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  Engineering geological observations during construction of Caldecott 4 Tunnel State route 24, San Francisco, California

  The 4 Bore is a highway tunnel on California State Route 24 (SR24), currently under finalization in the eastern San Francisco bay area. The 4 Bore is an undertaking by the California State Department of Transportation (CALTRANS) and the Contra Costa County Transportation Commission (CCTC). Its purpose is to alleviate traffic congestion on SR24, which connects the cities of Oakland and Walnut Creek. The cost for the 4 Bore is estimated at $ 390.8 Mill, of which the American Recovery and Reinvestment Act is providing $ 179.8 Mill. The 3,389 ft (1033m) long 4 Bore tunnel will have an excavated dimension of approximately 40 ft (13m) height and 49 ft (15m) width. A total of 7 cross passages will run between the 3 and the new 4 bore. This paper will discuss lithologic and hydrogeologic observations, the encountered ground challenges during the excavation, and the influence of fault zones on groundwater. It will also compare the geology of 4 bore to three existing Caldecott tunnels.

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