Slow and steady progress at Devil's Slide

June 19, 2009 6:53:06 PM PDT
Slow and steady progress on one of the Bay Area's most ambitious, and many would say, most needed transportation projects. Workers have just about reached the halfway point in excavating the new Devil's Slide tunnels on Highway 1, south of Pacifica.

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The Devil's Slide tunnel excavation began at the south portals and will end when equipment punches through the other side, 4,200 feet later. Right now the southbound tunnel is excavated about halfway, the northbound just a little more than that.

At the end of the line, a laser surveying system helps produce 3D imaging of the rock surface and of the light weight concrete, called shotcrete, shot onto the wall. The comparison measures shotcrete depth.

Tunnel manager Paul Madsen has brought his soil and rock mechanics expertise all the way from Denmark.

"It looks like just a muddy old hole in there, a bunch of stone age, but it's not," said Madsen. "There's quite a bit of technology behind everything we do here."

On the southbound side, bolts four meters long are inserted into the mountain to support the tunnel. The bolts are unusual because once pushed in, they are puffed up with water that is injected in at very high pressure.

"So it swells up inside the hole that was previously drilled and then creates a friction between the steel of the rock bolt and the rock, and that way it stitches everything together," said Madsen.

The completed bridges now provide a platform for work on the north portals where the hillside is mostly intact.

The rock at the north portals is so loose and soft, they have to put giant nails deep into the hillside to support it for excavation.

"Some of these nails go about 70 feet into the mountain and then we put wire mesh over it and shotcrete it and it supports this hill almost vertically," explained lead engineer Sergio Gomez.

Caltrans says so far the project is on schedule and on budget. It is scheduled to open to traffic in late 2011.

LINK: Devil's Slide project overview

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