The Yerba Buena Island Tunnel links the I-80 freeway east and west spans of the San Francisco-Oakland Bay Bridges. The tunnel consists of a reinforced concrete liner with two decks 65 ft wide cast against hard jointed Franciscan rock. Due to a vital access to the entire Bay Bridge, a peer-reviewed retrofit program was funded by the California Department of Transportation to study the seismic vulnerability of the tunnel. The primarily concern was a potential separation of the upper deck from the tunnel liner.

Geotechnical Investigation
For this evaluation, EMI conducted a comprehensive field investigation program to study in-situ rock conditions and behavior of the surrounding rock. Spectral Analysis of Surface Wave (SASW) geophysical testing was performed along the tunnel to understand the rock engineering properties behind the liner.

Liner Performance
Based on the rock properties observed, sensitivity analyses were conducted to evaluate seismic liner performance in two steps. The first step was to compute seismic-induced deformations of the tunnel subjected to seismic wave propagation through the island rock without the presence of the concrete liner. The second step involved imposing the deformations of the tunnel onto the structural liner through spring elements that accounted for interaction between the liner and the surrounding rock. From the studies, performance of the existing tunnel supports was found to be acceptable.
 

Portal Slope Stability
To evaluate seismic stability at the west portal slope, Key Block theory in conjunction with a Newmark-type analysis was used to assess movements of a potential failure wedge. For the east portal slope, Discontinuous Deformation Analysis (DDA) was used to evaluate a rotational mode of failure.