The San Andreas Fault is the place where the Pacific Plate and the North American Plate meet. While most tectonic boundaries are found in the depths of the oceans, the San Andreas Fault is onshore, in California on the American west coast. It frequently causes eartquakes and it determines the geological properties of the underground. That is why the geology of the Arrowhead Water Tunnel is characterized by high groundwater pressure und strongly decomposed rock. The two-section Arrowhead Water Tunnel is part of the project "Inland Feeder" securing Southern California's supply with water.
Two Single Shield TBMs by Herrenknecht were employed in California, tailor-made for this project's challenges. The machines could be operated in open mode as well as in closed mode. Depending on the quality of the excavated material and the water pressure, a slurry system for muck discharge could be activated. The most important feature of the machine for an effective tunnelling proved to be a system that could perform injection drillings to stabilze the rock and to controll the groundwater pressure. The machines were designed to withstand water pressure of up to 10 bar.
On May 2, 2008, the excavation on the lot "Strawberry Creek" and on August 20, 2008 the excavation on lot "Waterman Canyon" was successfully completed.