Claremont Tunnel Seismic Upgrade
One-hundred-thirty feet underground, the Claremont Tunnel crosses the actively creeping Hayward Fault. Studies indicate that a large-scale seismic event along this fault line could result in a sudden 7.5-foot horizontal offset, severely damaging the tunnel. To avoid the high costs and environmental impacts associated with constructing a completely new tunnel, Jacobs Associates designed a 1,570-foot-long bypass tunnel through the fault zone.
At the main active trace of the fault, the bypass tunnel includes a 26-foot-diameter, 100-foot-long enlarged vault section to accommodate horizontal and vertical offsets. Within this vault, an 85-foot-long, 6-foot-diameter, 3-inch-thick, freestanding steel pipe serves as a back-up system in the event of tunnel damage, protecting the water supply from rock debris and allowing temporary passage of emergency flows. Within the remainder of the 920-foot fault zone, the tunnel lining was thickened to accommodate secondary fault offsets, expected to cause an earth shift up to 2.25 feet.
The American Society of Civil Engineers (ASCE) San Francisco Section awarded the Claremont Seismic Upgrade Project its Special Innovation Award in 2006. The chapter specifically noted Claremont’s innovative use of materials and creative bypass tunnel design, which proactively mitigated seismic impacts.
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