For every five feet the road header carves through the East Bay hills, a thick coat of shotcrete is sprayed on the walls, bolts are drilled in for support, and another six inches of shotcrete finishes the process. It is repeated 24 hours a day.
When it is finished, the Caldecott Tunnel's fourth bore will be 4,000 feet long. But each day, the miners only advance 10-15 feet, hauling out 1,500 tons of muck.
So far, they are almost one-third of the way through and every day poses new challenges. The site is considered a "gassy" tunnel, which means something as simple as the electrostatic spark from a cell phone could turn a pocket of methane gas into a deadly fireball.
"It's all about safety; i's not an easy job, it could be dangerous if you're not smart," project superintendent Bill Monahan said.
A process called "brassing in and out" is mandatory. Brass tags indentify who is "in" and who is "out" of the bore. In the early days of tunneling, brass was used because of its high melting point of 1,700 degrees.
"In the event there is a fire, that won't melt; it'll survive the fire and the identifying imprint will still exist," project engineer Bob Smythe said.
Methane gas is also poisonous, so giant ducts constantly blast air into the tunnel.
"So if there was a pocket of methane or poisonous gas, it would be diluted before it got to the sensors," Smythe said.
As the crew tunnels through the Orinda formation and into the Claremont formation, geologists expect to find larger pockets of gas and more dangerous rock formations.
"Along the slick and slides blocks can fall down, break out of the face, and that makes it dangerous for the miners," geologist Gerhard Neuhuber said.
Neuhuber is the geologist hired to oversee the construction company's geologist. Despite the redundancy in safety measures, they had a sobering moment last November, when a large chunk of shotcrete broke off and injured two miners.
"It was about five feet long, and probably six to eight inches thick and three foot in width," Monahan said.
"They were putting the second level on there and they jumped on it too quick, well it just hadn't cured," Smythe said.
You know, you have always have to take care when you work inside a tunnel because something can happen," Neuhuber said. "You never can measure everything."
With all of his geological technology, Neuhuber says he could not have predicted the temperature's effect on the quality of shotcrete and leaking mountain water that may have loosened the shotcrete before it could dry. It is just another dangerous unknown the crews face as they mine their way through the mountain.