10901 27TH AVE SE Geotech Report 2025-07-29

' Proposed James Monroe Elementary School Replacement Subsurface Exploration, Geologic Hazards, andRevised Geotecltnical Engineering Report Everett, Washington Geologic Hazards and Mitigations 6.1 Surficial Ground Rupture ' Generally, the largest earthquakes that have occurred in the Puget Sound area are sub-crustal events with epicenters ranging from 50 to 70 kilometers in depth. Earthquakes that are generated at such depths usually do not result in fault rupture at the ground surface. However ' current research indicates that surficial ground rupture is possible in the South Whidbey Island Fault Zone. The South Whidbey Island Fault Zone is not thoroughly mapped or well understood. We are not aware of detailed maps of active faults in the project area. The best available mapping depicts multiple traces of the South Whidbey Island Fault, all oriented northwest-southeast and passing no closer than approximately one mile to the south of the site. We are available to discuss current research related to surface faulting in the project area. 6.2 Seismically Induced Landslides In our opinion, the risk of seismically induced landslides at this site is low due to the moderate inclination of site slopes and the presence of relatively dense soils at shallow depths. ' 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 is underlain by lodgement till that is very dense and not saturated. The liquefaction potential of the site is therefore low, in our opinion. We did not complete a detailed ' liquefaction analysis, and none is warranted, in our opinion. 6.4 Ground Motion ' Structural design of buildings should follow the current applicable building code. The applicable code at the time this report was written is the 2006 International Building Code ' (IBC). The site soils are consistent with 2006 IBC Site Class "C". The 2006 IBC seismic design parameters for short period (Ss) and 1-second period (Si) spectral acceleration values were determined by the latitude and longitude of the project site using the U.S. Geological Survey (USGS) software utility Seismic Hazard Curves and Uniform Response Spectra. The IJSGS software interpolated ground motions at the project site as follows for periods of 0.2 and October 5, 2009 ASSOCIATED EARTH SCIENCES, INC_ DWG/Id-KE090.312A.3-Projeccs1200903121KEIWP Page 7 1