ORINDA -- One more scoop of earth, a few inches closer to Oakland.
A giant excavation machine with whirling metal teeth is slashing and churning away, making steady progress in digging a new tunnel to widen Highway 24 at a major chokepoint: the Caldecott Tunnel.
A year after breaking ground, Caltrans reports that construction crews have dug out more than 900 feet -- or 27 percent -- of the 3,389-foot-long Caldecott Tunnel fourth bore, a project now estimated to cost $391 million after favorable construction bids lowered the cost from $420 million. The new bore will add two lanes to the existing six lanes in the tunnel and ease reverse commute traffic congestion.
The contractor, Tutor-Saliba Construction, remains on pace to finish the project by late 2013, ending the daily reversal of traffic directions through the middle tunnel, Caltrans said.
"The work has gone very well so far," said Cristina Ferraz, Caltrans' fourth-bore project manager. "Whenever you do a construction project underground, you never know exactly what you are going to find. We'll keep our fingers crossed."
All the excavation so far has been on the Orinda end, or east end, of the new bore. Later this month, Caltrans expects digging to start on the Oakland side.
Tunneling is a difficult, risky and highly specialized type of construction. Workers use powerful machinery in tight, dark spaces where cave-ins,
The Caldecott Tunnel is no different. Complicating this project, state safety regulators have classified the fourth bore as a "gassy" tunnel with the potential to encounter methane or other dangerous gases. That means extra safety precautions, preparations, labor and cost for contractors.
"It makes the project more expensive and challenging," said Bill Monahan, a Tutor-Saliba construction manager who is a veteran of tunnel building. "Gassy or not, we would do (air) testing here. But this means more testing, more labor, and different equipment."
Cell phones, gas motors and conventional cameras are banned from the tunnel interior because of the potential to spark explosions.
Weiss and Ferraz last week visited the Orinda tunnel opening, a 41.3-foot-wide gap -- about the size of a four-story building laid on its side -- in the earth. Mostly obscured by a wall, the new tunnel is hard to see by the 160,000 daily motorists who use the Caldecott.
This portal is where two shifts of construction workers enter each workday to excavate, removing enough soil and rock to fill 60 to 80 trucks a day.
The 21,665 cubic meters of earth and rock excavated from the Caldecott fourth bore so far would cover a football field 16 feet deep. When the project is finished, the excavated earth would cover a football field 134 feet deep, officials say.
Before walking or driving into the tunnel, workers are required to sign in; each carries a brass tag, a tangible reminder of the danger of the work.
"The brass has a very high melting temperature. If the worst case, if your body is torched, they use the brass tag to identify you," Weiss said. "It's a very practical but sobering safety measure."
Two workers have suffered minor injuries since construction began. One worker was struck outside the tunnel by a piece of flying metal. The other was hit when a chunk of spray-on ceiling broke off and fell on him.
Dig, spray, repeat
The work in the tunnel is about more than just digging. It's a finely choreographed series of actions that involve excavating a few feet forward, stopping, and fortifying the walls and ceilings with metal rods and a spray-on mortar called shotcrete.
When one segment is fortified, crews begin digging into another.
"Each round is done very meticulously," Weiss said. "The idea is to let the mountain support itself as much as possible. You assess what you've done and add more support if you need it."
Geologists assess each new area to be dug to determine earth hardness and other qualities.
During recent weeks, excavation crews dug out 12 to 14 feet a day of the top half of the new tunnel as they carved through the relatively hard Orinda Formation, an earth layer with sediment from ancient creeks. Crews later will return to dig out the lower part of the tunnel.
The excavation is slowing, though, as crews approach the Claremont Formation, an ancient ocean bed with silt that requires additional measures to fortify tunnel walls and ceilings, said Ivan Ramirez, a Caltrans engineer.
"I think, for the most part, the rock has been very good," Ramirez said. "But we expect the ground to get a little worse."
When brute force is needed, crews turn to a giant excavation machine called a roadheader with whirling teeth to crack, cut and dig away at the earth.
The roadheader has dug out about 150 metric tons of soil and rock a day, and loaded it via conveyor belt onto waiting trucks.
The excavated earth is checked, and if hazardous impurities such as petroleum are found, the soil is shipped to a hazardous-waste dump.
No hazardous concentrations of impurities have been found thus far, though, so all the tunnel spoils have been trucked to Treasure Island for building projects, Ramirez said.
When the tunnel core is finished, contractors will cover the ceiling and walls with a light-colored enamel that is easier to clean than the tile or concrete finish on the existing three Caldecott bores.
"It will look good," said Ferraz, the project manager. "We guarantee it."
Contact Denis Cuff at 925-943-8267.