Posey and Webster Tube Rehabilitation ProjectSeismic Evaluation and Retrofit Design of Immersed Tubes
The Posey and Webster Street Tubes provide pedestrian and vehicular connection between Alameda Island and the city of Oakland, California. As part of Caltrans’ seismic retrofit program following the 1989 Loma Prieta earthquake, critical infrastructure such as the Posey and Webster Street Tubes were evaluated and retrofitted to ensure operational reliability following the next large seismic event.
Each of the reinforced concrete tubes are located under the Alameda Estuary at a maximum depth of 70-ft below water. The Posey Tube, constructed in 1928, consists of both cast-in-place and immersed tunnel sections. The pre-cast concrete sections were constructed in approximately 200-ft long segments. At the time of construction, the Tube was the largest and the first to utilize pre-cast reinforced concrete in the United States. The Webster Street Tube, completed in the early 1960’s, has a similar configuration to the Posey Tube. A typical cross-section of the tubes has a 37-ft circular outside diameter with a 2.5-ft thick circular wall. Each of the tubes is more than 4400-ft long, including more than 3500-ft of buried ventilated sections.
The Tubes are located in a high seismic zone approximately 14 miles east of the San Andreas Fault and 4 miles west of the Hayward Fault. One of the challenges is to properly consider wave propagation along the length of the Tubes. Other challenges include high soil variability along the alignment and complex portal interfaces.
SC Solutions’ provided seismic analysis expertise to the project team. Our responsibilities included seismic evaluation of the Posey and Webster Street immersed tunnels for the as-built and several proposed retrofit configurations. For each of these configurations, nonlinear time history analyses with multiple support excitations were conducted and the structural integrity of each of the tubes was evaluated. Nonlinear effects simulated in these models include representation of global geometric effects, joints between tube segments, and soil representation. Our advanced analysis approach reduced unnecessary conservatism and expedited peer review approval.