The 3.5-mile long New Irvington Tunnel provides a seismically sound conveyance connecting the San Francisco Public Utility Commission’s water sources in the Sierra Nevada and Alameda County to the Bay Area’s water supply systems. The project required one 41-foot diameter, 115-foot deep temporary shaft to create access for 13-foot diameter tunnel drives in two directions. Secant piles were advanced through approximately 30 feet of water bearing alluvium into extremely weathered to decomposed Franciscan shale and sandstone. Harder rock conditions, with sheared water bearing zones were encountered below 90 feet. The shaft was adjacent to the I-680 freeway at Vargas Road in Fremont, California.
Malcolm used Loadtest’s Sonic Caliper technology to verify the verticality of every pile during installation. The Sonic Caliper technology utilizes sonar signals to provide a full 360-degree profile of each pile. The combination of stiff sectional drilling casing, powerful oscillators and rotary drilling machines enabled the installation of very deep secant pile walls with unmatched accuracy. The access shaft was installed with verticality tolerances ranging from less than 0.2% to maximum 0.5%. Water seepage into the shaft was successfully minimized by advancing all shafts through the shear zones and seating them deep into the fractured bedrock.