2320 RAILWAY AVE Geotech Report 2023-02-01

Interwest Construction Co.-Everett Contractor Yard Site Improvements, Everett, WA March 12,2021 Geotechnical Engineering Report P2184-T20 Underlying this material, we first encounter alluvial sands, which were relatively clean, but encountered in a loose in situ condition. These deposits were 41/2 to 6 feet thick across the southern portion of the project area, and greater than 6-feet-thick across the northern portions of the site. Along the southern portion of the site,granular soils transitioned to fibrous peat,which extended through the termination of test pit explorations TP-1 and TP-2;a maximum depth of 13 to 14 feet. Peat was not encountered through the termination of our northern explorations,but may be present at greater depth. In the Geologic Map of the Everett 7.5-Minute Quadrangle,Snohomish County, Washington,published by the United States Department of the Interior(USDI) (1985), the project area is mapped as Qyal,or younger alluvium deposited during the Holocene. These deposits lie in and along present streams and are partly subject to seasonal flooding. The sediments consist mostly of unconsolidated, stratified,gray to brown clay,silt,and very fine sand to fine sand,with abundant organic material. The younger alluvium can be anywhere from 1 meter to possibly 20 meters thick and is mostly poorly drained. Peat deposits and buried logs and stumps are common in the flood plain of the Snohomish River. Our field observations generally conform with the site classifications prepared by the USDI. The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata encountered in our subsurface explorations. 3.3 Groundwater Conditions We encountered groundwater seepage in each of our explorations at depths 10 to 11 feet below existing grade. We understand this level corresponds with field observations of Interwest personnel. Given the fact that our explorations were performed outside of what is generally considered the "rainy season", groundwater levels could rise slightly higher than that which we ' observed. Regional groundwater levels are expected to fluctuate with localized precipitation and geological conditions. 3.4 Seismic Conditions The site is in the Puget Sound basin which has experienced several earthquakes. A detailed description of the regional seismicity is beyond the scope of this report,however,previous regional earthquakes can be split into two general categories: 1.) large earthquakes with a moment magnitude greater than 8.0(Mw>8.0),and 2.)modest size earthquakes with a moment magnitude generally less than 7.25(Mw<7.25). In all cases, the thickness of the soil between the bedrock and the ground surface can change (usually amplify) the seismically induced ground motions and therefore the inertial loads acting on surface structures. "Site Class" is a classification system used by the IBC and ASCE 7 to provide some insight to the potential for ground motion amplification. The site class is based on the properties of the upper 100 feet of the soil and rock materials at the site. MGI used a combination of onsite explorations and our review of the geologic mapping of the site to derive a site class for the site. Based on evaluation and the definitions of Site Class as provided in Table 20.3-1 of ASCE 7-16(as required by the 2015 International Building Code), the soil conditions on this site satisfy the definition of Site Migizi Group, Inc. Page 4 of 16 NtIGIZI I�I�I�I i