10200 19TH AVE SE COSTCO FUEL STATION Geotech Report 2022-10-25

GEOTECHNICAL ENGINEERING STUDY Costco Wholesale c/o Barghausen Consulting Engineers E-8556 January 13, 1999 Page 7 Seismic Design Considerations The Puget Sound region is classified as Zone 3 by the Uniform Building Code (UBC). The largest earthquakes in the Puget Sound region have been subcrustal (intraplate) events, ranging in depth from fifty (50) to seventy (70) kilometers. Such deep events have exhibited no surface faulting. Weaver and Shedlock (1989) researched the probable or known source areas for the crustal, intraplate, and subduction zone earthquakes in the Washington and Oregon area. Crustal and intraplate earthquakes are the only events in Washington and Oregon in which there is a historical record. Shallow crustal earthquakes occur within the North American Plate, and typically do not exceed focal depths of approximately 20 kilometers. Intraplate earthquakes occur in the subducting Juan de Fuca plate, and typically occur below depths of 40 kilometers. The subduction zone earthquake, in which there is no historical record in the Washington and Oregon area, would have its source along the interface between the North American Plate and the subducting Juan de Fuca Plate. Magnitude 8+ earthquakes are thought to be possible along this interface, and would occur at depths of approximately 50 to 60 kilometers (Weaver and Shedlock, 1989). The UBC Earthquake regulations have established a series of soil profile types that are used as a basis for seismic design of structures. Based on the encountered soil conditions, it is our opinion that soil type Sc from Table 16-J of the 1997 UBC should be used for design. Liquefaction is a phenomenon in which soils lose all shear strength for short periods of time during an earthquake. The effects of liquefaction may be large total and/or differential settlement for structures with foundations founded in the liquefying soils. Groundshaking of sufficient duration results in the loss of grain-to-grain contact and rapid increase in pore water pressure, causing the soil to behave as a fluid for short periods of time. To have potential for liquefaction, a soil must be cohesionless with a grain size distribution of a specified range (generally sands and silt); it must be loose to medium-dense; it must be below the groundwater table; and it must be subject to sufficient magnitude and duration of groundshaking. Based on the soil and groundwater conditions observed at the site, it is our opinion that the site has a low susceptibility to liquefaction. The presence of dense to very dense soil conditions at the site is the primary basis for this conclusion. Earth Consultants, Inc.