ESBGU <br />SEISMIC DESIGN <br />Seismic Response Spectrum <br />A site response spectrum was developed for the site in accordance with the Oregon <br />Structural Specialty Code (OSSC 2014), which is based on Section 1613 of the <br />International Building Code (IBC 2012). The design maximum considered <br />earthquake ground motion maps in the IBC (2012) are based on modified <br />USGS (2008) maps with a 1% probability of exceedance in 50 years (i.e., a ±4,975-year <br />return period). The modifications include factors to adjust the spectral accelerations <br />to account for directivity and risk. <br />The subsurface profile consists of fine-grained soil followed by gravel. Based on <br />these conditions, a Site Class D is appropriate for design. The seismic design <br />parameters and OSSC response spectrum are shown on Figure 4A (Appendix A). <br />Liquefaction <br />Liquefiable soils typically consist of saturated, loose sands and non-plastic or low <br />plasticity silt (i.e.a PI of less than 8). Loose sand was encountered in BH-1 and TP- <br />8 to depths ranging from ±11.5 to 12feet. The loose soil maybe susceptible to <br />liquefaction if a moderate to large magnitude earthquake were to occur during the <br />wet winter and spring months. However, based on the limited thickness of the <br />saturated soil, we anticipate the liquefaction induced settlement would be ±2 inches <br />or less. The loose sand was encountered in our explorations outside the building <br />and pavement areas. Therefore, we believe the liquefaction risk is relatively low. <br />ENGINEERING ANALYSIS AND DISCUSSION <br />Building Foundations <br /> We anticipate the building foundations will bear on medium stiff <br />to stiff silt. We estimated the bearing capacity for the foundations assuming an <br />undrained shear strength of ±0.6 tsf and a moist unit weight of 115 psf for the <br />foundation soil. The calculations indicate an allowable bearing pressure of ±2,000 <br />psf with a typical factor of safety of 3. The recommended bearing pressure is <br />conservative for footings built on the existing bar-run fill. This bearing pressure <br />may be increased by one-third for transient (seismic and wind) loads. <br /> We anticipate the planned building will be a relatively light, single-story <br />structure. Based on the stiffness of the soils we anticipate the foundation <br />settlement will be modest. For design, we recommend assuming total and <br />differential settlement of ±½inch and ±¼ inch, respectively. <br /> <br />Santa Clara Transit Center January 15, 2019 <br />Geotechnical Investigation 4 Project 2181060 <br />Eugene, Oregon Rowell Brokaw <br />