ENGINEERING GEOLOGY OF A TUNNEL THROUGH THE HAYWARD FAULT ZONE

The Claremont Tunnel, an 18,000 foot long water tunnel, was built in the late 1920s and crosses the Hayward Fault Zone. Reinforcing in the original cast-in-place final lining is limited to areas of bad ground and the fault zone. The East Bay Municipal Utility District began a project to make repairs to the existing tunnel to construct a bypass tunnel through the Hayward Fault Zone in the summer of 2004. The bypass tunnel includes a special vault section through the Hayward Fault Zone which is intended to allow significant horizontal offset due to a major seismic event on the Hayward Fault.

This paper will discuss the characterization of the ground during the geotechnical investigation program and the application of modified Terzaghi ground classification in order to select initial tunnel support during construction. Most of the bypass tunnel was supported using steel sets and shotcrete lagging. The excavation and support of the vault section was performed in stages, with two side drifts supported by fiberglass rock dowels and shotcrete, the drifts backfilled with concrete, and then the main heading excavated sequentially and supported with custom steel sets.

The tunnel was classified by Cal/OSHA as Gassy and was excavated primarily using a roadheader, occasionally utilizing drill-and-blast excavation and customized mini excavators through the Hayward Fault Zone. We will describe the ground conditions as the tunnel progressed through Franciscan Mélange, serpentinite and silica carbonate, fault gouge, and the interbedded sandstone and shale of the Joaquin Miller formation. Geologic mapping was performed throughout the project, including a comprehensive mapping effort through the Primary Hayward Fault Zone, to confirm the location of the special vault section and a steel pipe that will be installed to ensure water delivery to 800,000 customers following a major seismic event.