TOP 10 ROADS: No. 2—A devil of a time

Tunnels protect roadway from landslides on California coast

Article October 04, 2012
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Devil’s Slide would be a good title for a horror movie, or a Fifties song in which a teenager crashes his hot rod and dies in the fiery wreckage.

In this case, Devil’s Slide is a stretch of California Highway 1 famous for landslides. The roadway was realigned between Pacifica and Half Moon Bay as part of the Devil’s Slide Tunnels, Roadways and Operations Maintenance Center project.

The project slides into spot No. 2 on this year’s ROADS & BRIDGES Top 10 Roads list.

The project is constructing twin tunnels through the San Pedro Mountains using the New Austrian Tunneling Method. A wise man said the battle plan is the first casualty of war. For the Devil’s Slide tunnels, the plan was crunched by site conditions that were different from what was expected: mixed rock face conditions, block fallouts and ground support categories that were not consistent with the contract plans. Kiewit Infrastructure West was flexible enough to adjust to the different conditions in the field with additional support measures, such as shotcrete, rock bolts, lattice girders and spiles. The project team of engineers met daily to assess the latest issues and their solutions. They excavated more than 418,000 cu yd of rock from the mountain.

Groundwater was one condition that was anticipated. Kiewit expected the water to be worst at the north portal, so they filed a change order to dewater the tunnel by installing horizontal drain pipes in the northern portal area.

Kiewit also dealt with complicated geometry for the tunnel entrance structure. The portal entrances were designed with a 22º vertical skew and a 57º horizontal skew. Each entrance has a bull nose that is composed of up to 21 different elliptical shapes gradually transforming from one end to the other. The skews and elliptical shapes required complicated 3-D modeling for fabricating forms and rebar and installing the forms and rebar.

The tunnels will be 4,150 ft and 4,048 ft long and 30 ft wide with 10 cross passages for use in emergencies. The tunnels will be operated from an 11,000-sq-ft operations, maintenance and control building constructed on a site close to the tunnels.

At the portals, Kiewit was required to match natural rock formations in the vicinity. They created molds by casting resin material onto natural rock, then used the molds to fabricate form liners. They practiced painting a mockup to find the right colors and techniques to duplicate natural rock before they applied it to the tunnel portal.

Devil’s Slide is an environmentally sensitive area and is home to numerous types of wildlife. A team of biologists was on-site to monitor construction activities to ensure that protected species such as the peregrine falcon and the California red legged frog were not being harmed. R&B

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