Bay Bridge and Yerba Buena Utility Tunnel – Bay Area, CA

Holdrege & Kull provided shoring design to retain rock exposed in vertical cut faces of the new San Francisco-Oakland Bay Bridge foundation excavations. Two footings on Yerba Buena Island were roughly 70 x 70 x 90 feet deep and extended nearly 30 feet below sea level. H&K’s preliminary investigations included geologic mapping, use of stereonets, and computer modeling to determine wedge, planar, and toppling failure potential. Excavation of the footings was accomplished by drilling, blasting, and mucking fractured Franciscan Formation material using heavy equipment and 750,000-pound cranes. H&K designed a tensioned anchoring system that prevented slope failure and catastrophic collapse by compressing rock material exposed in the excavations. Manual scaling dislodged loose material not secured by the anchoring system. Maccafferi rock-fall netting was draped over the vertical faces of the excavation to prevent material dislodged by blasting from free-falling into the hole.

To re-route existing utilities for the new bridge alignment, a tunnel was constructed under I-80 just below the east portal of the YBI tunnel. H&K provided soil sampling and testing, slope stability analysis, and shoring design for the 60-foot-deep entry and exit pits excavated in poorly cemented sandstone on the entrance side and fractured Franciscan formation greywacke on the exit side. The three-foot-diameter tunnel was drilled using jack-and-bore techniques that did not interrupt the flow of traffic on I-80. H&K’s shoring design included tensioned rock bolts, split-set anchors, rock-fall netting, and geo-fabric. Shoring design calculations included consideration for live loads generated by the constant traffic on I-80, slope stability for poorly cemented sandstone, as well as precise tendon alignment to prevent interference with jack-and-bore drilling. H&K also provided vibration monitoring and footing distress monitoring for adjacent bridge footings and I-80 footings during tunnel bore and jack operations.