4800 DOGWOOD DR Geotech Report 2022-10-25

Subsurface Exploration, Geologic Hazards, View Ridge Community Church Addition and Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations Generally, there are three types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture, 2) liquefaction, and 3) ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 6.1 Surficial Ground Rupture The project area is located approximately 5 miles north of the suspected traces of the southeastward extension of the Southern Whidbey Island Fault Zone (SWIFZ). A recent study by the U.S. Geological Survey (USGS) (Sherrod et al., 2005, Holocene Fault Scarps and Shallow Magnetic Anomalies Along the Southern Whidbey Island Fault Zone near Woodinville, Washington, Open -File Report 2005-1136, March 2005) indicates that "strong" evidence of prehistoric earthquake activity has been observed along two fault strands thought to be part of the southeastward extension of the SWIFZ located southeast of the site. The study suggests as many as nine earthquake events along the SWIFZ may have occurred within the last 16,400 years. The recognition of this fault splay is relatively new, 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 one thousand years. Due to the suspected long recurrence interval it is our opinion that the potential for damage to the proposed addition by surficial ground rupture along the SWIFZ is considered to be low during the expected life of the proposed addition. Mitigation should consist of complying with the 2015 International Building Code (IBC) for this site. 6.2 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. Although ground water seepage and/or mottling indicative of possible seasonal saturation was encountered in our exploration boring, the encountered advance outwash stratigraphy has a low potential for liquefaction due to its dense state. The subsurface conditions encountered at the site pose little risk of liquefaction. The site is not mapped as an area of potential seismic August 29, 2016 ASSOCIATED EARTH SCIENCES, INC. SKL/Id-KE160351A2-ProjectsJ20160351�KE�WP Page 7