1901 W MUKILTEO BLVD Geotech Report 2022-10-25

Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 7 Due to the presence of loose, undocumented fill soils within the yard area and within the vicinity of the top of the steep slope, it is also our opinion that these soils may experience continued settlement and failures potentially impacting the back yard area and residence, especially during a seismic event or periods of extended rainfall even after the proposed grading activities along the top of the steep slope are performed. If this potential can be understood and managed by you and future property owners, we do not anticipate the need for elaborate stabilization measures at this time along the upper portion of the slope, provided our grading and drainage recommendations as discussed in this report are strictly followed. If any indications of settlement or movement are observed after the proposed improvements are constructed, we recommend that NGA be promptly notified to evaluate and provide stabilization recommendations. If the risk of future failures along the top of the slope cannot be tolerated, it is our opinion that construction of a soldier pile retaining wall along the top of slope is the only feasible method to permanently stabilize the top of slope area along the yard area and limit potential impacts associated with potential slope movements on the yard area and residence. Due to site constraints, and based on our explorations and site observations, we recommend the use of a"light"retaining wall to protect the top of the slope along the entire extent of the yard area, along with erosion control and drainage systems to be utilized below the new retaining wall. We have provided recommendations for design and installation of a new soldier pile wall with timber lagging and tie-back anchors to protect the top of the slope. The anchor system should consist of grouted tiebacks anchors or mechanical helical anchors. The approximate location of the proposed retaining wall along with a schematic detail of the proposed new wall are shown on the Schematic Stabilization Site Plan and Schematic Retaining Wall Details in Figures 7 and 8,respectively. Detailed recommendations regarding the proposed retaining wall construction have been included in the Retaining Wall Design and Construction subsections of this report. Our explorations within the proposed addition area encountered approximately 7.5 feet of undocumented fill soils with medium dense or better glacial soils at depth. These glacial soils should provide adequate support for the addition foundation loads. Due to the limited access and depth of the undocumented fill within the proposed residence addition area, it may be unfeasible to remove all of the unsuitable soils from this area and extend the foundations down to the underlying medium dense or better soils at depth. To minimize the need for a deep over-excavation,we recommend that the proposed residence addition be supported on deep foundations consisting of 2-inch driven steel pin piles to transfer building loads through the loose undocumented fill soils to the underlying medium dense or better native glacial soils. Recommendations for the deep foundation design and installation are included in the Foundation NELSON GEOTECHNICAL ASSOCIATES, INC.