SEISMIC RETROFIT OF BORED AND CUT AND COVERED TUNNELS

The purpose of this paper is to present the geotechnical earthquake engineering analysis and the “innovative” seismic retrofit solution of the Yerba Buena Island and Posey and Webster Street Tunnels located in the San Francisco Bay area. Yerba Buena Island Tunnel is bored through solid rock and the Posey and Webster Street Tunnels are parallel cut and cover tunnels. The tunnels are situated between two major faults, the Hayward Fault and the San Andreas Fault. A two-step analysis procedure was adopted to assess the performance of the tunnels liner subjected to a design earthquake. The first step was to compute seismic induced deformations of the tunnels without the presence of the liner due to seismic wave propagation through the rock and or soil medium. The second step involved imposing the deformations of the tunnel onto the structural liner through springs which accounted for interaction between the liner and surrounding medium. The objectives of the analysis were to estimate the responses of the as-built and retrofit configurations of the tunnels due to seismic excitations. The 1989 Loma Prieta earthquake was the most recent major seismic event that occurred near the Yerba Buena Island and the Posey and Webster Street Tunnels. The earthquake damage to the east span of the San Francisco-Oakland Bay Bridge resulted in a number of injuries and the bridge was closed to traffic for one month. The Yerba Buena Island Tunnel is a link between the east span and west span of the Francisco-Oakland Bay Bridge. Therefore, the Yerba Buena Island Tunnel seismic performance criteria was established to be consistent with that of the San Francisco-Oakland Bay Bridge. The Post Earthquake Investigation report indicated also evidence of liquefaction on the Island of Alameda and in the vicinity of the Posey and Webster Street Tunnels.