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Review of Underground Tunneling Techniques at the New Irvington Tunnel in San Francisco, California and the Virginia and Truckee Railroad in Virginia City, Nevada
Geological Sciences and Engineering
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The Irvington Tunnel located beneath San Francisco, California transports drinking water from Hetch Hetchy Reservoir to almost 2.5 million people. Due to seismic activity concerns and lack of renovation, portions of the tunnel are being replaced. The New Irvington Tunnel project is currently underway; tunneling mechanisms include blasting and traditional mining techniques. However, support after excavation is not widely known. Research on tunneling through weak rock masses was conducted in Virginia City, Nevada in order to determine the best practices for support of deep tunnels. The Virginia and Truckee Railroad company has one tunnel that has had multiple failures since the 1950s. This tunnel, Tunnel 3, was visited to collect data and determine the cause of failure. The Tunnel is located in moderately to highly altered Alta Formation andesite and has silty clay surrounding its failure zone. The west side of the tunnel has a blocky joint condition with the joints crossing each other. The joint orientations on the east side are parallel to each other and the material is of a higher degree of alteration. This is assumed to be the cause of failure on the east end of Tunnel 3. The properties of the weak rock at Tunnel 3 were used to create strength and displacement finite element models for a deep tunnel. This is related to the New Irvington Tunnel and shows that multiple supports are needed, such as rock bolts, infill, and composite liners, to avoid failure. Background