3rd Street Light Rail Program - Central-Subway Tunneling
Joint Venture Members
Salini Impregilo S.p.A.
S. A. Healy Company
The San Francisco Municipal Transportation Agency awarded the $234 million Central Subway Tunneling Project in August, 2011, to the joint venture of S. A. Healy Co., Barnard Construction Company, and Impregilo S.p.A. Construction is scheduled to be completed by the end of 2014.
The Project consists of construction of a Launch Box for two Tunnel Boring Machines under 4th Street and the I-80 freeway, and excavation and lining of parallel tunnels and cross-passages under 4th Street, Stockton Street, and Columbus Avenue. Headwalls will be constructed at the future Moscone and Union/Market Square Stations through which the TBM’s will bore. A Portal Structure will be built in the Launch Box after tunnel excavation is complete.
The twin bore tunnels will be excavated with two Robbins Earth Pressure Balance Tunnel Boring Machines (EPB’s) with excavation diameter of 20’-8”. During excavation, the EPB’s will also install a final one-pass lining of precast concrete bolted and gasketed segments with an inside diameter of 17’-10”. The tunnel will be excavated and lined through the locations of three future stations. During construction of the stations, the precast segments in that area will be removed.
Underground construction also includes the excavation and reinforced concrete lining of five crosspassages for future emergency use.
EPB TBM’s were selected on the basis of the soil and rock conditions that will be encountered. The geology of the tunnel alignment is divided into thirds. The first third of the tunnel is through the Colma Formation, consisting of well-bedded dense to very dense sand with interbedded stiff to very stiff clay and sandy clay. The central third of the tunnel will be driven through the Franciscan Complex Bedrock, a highly variable unit including sandstone, meta-sandstone, sandstone breccias, shale, shale breccias, siltstone, claystone, mudstone, chert, serpentine and mélange. The final third of the tunnel is through the Colma Formation.