4730 COLBY AVE YMCA Geotech Report 2025-04-14

Subsurface Exploration, Geologic Hazard, and Snohomish County YMCA Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations 6.2 Seismically Induced Landslides It is our opinion that the potential risk of damage to the proposed development by seismically induced slope failures is low during a design-level seismic event. No quantitative slope stability analysis was completed, and none is warranted for the project as currently proposed, in our opinion. 6.3 Liquefaction Liquefaction is a process through which unconsolidated soil loses strength as a result of vibrations, such as those which occur during a seismic event. During normal conditions, the weight of the soil is supported by both grain-to-grain contacts and by the fluid pressure within the pore spaces of the soil below the water table. Extreme vibratory shaking can disrupt the grain-to-grain contact, increase the pore pressure, and result in a temporary decrease in soil shear strength. The soil is said to be liquefied when nearly all of the weight of the soil is supported by pore pressure alone. Liquefaction can result in deformation of the sediment and settlement of overlying structures. Areas most susceptible to liquefaction include those areas underlain by non-cohesive silt and sand with low relative densities, accompanied by a shallow water table. Our explorations suggest that the potential risk of damage to the proposed development by liquefaction is low, due to the dense, glacially consolidated sediments observed at shallow depths. 6.4 Ground Motion/Seismic Site Class(2015 International Building Code) Structural design of the building should follow 2015 IBC standards. We recommend that the project be designed in accordance with Site Class "D" as defined in Table 20.3-1 of American Society of Civil Engineers (ASCE) 7-10- Minimum Design Loads for Buildings and Other Structures. 7.0 EROSION HAZARDS AND MITIGATION Project plans should include implementation of temporary erosion controls in accordance with local standards of practice. Control methods should include limiting earthwork to seasonally drier periods, typically April 1 to October 31, use of perimeter silt fences, stabilized construction entrance, and straw mulch in exposed areas. Removal of existing vegetation should be limited to those areas that are required to construct the proposed warehouse, and new landscaping and vegetation with equivalent erosion mitigation potential should be established as soon as possible after grading is complete. During construction, surface water February 26,2018 ASSOCIATED EARTH SCIENCES,INC. AWR/ms-170671E001-2-Projects12 01 706 71\KEIWP Page 8