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ORINDA -- After two years of carving a new passage deep inside the hills, work has picked up speed and crews have a clearer path to construct the $391 million Caldecott Tunnel fourth bore. 


The additional freeway lanes between Orinda and Oakland are expected to open in late 2013. 


Until now, operators of giant scrapers and cutters had been forced to estimate where to dig in creating the bore. No longer. 


Now they know more precisely where to dig because of a breakthrough made seven weeks ago, when workers excavating from opposite directions connected the two tunnel sections. 


The top portion of the tunnel is dug from end to end. Now crews need an additional six months to dig the lower portion -- called the bench -- where two new lanes will be paved to relieve reverse commute traffic congestion. 


That excavation is about 24 percent complete. To date, about 639 feet has been excavated near the western portal and about 167 feet on the eastern side. 


"The breakthrough changes things. We now have more predictability in excavating this big cavity in the earth," said Ivan Ramirez, a senior Caltrans engineer. "In tunneling, there are always risks and unknowns. Now we have less of them." 


Able to see all 3,389 feet of the upper tunnel, equipment operators can focus more precisely on how and where to scrape out and reinforce the lower section. 


"Before," Ramirez said, "no matter how thorough our geologists were in preparing  for the rock ahead of us, we didn't exactly know what the earth would look like until we got there." 


In another favorable development for Caltrans, state safety regulators recently ruled the fourth bore project is no longer classified as a "gassy" tunnel, where methane and other gases could trigger explosions or fires. 


This change means a relaxation of restrictions on tunnel equipment and operations, speeding up the work. 


"This project is progressing as we expected," said Randy Iwasaki, executive director of the Contra Costa Transportation Authority, a partner and co-funder in the project. 


Caltrans last week guided a reporter and photographer from this newspaper on the first media visit inside the tunnel since the breakthrough Nov. 29. 


Ceiling lights cast an eerie glow, reflected in the wet and muddy floor dampened by groundwater flows percolating in the man-made cavern. A reporter nearly lost a boot in the thick mud. 


Water pumps emitted a sharp and loud pounding that made it nearly impossible to be heard in some areas. 


Miners wearing head lamps toiled far apart in a different tunnel area. Some operated excavators, dirt haulers and scrapers to remove dirt and rock. 


At the Oakland opening of the tunnel, crews constructed large metal gantries that will slide on a rail so crews can apply a final concrete lining to stabilize the ceiling. 


In one niche far inside, crews sprayed a watery mixture called shotcrete on one of seven cross-passages that will provide an emergency escape route for motorists using the third and fourth bores. 


The seven cross-passages will be important safety improvements, Ramirez said. 


During fires or explosions such as the 1982 gas tanker explosion that killed seven people inside the tunnel, trapped motorists must walk to either end of the passage. 


The new cross-passages connecting the two northernmost bores will let trapped motorists walk from the blocked bore into the clear one. The passages are every 393 feet within the bores. 


"The doors on the passages will open automatically when they detect a person," Ramirez said. "Then they will close again to block out the smoke." 


Tunnel builders -- led by prime contractor Tutor-Saliba -- expect to finish removing dirt from the tunnel in about six months, Caltrans spokeswoman Ivy Morrison said. 


Much work remains to install a plastic liner to collect water and a final concrete coating to strengthen the tunnel. 


"We're roughly halfway through the project," Morrison said. "Even when the excavation is done, there is lot of work left to be done." 

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