ORINDA -- Deep inside a dark and damp cavern being excavated through the Berkeley hills, construction workers Monday used a drill on a long mechanical arm to slice out plugs from a rock wall.
The workers, using miner's lamps, carefully examined the color and fineness of earth particles and the hardness of rock clumps.
Then they drilled again, searching for clues to their mission.
It wasn't riches they sought, but information about rock strength, depth and brittleness so they can fine-tune plans for digging and reinforcing the next 42-foot segment of the Caldecott Tunnel fourth bore, a two-lane project to ease congestion on Highway 24 between Orinda and Oakland. Tunneling began in August on the project, which is expected to be completed in 2013.
The tunnel will stretch 3,389 feet -- more than 11 football fields stacked end to end -- making it all the more important for crews to reinforce each new segment with metal rods and a spray-on concrete called shotcrete.
"We do these probes because we want to know what lies ahead of us," said Ivan Ramirez, a senior Caltrans engineer assigned to the $391 million project. "We will do the probing every time the roadheader (excavating machine) goes into a new section."
Ramirez spoke loudly to be heard over the clanging, banging and whirring of the heavy machinery inside the tunnel as crews prepared for another finely choreographed series of actions to dig, reinforce, and dig some more.
Because of concerns about explosion risks from naturally occurring methane gas, Monday was the first time a small group of media members was allowed deep into the tunnel for a tour following an in-depth safety instruction.
As of the morning visit, crews had excavated 1,230 feet of the top half of the passageway from the Orinda side with the help of the biggest roadheader in the country. The German-built machine has rotating metal teeth to dig out the rock and earth.
Crews earlier this month began excavating from the Oakland side with smaller equipment that won't dig as far or as fast.
Inside the Orinda opening, crews backed the big roadheader away from the face of the tunnel wall to make room for the probing drill, called a jumbo.
As the probe pulled out rock samples, water injected into the wall to cool the drilling bit sprayed and poured down the side of the tunnel.
The machine noise was so loud that it was hard to hear anyone talk. The dark environment, illuminated by a series of lights along the tunnel ceiling, looked eerie. Two large ventilation pipes spanning the entire tunnel pumped out stale air so clean, safe air flowed in.
"Some people don't like to work underground. It's just like some people don't like to work high on a bridge or the top of a building," said Bill Monahan, a longtime tunnel construction manager with Tudor Saliba Corp., the lead contractor for the fourth bore project. "Workers will admit it. They'll come up to you and say, 'This is not for me.' "
Other workers like the challenge and variety of tunnel work, even though in winter it can mean not seeing sunlight for days because some crews start before sunrise and finish after sunset. Tunnel work also means steady paychecks because storms do not stop construction underground.
James Martin, a roadheader operator, said he enjoys the sense of accomplishment at digging tunnels.
"I like knowing that I've been part of building something so big."