Seismic Design Analysis of a Shallow-Buried Subway Station in Liquefiable Soils

Abstract eng:
During the recent design of a 16.3-mile (26.2-km) long rapid transit project which extends the Bay Area Rapid Transit system into Santa Clara Valley and San Jose areas in northern California, shallow-buried (cut-and-cover) box structures were proposed for the subway stations. The Downtown San Jose Station is located in alluvial soils including layers of saturated sand with corrected SPT N-value of 14 to 27, indicating high potential of liquefaction subject to the design earthquake. To characterize the site, a field exploration program was performed which consisted of cone penetration test soundings and drilled borings with standard penetration tests. Laboratory tests were assigned on selected soil samples to develop soil properties including cyclic triaxial testing to determine the cyclic strength of the layers susceptible to liquefaction. This paper focused on geotechnical hazards and seismic design methods, geotechnical design parameters, a practical-oriented pore pressure generation model, and seismic time-history design analyses of the subway structure. The analysis results indicated that the ratio of the maximum seismic racking deformation versus the height of the box structure is approximately 1 percent, which compares to about 0.4 percent if liquefaction would not occur. It was recommended that such a large racking deformation due to liquefaction should be designed with adequate structural ductility and strength; rather than more expensive ground improvements in the Downtown area.

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