VTA BART Tunnel Liner Study

Santa Clara Valley Transportation Authority (VTA) is constructing the Silicon Valley BART Extension to San Jose, California.  This will be an approximately 16 mile long extension of the Bay Area Rapid Transit (BART) heavy rail rapid transit system from its planned terminus at the end of the Warms Springs Extension in Fremont to San Jose. The alignment includes six stations – three above and three below grade – separated into two main segments:

• Approximately 11 miles of at-grade, elevated and cut-and-cover track from the Warm Springs Station to the proposed Berryessa Station.
• Approximately 5 miles of tunnel segment predominately consisting of twin bored tunnels and cut-and-cover structures through downtown San Jose and at grade track to its terminus in Santa Clara.

VTA was considering twin circular tunnels constructed by closed face tunnel boring machines (TBMs) to interconnect the stations and portals.  Approximate total length of each the bored tunnels is 23,000 feet.  The TBMs will be used to install a single pass lining system consisting of pre-cast concrete segmental rings in lightly over-consolidated alluvial soils with the water table generally lying 5 to 15 feet below the ground surface.

SC Solutions provided seismic analysis and tunnel expertise to the project team during the preliminary planning phase.  Our responsibilities include performing 3D seismic soil-structure interaction (SSI) analyses of the proposed tunnels to assess design alternatives.  Analytical investigations were performed to evaluate two liner alternatives while considering  variations in soil properties.  The structural models include 3D characterization of the tunnel liner segments and friction interfaces at contact between adjacent segments. Construction sequence, followed by the ovaling analysis was carried out and stability of each geometric configuration was evaluated for seismic performance.