1517 132ND ST SE 2024-12-09 - Laserfiche WebLink

Proposal for Geotechnical Engineering Services Proposed Bank: 1515 132nd Street Southeast, Everett, Washington March 15, 2019 4.0 INFILTRATION CONSIDERATIONS The native soils underlying the site consist of Vashon till, a dense to very dense deposit comprised of clay, silt, sand, and gravel. Till is generally not a suitable receptor soil for infiltration due to the high fines content and density. Where loosened by weathering infiltration rates of about 1/2 inch per hour may be obtained. Where unweathered, infiltration rates are typically less than 1/4 inch per hour. Perched groundwater, which can limit infiltration, also develops in till soils during wet season at the transition from weathered to unweathered soils. In summary, it is our opinion that non-infiltration alternatives should be considered to manage the surface runoff at the site. 5.0 GEOTECHNICAL RECOMMENDATIONS 5.1 SEISMIC DESIGN PARAMETERS The 2015 International Building Code (IBC) seismic design section provides a basis for seismic design of structures. Table 1 below provides seismic design parameters for the site that are in conformance with the 2015 IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1 —Seismic Design Parameters Spectral Spectral Design Spectral Site Control Periods Site Acceleration Acceleration Coefficients Response [sec.] Class at 0.2 sec. [g] at 1.0 sec. [g] Parameters Ss S1 Fa F,, SDs SDI To Ts C 1.404 0.545 1.000 1.300 0.936 0.472 0.101 0.504 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. Liquefaction Potential. Liquefaction is a process that can occur when soils lose shear strength for short periods of time during a seismic event. Ground shaking of sufficient strength and duration results in the loss of grain-to-grain contact and an increase in pore 19-049 1515 132nd St SE Page 5 PanGEO,Inc.