After eight months of intense excavation on the Caldecott Tunnel fourth bore from the east end in Orinda, miners have begun to excavate from the west end in Oakland. The two holes are anticipated to connect by this fall.
Work began on the west end on March 10, 2011, and by a press tour today, miners had excavated about six feet into the mountain on that side of the tunnel.
Drilling on the west is somewhat more complex than from the east on account of softer, more crumbly rock and a steep hillside bearing down on the north side of the bore. Before the excavation began on the west side, months of work went into shoring up the tunnel site. Crews first installed stiff walls forming the portal and a canopy of steel pipes outlining the shape of the tunnel. The pipes are each 8 inches in diameter and 180 feet long, and support the mountainside while the miners excavate underneath the steel canopy.
The tunneling methodology is similar on both sides, in that crews must spray the tunnel walls and tunnel face with shotcrete (essentially concrete) every few feet in order to keep the walls from caving in. Once the face and walls are installed, the miners can then again break through the face and excavate the next round.
Still, “The progress won’t happen as quickly (on the west side) as on the eastern side,” explained tunnel designer Bhaskar Thapa, an associate with the engineering consulting firm Jacobs Associates in San Francisco. The difference is that on the west end of the tunnel, miners have to pause to apply the shotcrete every 3.3 feet as opposed to every 4.5 to 6 feet on the east side, and the shotcrete must be 12 inches thick on the west side vs. 8-10 inches thick on the east side. Also slowing down the work on the west end is the need to pause every 10 feet to apply the shotcrete to the floor of the bore as well as to the sides, ceiling and rock face, so as to form a complete circle of reinforcing concrete.
Crews have been putting in back-to-back shifts on the east side, and by this morning’s press conference, had progressed 1,175 feet along the 3,400-foot tunnel from that side. Now mining crews are working their way up to a 24-hour operation on the west side as well. Sound walls and restrictions on night-time hauling of tunnel spoil will protect the adjacent neighborhood from noise.
In an important milestone, the massive, German-made roadheader operating on the east side recently finished chewing its way through the Orinda Formation, one of three rock formations that occur along the tunnel pathway. In the middle of the tunnel path is the Claremont Formation, while the west side is known as the Sobrante Formation. Each represents a different paleontological era, with the Sobrante Formation being the oldest and the Orinda being the youngest, according to Thapa.
Somewhat paradoxically, at the same time as being strong enough to support the crushing load of the mountain, the new bore lining is also flexible enough to withstand a seismic event of the magnitude that might occur every 1,500 years. “That’s a very high level of shaking that the tunnel is designed to perform in,” Thapa said, explaining that the tunnel walls themselves are flexible enough to move with the ground.
The current phase of tunneling is excavating just the upper half of the 50-foot wide tunnel. The excavator on the west side will drill a quarter of the way in, while the roadheader on the east side will go three-fourths of the way through. Once they meet, the machines will back out and begin to excavate the lower layer of the tunnel.
“It’s a challenging tunnel considering its size, and the weak and fractured rock that it’s being built through,” Thapa said.
The Caldecott Tunnel runs along State Route 24, connecting Alameda County with Contra Costa County. Currently Caltrans must daily shift the direction of the traffic lanes among the three existing bores to accommodate the ebb and flow of traffic. With the opening of the fourth bore in the fall 2013, travelers will permanently have access to two lanes in each direction, which should alleviate the perennial back-ups at this key gateway. The $391 million fourth bore project is being built with the help of $50 million in bridge toll money from MTC, along with federal stimulus money and local sales tax funding, among other sources. -- Brenda Kahn
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