Geotechnical Investigations and Ground Characterization―The New Irvington Tunnel Project
Abstract
The Irvington Tunnel was completed in 1931 as part of San Francisco’s Hetch Hetchy Water System. The 3.5-mile long tunnel currently delivers water to about 2.5 million customers. Due to high demands, the City has not inspected the tunnel for over 40 years. Although soundly built, the existing tunnel was not designed to withstand strong ground shaking and potential fault offset. These facts led the City to initiate the New Irvington Tunnel project under its $4.5 Billion Water System Improvement Program. The new tunnel will be designed to remain in service following the design earthquake. The new tunnel facilities will also provide redundancy so either tunnel can be inspected or repaired without interrupting water deliveries.
The new tunnel construction will encounter highly variable and difficult ground conditions including very weak to strong rock, sheared and shattered to massive rock, squeezing ground, and high groundwater inflows. Over 2000 gpm was encountered during construction of the existing tunnel. The geologic formations along the alignment include Cretaceous and Tertiary shale, siltstone and sandstone, which have been folded and faulted into broad, northwest trending synclines and anticlines. These features form an uplifted range bounded by the active Hayward and Calaveras Faults. In addition to very strong shaking from these active faults, secondary offsets up to 6 inches are anticipated on four minor inactive faults along the alignment.
A comprehensive program of geotechnical and geophysical investigations was carried out to characterize the conditions along the tunnel alignment. The investigations included over 7500 feet of core drilling along with extensive downhole testing and logging. This paper will discuss the investigation program, the main findings, the interpreted ground characterizations, and some of the key design measures implemented to help manage and control critical geotechnical construction risks.
2010
World Tunnel Congress (WTC) Proceedings
Glenn Boyce
Jacobs Associates
T. Feldsher
URS Corporation
S. Gambino
URS Corporation
C. Jaramillo
URS Corporation
D. Tsztoo
San Francisco Public Utilities Commission
