California High Speed Rail

Jacobs Associates is performing preliminary tunnel design for the Parsons-led Pacheco Pass project, part of a high speed train (HST) system that will serve and link the major metropolitan centers of northern and southern California. This project will focus on the existing Union Pacific Railroad corridor between San Jose and Gilroy, the Pacheco Pass, and the Henry Miller Road alignment to Merced. The California High Speed-Rail Authority undertaking envisions approximately five twin bore tunnels totaling some 63,800 linear feet through Pacheco Pass. The tunnels will range in length from 3,000 to 26,000 feet. Geologic conditions vary from sandstone, siltstone, shale, and conglomerate for the eastern tunnel (Elephant Head area), to a chaotic mixture of sheared rocks in the Franciscan Complex mélange for the western tunnels (Pacheco Lake and Cottonwood Bay areas).

The HST tunnels along the Pacheco Pass corridor will be located in Seismic Zone 4— in close proximity to several active earthquake faults, including the San Andreas, Calaveras, Silver Creek, and Ortigalita faults. As such, it is likely that during their serviceable life the tunnels will be subjected to significant ground shaking caused by a major earthquake. Therefore, the preliminary design efforts for the tunnels will be focused on a final lining design that will assure serviceability following the design earthquake.

These highly variable geologic conditions will present several challenges to tunnel construction and will require an approach to tunnel excavation and ground support that is not only highly adaptable to conditions encountered during construction, but that will also result in a timely project completion.