607 RIVERSIDE RD Geotech Report 2022-10-25 (2)

1 March 22,2016 Project No.T-7388 A map titled Geologic Map of the Marysville Quadrangle, Snohomish County, Washington by J.P. Minard, dated 1957, shows the site soils mapped as Qyal, Younger alluvium. This map unit is consistent with the native soils found beneath the fills in our field observations. The preceding discussion is intended to be a brief review of the soil conditions observed at the site. More detailed descriptions of the subsurface conditions we observed are summarized on the Test Pit and CPT Logs attached in Appendix A. Logs of test borings advanced at the site by others are provided in Appendix B. The approximate locations of the test pits,CPTs,and test borings by others are shown on Figure 2. 3.3 Groundwater We observed heavy groundwater in 4 of the test pits at depths ranging from 5 feet to 10.5 feet below the existing site elevations. Dissipation tests at CPT-2 through CPT-5 determined hydrostatic groundwater levels of 10.4 to 12.8 feet below existing grades at those locations. We expect that the groundwater levels noted during our investigation are near their seasonal high. Groundwater levels would be expected to be lower during late summer to early fall(July through September). 4.0 SEISMIC Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength due to an increase in water pressure induced by vibrations. Liquefaction mainly affects geologically recent deposits of fine-grained sands underlying the groundwater table. Soils of this nature derive their strength from intergranular friction. The generated water pressure or pore pressure essentially separates the soil grains and eliminates this intergranular friction;thus,eliminating the soil's strength. We completed a liquefaction analysis using the computer program Liquefy 5 published by CivilTech Corporation. For the analysis, site-specific determinations of the soils' shear wave velocity and de-aggregated earthquake magnitude were obtained from USGS online map-based calculators. The analysis was completed using a ground acceleration of 0.35g, and an earthquake of Magnitude 7.0, which represents ground shaking that could be expected for an earthquake having a 10 percent probability of exceedance in a 50-year period (return period of once per 500 years). The results of the liquefaction analysis are attached in Appendix C. The results of our analysis indicate soil liquefaction could occur during the design earthquake event. Impacts to site structures and infrastructure should liquefaction occur would be in the form of subsidence with total settlements in the amount of 2 'h to 4 inches and differential movement in the range of 1 %to 2 inches expected. This amount of settlement would cause cosmetic cracking of the structure and pavements, but would not structurally impair its use, in our opinion. The project's structures will be located approximately 600 feet west of the Snohomish River. At this distance, there is no risk to the structures due to liquefaction-induced lateral spreading. I Page No. 3 I