Under pressure

The Arrowhead East and West contracts for the Inland Feeder water conveyance system being built in California consist of 9.8 mi of tunnels that cross the San Andreas fault east of Los Angeles. The tunnels reach depths of 2,040 ft as they pass under mountainous terrain, with groundwater levels extending as high as 1,100 ft above the tunnels. The final tunnel liner must resist external groundwater pressures of up to 480 psi and internal water pressures of up to 100 psi, as well as ground loads and seismic strains. Because of these difficult and unusual requirements, the design team developed a new type of pipe that consists of an external steel cylinder with an inner concrete core that is reinforced on the inside face.