The Claremont Tunnel was built in 1929. After the Loma Prieta earthquake in 1989, EBMUD began studying the potential effect of a large seismic event along the Hayward Fault. In the worst-case scenario, slip along the fault would sever the tunnel, disrupting the water supply to approximately 50 to 70% of EBMUD's customers for up to six months. In response, EBMUD approved a $189 million Seismic Improvement Plan in 1994, which included seismic upgrades to the Claremont Tunnel. Seismic upgrades began in 2004 and completed in 2007.
Design
The original tunnel connects the EBMUD treatment plant in Orinda with the distribution system west of the Oakland Hills. It featured a bore square with a mostly unreinforced concrete lining thick. At peak capacity, the tunnel can deliver. The tunnel crosses the Hayward Fault at a nearly perpendicular angle from its western portal, and creep along the fault resulted in an offset of by 2008. The maximum credible earthquake predicted in studies would cause an offset of. For the seismic upgrade, a reinforced bypass tunnel was constructed across the Hayward Fault starting in 2004. The seismic upgrade also injected grout to improve contact between the 1929 concrete tunnel and the surrounding earth. For the bypass, first a access tunnel was bored into the fault zone. Next, the space for the long bypass tunnel was excavated using the New Austrian tunnelling method. A bypass tunnel was used to avoid disrupting service through the main tunnel and to minimize cost and time. The bypass tunnel is in diameter, except for a reinforced section crossing the fault, where it takes on an oval cross-section wide for a length of. In the reinforced section, the concrete liner of the tunnel is thick, instead of the thickness elsewhere in the bypass tunnel. The reinforced section was excavated by first boring two drifts to either side of the planned section. The space between the two drifts was excavated, and then the drifts were filled with concrete so that if the reinforced section breaks, the water will not wash out the dirt surrounding the tunnel. In the reinforced section, the reinforcement is weakened every along its length to allow the reinforced section to shear predictably in the event of a slip along the fault. The reinforced section is fitted with a steel carrier pipe thick with a inner diameter, capable of delivering. Up to an offset can be accommodated without breaking the pipe or losing water delivery. The bypass tunnel was designed by Jacobs Associates for EBMUD, and cost $38 million. It went into service on May 30, 2007.
