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TECHNOLOGY Tunnel safety


“Having tunnel vision is sometimes the broadest vision of them all”


Pacifica


lines, the tunnels had to withstand a 7.5 to 8.0 magnitude earthquake, the maxi- mum movement estimated for this arm of the San Andreas Fault. After a design seismic event, the tun-


nels were to remain serviceable and have experienced only “repairable” damage, completed within 90 days and with lane closures during off-peak traffic periods only.


3. Environmental sensitivity. Federally protected Peregrine falcons nested above the alignment, and Califor- nia’s endangered red-legged frogs were found living in the Shamrock Valley wet- land area. The tunnels’ alignment was carefully chosen to respect these environ- mental concerns. In addition, no equip- ment was allowed to enter the wetland area. Crews cordoned the area with frog fences and conducted extensive wetland monitoring to ensure the animals’ safety. To further minimize the project’s


impact to the environment, twin, bal- anced, cantilever cast-in-place segmental concrete box girder bridges were con- structed over Shamrock Ranch. Designed in-house by Caltrans, they are the first bridges of their kind in California.


CALIFORNIA’S FIRST NATM HIGHWAY TUNNELS The tunnels’ length and the challeng- ing ground conditions did not lend themselves to a tunnel boring machine technique. Instead, the New Austrian Tunneling Method, a sequential excava- tion and support tunnel-mining tech- nique, was chosen. This flexible, adaptable method relies


on observation of ground behavior dur- ing excavation, which facilitated unprec- edented in-field team collaboration. Based on instrumentation and monitor- ing of the rock deformation, geologists and engineers adjusted the rock supports


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on the fly. This allowed the tunnels to be constructed safely, quickly and at a rea- sonable cost. NATM incorporates the strength of


the surrounding rock during construc- tion and uses a double-shell support system. The system consists of an initial lining of sprayed fiber-reinforced shot- crete, rockbolts and lattice girders. Once a section was excavated, crews installed lattice girders and then quickly applied an initial six-inch layer of shotcrete. The shotcrete included structural syn- thetic fiber to meet specified deflection requirements, a novel approach in the United States. The final lining, designed to accommodate seismic forces, was con- structed from reinforced concrete. It was applied after rock dowels and other pre- support measures were implemented. The initial support and final lining are separated by a waterproofing and tunnel drainage system. Tunnel excavation was accomplished


using top heading, bench and invert. Both tubes were excavated 60ft apart and simultaneously, beginning at the south- ern portals. Each time crews encoun- tered a different rock type, they had to swap out an entire set of machinery. To improve excavation efficiency, the bench was excavated the full length of the reach prior to the installation of the invert. In addition to two-dimensional


numerical models of tunnel cross- sections, the HNTB team prepared 3D structural analysis models for a typical cross-passage and the transition of the tunnel to the north portal. Construction of the 10 cross-passages, which connect the two tubes, changed the rock stress field that impacts both tubes; therefore, to assess the impact of constructing the cross-passages on the stresses in the rock and, subsequently, the stresses in the tunnel liner, 3D numerical modeling was required.


thinkinghighways.com Pedro Point


Pacific Ocean


DEVIL’S SLIDE AREA


1


APPROACH WITH BRIDGE


San Pedro Mountain


1


Half Moon Bay ROADWAY


APPROACH


TUNNEL VISION The new US$430m Tom Lantos Tunnels, named after the late Congressman who secured the emergency federal funding, are among the safest in the nation, meet- ing every requirement of the National Fire Protection Association’s 502 Code and the Americans with Disabilities Act. In addition, Caltrans will have a greater level of confidence going into the next tunnel project. “With modern specifications now in


place, the next tunneling project should be much easier, comparatively,” Amini said. “The tunnels set the standard for the state. Caltrans simply can tell future designers and contractors to


follow the design standards and speci- fications developed for the Tom Lantos Tunnels.” As one resident put it, “Having tunnel


vision is sometimes the broadest vision of them all.”


||||||||| |||||||| ||||||||


ywang@hntb.comwww.hntb.com


 This article first appeared in Issue 99 of HNTB’sDesigner magazine and has been reproduced in this form with the kind permission of HNTB


Vol 8 No 3 North America |||||||||


 Y Nien Wang is Project Manager at HNTB


TUNNEL


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