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00The design incorporates several unique features: The tunnel lining for the Arrowhead Tunnels is designed to withstand an earthquake of magnitude 8.0, a likely event during the design life of the tunnel. In addition, one portion of the alignment crosses an active splay of the San Andreas Fault, where a special lining section is being used to accommodate fault rupture. A primary lining of bolted and gasketed precast concrete segments was designed to withstand a pressure of 900 feet of hydrostatic head - the highest pressure ever considered in design of a segmental tunnel lining. An extensive testing program was performed by MWD and the University of Illinois to confirm the ability of the gaskets and segments to perform under these unprecedented conditions. Two closed-face TBMs designed to withstand a pressure of 300 feet of hydrostatic load are currently being used to excavate the Arrowhead Tunnels. In combination with the watertight primary and final linings, extensive probe-drilling and pre-excavation grouting ahead of the tunnel face is being conducted to protect the groundwater resources under U.S. Forest Service land.
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