5200 EVERGREEN WAY SEPA 017 - 010 Geotech Report 2025-04-23

1 Subsurface Exploration, Geologic Hazards, and Former Brien Ford Preliminary Geotechnical Engineering Report Everett, Washington Preliminary Design Recommendations III. PRELIMINARY DESIGN RECOMMENDATIONS 8.0 INTRODUCTION A portion of the site is underlain by existing fill that was encountered to a depth of 13.5 feet below the surface in exploration EB-1. Other explorations previously completed at the project site also encountered existing fill soils or ice-contact deposits near the surface. We anticipate that underpinning of existing foundations will be required in locations where the proposed excavation will undermine the supporting soil prism for existing foundations. Underpinning of foundations adjacent to the proposed excavation for environmental remediation will need to fully penetrate existing fill soils and ice-contact deposits in order to be supported by the Vashon advance outwash sediments. We have provided recommendations for underpinning in the form of small-diameter pipe piles in the "Foundations" section of this report. 9.0 SITE PREPARATION Existing or demolished paving, concrete and other construction debris, buried utilities, vegetation, or any other deleterious materials should be removed from areas near proposed pipe piles. Erosion and surface water control should be established around the project limits to satisfy local requirements. 9.1 Site Drainage and Surface Water Control The site should be graded to prevent water from ponding in construction areas and/or flowing into excavations. Exposed grades should be crowned, sloped, and smooth drum-rolled at the end of each day to facilitate drainage. If an effective drainage system is not utilized, project delays and increased costs could be incurred due to the greater quantities of wet and unsuitable fill, or poor access and unstable conditions. Surface water that comes in contact with potentially contaminated soils should be collected in a Baker Tank and properly treated. 9.2 Temporary and Permanent Cut Slopes In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction. For estimating purposes, however, we anticipate that temporary, unsupported cut slopes in unsaturated existing fill and unsaturated native ice-contact deposits or Vashon advance outwash sediments can be made at a maximum slope of 1.5H:1V (Horizontal:Vertical) or flatter. As is typical with earthwork operations, some sloughing and raveling may occur, and cut slopes may have to be adjusted in the field. If ground water seepage is encountered in cut slopes, or if surface water is not routed away from temporary cut slope faces, flatter slopes will be required. In addition, WISHA/OSHA regulations May 16,2017 ASSOCIATED EARTH SCIENCES,INC. AWR/Id-160594V003-2-Projects\201605941 KV\WP Page 9