1707 SCENIC DR Geotech Report 2022-10-25

{ t t Snohomish County Public Utility District No.1 March 31,2015 Page 2 SUBSURFACE CONDITIONS Borings B-8 and B-9 were drilled in the vicinity of the planned wall alignment.The subsurface soil conditions along the proposed wall alignment consist of fill overlying dense native glacial till. Glacial till is anticipated near the bottom elevation of the east end of the wall,as observed in boring B-9. However,fill consisting of very loose to medium dense sand with varying amounts of silt and gravel was observed to a depth of about 4.5 feet in boring B-8 located near the west end of the proposed wall.The observed fill appears to typically consist of reworked glacial till soil-placed during past grading at the site.The glacial till soils consist of very dense/hard silty sand and sandy silt with gravel. Groundwater is anticipated to be perched within the fill soils near the contact with the underlying native dense glacial till. DESIGN RECOMMENDATIONS Analysis Method • Engineering analysis to develop the design recommendations for the MSE wall was completed using the commercial computer program MSEW (3.0) developed by ADAMA Engineering, 2010. Our engineering analysis was completed in accordance with the National Concrete Masonry Association (NCMA) design recommendations. Subgrade Preparation The wall subgrade should be prepared as described in the "Subgrade Preparation" section of the. October 24,2014 geotechnical report and per Sheet S-10A C38 of the project drawings.If soft silt,organic, or otherwise unsuitable soils are encountered at the wall subgrade we recommend these soils be removed and replaced with structural fill. We anticipate that dense glacial till will be exposed at wall subgrade level near the east end. However,we anticipate that existing loose fill soils will be exposed at the subgrade elevation near the west end of the wall. Therefore, the contractor should be prepared to over excavate up to 2 feet of unsuitable soils from below the western and middle portions of the wall.The unsuitable soils should be replaced with properly compacted imported structural fill.The structural fill zone should extend horizontally beyond the face of the wall by the depth of the excavation. The exposed subgrade should be compacted prior to placing the structural fill. • Soil Properties The design parameters summarized in Table 1 were used for design of the proposed MSE wall.The fill and glacial till soil parameters are based on explorations completed at the site,our experience in the area,and are summarized in Table 1. The design values shown in Table 1 assume the backfill soils in the reinforced zone are compacted to at least 95 percent maximum dry density(MDD) per ASTM D 1557. Wall backfill within the reinforced zone will consist of imported Gravel Borrow as described in Section 9-03.14(1) of the Washington State Department of Transportation (WSDOT)Standard Specifications. GEOENGINEER. File No.0482-027-02