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TECHNOLOGY Tunnel safety


A safer slide A


fter a heavy winter rain, residents of Pacifica and Half Moon Bay, California would wake up won-


dering if the highway they depended on daily still would be there, or if it had slid once again into the Pacific Ocean. Notorious for rockslides and collapse,


a section of scenic State Route Highway 1 — aptly named the Devil’s Slide — had been closed nine times in the past 28 years, with the longest closure lasting 158 days. Commutes between the two cit- ies, an easy 7.5 miles normally, became 45-minute slogs for the 20,000 motorists who traveled the route daily. When twin, state-of-the-art tunnels


opened in March 2013, providing safe passage through the Devil’s Slide, they were dubbed the ‘people’s tunnels’ and hailed as an unprecedented grassroots victory because no highway tunnel had been constructed in California since 1964.


BETTER THAN THE ALTERNATIVE In 1995, public opposition to a pro- posed four- to six-lane highway bypass prompted a Federal mandate for the California Department of Transporta- tion (Caltrans) to reconsider a tunnel alternative. “Following a reaffirming feasibil-


ity study, San Mateo County voters approved by 74 percent a ballot measure in 1996, requiring adoption of the tunnel alternative,” said Moe Amini, Caltrans’ design contract manager/task manager for the tunnels project. “It is one of only a few projects ever determined by popular referendum in California.” In 2001, Caltrans retained HNTB to


serve as lead tunnel designer and intro- duce the department to modern tun-


12


Y Nien Wang on how Caltrans overcome seismic, ground and environmental challenges to build twin tunnels as an alternative to the notorious Devil’s Slide


neling practices. HNTB worked with Caltrans to prepare plans, specifications and estimates and supported an exten- sive public outreach campaign. The solu- tion, one of the largest and most complex projects in California’s history, would include: • Twin tunnels, with the longest bore of 4,298ft, making them the longest high- way tunnels in California.


• A southern portal approach, measur- ing 1,000ft long, with a tall rock slope held in place by soil nails.


• Northern portal approaches, measur- ing 1,500ft long, with separated twin cast-in-place segmental concrete box girder bridges.


• An off-site operations and mainte- nance center.


DEVIL OF A JOB The project presented three substantial technical challenges:


1. Underground hurdles. The alignment proceeds south from Pacifica, departs from existing State Route 1 along a 7 per cent uphill grade, crosses a valley at Shamrock Ranch, passes through a small ravine, enters the horseshoe-shaped tunnels 650 ft below San Pedro Mountain and exits the tun- nels just south of the Devil’s Slide area, where it rejoins the highway. As part of an extensive geologic base-


line report, the project team drilled sev- eral bores along the proposed alignment and extracted rock cores. Laboratory tests performed on the rock cores pro- vided engineers the data necessary to identify and categorize several rock types that would be encountered during exca- vation. In addition, studies indicated sig- nificant groundwater flows. In the north


thinkinghighways.com


side of the tunnel, for example, crews encountered excessive water flow from a fault acting as a drain, collecting ground- water from the rock mass. Three holes were drilled through the fault, draining more than 10 million gallons of water to keep the tunnel headings dry.


2. High seismicity. Because they lie along the active San Andreas Fault and four inactive fault


Vol 8 No 3 North America


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