Arrowhead Tunnels of the Inland Feeder System

The Arrowhead Tunnels project consists of two 19-foot (5.7 m) excavated-diameter tunnels: the 6-mile (9.6 km) East and 5-mile (8 km) West contracts. These tunnels are a key segment of the Metropolitan Water District of Southern California’s Inland Feeder. This 44-mile (71 km) water conveyance system of tunnels and pipelines extends from the termination of the State Water Project in San Bernardino to the Colorado River Aqueduct.

Jacobs Associates assisted in planning the geotechnical investigation, provided tunnel and portal designs, authored the Geotechnical Baseline Report, served as Engineer of Record for all drawings and specifications, and provided construction engineering services.

Tunnel excavation was challenging, with highly variable ground conditions. The tunnel alignments cross active splays of the San Andreas fault in three locations, encountering sheared rock and raveling and squeezing ground. Controlling groundwater inflows was critical to meet environmental requirements.

The design incorporates several unique features. The lining design allows the tunnel to withstand a magnitude 8.0 earthquake. A special lining, where the alignments cross the fault’s active splays, will accommodate fault rupture. The segmental lining was designed to withstand a pressure of over 900 feet of hydrostatic head during excavation, the highest pressure considered in the design of a precast segmental tunnel lining at the time. The 12-foot (3.7 m) inside-diameter final lining consists of welded steel pipe and reinforced concrete cylinder pipe.

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