1210 W MARINE VIEW DR Geotech Report 2022-10-25

Proposed Port Gardner Substation Project No. 1779.01 15 November 2017 j Geoprofessional Consultants Soil Liquefaction Liquefaction is a phenomenon wherein saturated cohesionless soils build up excess pore water pressures during earthquake loading. Liquefaction typically occurs in loose soils,but may occur in denser soils if the ground shaking is sufficiently strong. ZGA completed a liquefaction analysis in general accordance with Section 1803.5.12 of the 2015 IBC and Section 11.8.3 of ASCE 7-10. Specifically, our analysis used the following primary seismic ground motion parameters. • A Maximum Considered Earthquake Geometric Mean (MCEG) Peak Ground Acceleration of 0.530g, based on Figure 22-7 of ASCE 7-10. • A Modified Peak Ground Acceleration (PGAM)of 0.477g based on Site Class E, per Section 11.8.3 of ASCR7-10 (Site Class modification to MCEG without regard to liquefaction in accordance with Sections 11.4.7 and 20.3.1 of ASCE 7-10). • A Geometric Mean Magnitude of 6.73 based on 2008 USGS National Seismic Hazard Mapping Project deaggregation data for a seismic event with a 2% probability of exceedance in 50 years (2,475-year return period). Our liquefaction analysis was completed using the computer program LiquefyPro Version 5.8. Our analysis was based on boring B-1 completed to a depth of 54 feet and site-specific laboratory tests. The approximate exploration location is shown on the enclosed Site and Exploration Plan, Figure 1. Our analysis indicates the potential for liquefaction from the groundwater table(about 3%feet below grade) to a depth of about 31%feet.Appendix C contains the liquefaction analysis output graphic. Liquefaction Settlement Based on our analyses,we estimate a total seismic settlement of approximately 6 to 8 inches. Potentially liquefiable soils are located a few feet below the ground surface. As such, soil liquefaction may be expressed at the ground surface as sand boils,ground cracks,and vertical settlements. Given the shallow nature of potentially liquefiable soils,we estimate a differential seismic settlement range of 41/2o 6 inches over a horizontal distance of 50 feet. Erosion Hazard: The project site lacks significant slopes and is mantled by fill material. The site does not meet the criteria for an erosion hazard area as described in EMC 19.37.080.A.3. Landslide Hazard: The project site and immediate vicinity lack significant slopes. The nearest slope of significant height(about 80 feet)is located approximately 150 feet to the east and is comprised of Vashon glacial till, advance outwash, and older glacially consolidated soils. Consequently, the risk of landsliding impacting the site is low and the site does not meet the criteria for a landslide hazard as described in EMC 19.37.080.A.1, in our opinion. Page l5