2202 100TH ST SW Geotech Report 2022-10-25

Subsurface Exploration, Geologic Hazard, and Stephens Everett Property Preliminary Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations and data pertaining to it are limited, with the studies still ongoing. The recurrence interval of movement along this fault system is still unknown, although it is hypothesized to be in excess of 1,000 years. Due to the suspected long recurrence interval, it is our opinion that the potential for damage to the proposed structures by surficial ground rupture is considered to be low. No mitigations other than complying with 2015 International Building Code (IBC) seismic design recommendations are recommended. 6.2 Seismically Induced Landslides It is our opinion that the risk of damage to the proposed structures by seismically induced landsliding during a design-level seismic event is low. 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. The site soils observed in our explorations either consisted of dense lodgement till or Vashon recessional outwash consisting of silty sands. The subsurface soils at the site pose low risk to liquefaction due to the relative density of the Vashon lodgement till and limited vertical extent of saturated recessional outwash. No quantitative liquefaction analysis was completed for this study, and none is warranted in our opinion. 6.4 Seismic Site Class (2015 International Building Code) In our opinion, the subsurface conditions at the site are consistent with seismic Site Class "D" in accordance with the 2015 International Building Code (IBC), and the publication American Society of Civil Engineers ASCE 7 referenced therein, the most recent version of which is ASCE 7-10. March 29,2019 ASSOCIATED EARTH SCIENCES,INC. TG/ms-180567E001-2-Projects1201805671KE\WP Page 8