analyses are generally consistent with the Standard of Practice for the area and have used reasonable soil parameters in the analyses. 5 FederalWay-1-05:050123 The crossing of Drainage “Y will provide a secondary access road into the development. The road appears to be a shared benefit as it will provide a secondary access point to the properties north of the proposed development. The proposed Drainage “Y” crossing can be considered for a partial exemption to FWRC 19.145 under FWRC 19.145.120. Site Grading: Proposed site grading activities indicated on the preliminary clearing and grading plan (Sheet 3) will generally be outside of the recommended 25-foot buffer throughout most of the development, except in the vicinity of the Drainage “Y” ravine crossing, as discussed above. Retaining Walls: Retaining walls and rockeries are proposed to address slope cuts and to support fill embankments required for the project. The slope cuts and fills are generally outside of Landslide Hazard Areas, except for the grading and cast-in-place wall construction proposed for the Drainage “Y” crossing, as described above. Rockeries are proposed at several locations and additional information or revisions to them are necessary to address FWRC requirements. FWRC 19.145.130 – Additional Information Requirements Revisions or additional information associated with the following retaining walls or proposed rockeries are required to address conformance with FWRC 19.145 and FWRC 19.120.120. Tract “A” Rockery: An up to 8-foot-high rockery is proposed along the north property line of Tract “A”. The proposed rockery is not in compliance with FWRC 19.120.120 and will impact adjacent properties. An engineered retaining wall, such as a large-block gravity wall, within the future right-of-way should be used to support the slope cut and to avoid long-term maintenance associated with rockeries. Plans should be revised to address this wall. FWRC 19.145.150 CRITICAL AREA TRACTS AND DESIGNATION ON SITE PLANS The Landslide Hazard Areas are identified on the project plans and are generally contained within tracts labeled as “Tract C (Critical Area).” FWRC 19.145.230 LANDSLIDE HAZARD AREA PROTECTION MEASURES The FWRC allows the standard buffer width of 50 feet to be reduced through the critical areas report based on recommendations by the geotechnical engineer. The submitted geotechnical report recommends a “buffer/setback” of 25 feet between the lots and adjacent slopes classified as Landslide Hazard Areas. The combined 25-foot buffer is less than the 50-foot, FWRC-prescribed buffer, but it is supported by the findings and recommendations of the geotechnical engineering report. The 25-foot buffer appears sufficient for the area based on the site conditions and slope stability analyses. FWRC 19.245.240 EROSION AND SEISMIC HAZARD AREAS PROTECTION MEASURES The geotechnical report provided recommendations to reduce the temporary erosion hazard during construction and indicated the site has low susceptibility to seismic hazards, as it is underlain by dense glacial deposits. 6 FederalWay-1-05:050123 FWRC 19.245.250 ADDITIONAL REPORT REQUIREMENTS – GEOLOGICALLY HAZARDOUS AREAS If the project is approved, we recommend the following stipulations be required by the director with regards to geotechnical engineer of record oversight. The geotechnical engineer should be required to review the final plans for conformance with their recommendations to reduce erosion and avoid adverse impacts to slope stability and, as part of the review, provide a minimal risk statement related to the development impacts on adjacent geologic hazard areas and nearby properties. Geotechnical observation of sufficient daily duration and oversight, by the geotechnical engineer of record, should be required throughout construction and development on each individual lot in accordance with FWRC 19.145.240 (2). The geotechnical engineer should be tasked with observation of the following:  Temporary erosion and sediment control element installation and performance, as well as verification that the measures are being maintained  Excavation conditions and temporary slope stability  Fill placement and compaction verification  Retaining wall and permanent slope construction, including subgrade preparation and placement and compaction of fill  Subgrade bearing verification below structures and hardscape elements  Utility installation  Subsurface drain installation  Confirm surface water drainage will not impact slope stability  Permanent erosion and sediment control stabilization  Preparation of a project close-out letter summarizing the earthwork activities and conformance with project plans, specifications, and geotechnical recommendations    7 FederalWay-1-05:050123 We appreciate the opportunity to provide our services on this project. Please call if you have questions regarding this submittal. Sincerely, NV5 Kevin J. Lamb, P.E. Principal Engineer KJL:kt One copy submitted Document ID: FederalWay-1-05-050123-geol.docx © 2023 NV5. All rights reserved. Signed 05/01/2023