The existing Irvington Tunnel is part of the San Francisco Public Utilities Commission’s (SFPUC) Hetch Hetchy Water System and delivers approximately 85% (220 million gal/day [832.8 million L/day]) of the San Francisco Bay Area’s water. Built around 1930, the tunnel cannot be taken out of service without impacting the water supply, and it has not seen an inspection since 1966. The New Irvington Tunnel project bypasses the existing tunnel, allowing the original tunnel to be removed from service for inspection and maintenance. Furthermore, the New Irvington Tunnel affords welcome redundancy, providing a continual water supply to the Bay Area in the event of earthquake or other major disruption.
Delve Underground was the lead designer for the tunnel, shafts, and portals, and provided engineering services during construction. We also prepared a Geotechnical Baseline Report. Facilities include the new tunnel with initial and final linings; overflow shaft; tunnel portals; a manifold and pipelines to facilitate the connection of Bay Division Pipelines 1, 2, 3, 4, and the new Pipeline 5; and related ancillary appurtenances.
Ground conditions along the tunnel alignment consisted of interbedded layers of sandstone and shale, with several smaller fault zones. Abrupt changes in ground conditions and mixed ground conditions that exhibit ground behaviors of multiple ground classes were also present along the tunnel alignment.
Groundwater inflows occurred throughout a significant portion of the tunnel. Design challenges included 700 feet (213 m) of cover and a high hydrostatic head with potential water inflows of up to 1,200 gpm (4,542 L/min).
Special steel pipe design measures were required at four fault zones, where there is a risk of sympathetic fault displacements.
American Public Works Association—NorCal Section
Outstanding Construction Project, ASCE Region 9