Since 1967, the Caldecott Tunnel, which lies on Highway 24 between Oakland and Orinda, Calif., has had three two-lane tunnels, or bores. In the morning, as travelers rush off to work in Oakland and San Francisco, the middle bore relieves congestion by becoming a second set of westbound lanes. In the afternoon, when the peak flow reverses direction, the middle bore’s direction is reversed to become a second set of eastbound lanes.
However, traffic has become so dense that the off-peak lanes of a single bore no longer are sufficient. A $420 million fourth bore project to alleviate congestion currently is underway through a partnership of the California Depart-ment of Transportation (Caltrans), the Contra Costa Transportation Authority (CCTA), and the Alameda County Congestion Management Agency (ACCMA). It will result in two two-lane bores permanently dedicated to each direction. In charge of excavation from the west end of the fourth bore is FoxFire Constructors of San Clemente, Calif.
Case-setting System
FoxFire’s job is to excavate 700 feet into the west side of the bore to meet the general contractor’s excavation already underway from the east end. They’ll meet each other about one-fifth of the way into the bore, as the general contractor started several months earlier. This was according to plan.First, FoxFire must stabilize the soil and poor-quality rock conditions that exist for nearly the first 180 feet. At that point, the formation transitions abruptly from shale to more consolidated sandstone.
Engineer Bill Martin coordinated with Atlas Copco representatives to configure a Symmetrix simultaneous drilling/casing advancement system for the project. With its reputation for drilling long, straight holes even in horizontal applications such as this one, Symmetrix provided an alternative that potentially could save time, fuel and labor. It meant that the canopy could be completed all at once, after which FoxFire then would be able to turn its attention solely to excavation.
The setup chosen for the project was the Symmetrix SE 219 system for 85⁄8-inch pipe with two hammers – the Atlas Copco Secoroc QL60 running on 120 psi, and the Terranox 6 hammer at 300 psi. The rig was a rental Casagrande C-8, whose air was boosted by an Atlas Copco XRVS 1000 compressor. Trey Martin and Tony Galvez were site superintendents supervising the drilling operations.
The project’s specifications allowed less than 9 inches of deviation from the target 180 feet away. Using Symmetrix, FoxFire generally met these specifications. Several holes were right on target, without any significant deviation at all. Courtney “Stumpy” Andrus, FoxFire’s dayshift crew foreman on the project, says, “Accuracy depends greatly on your initial setup.”
Simultaneous Drilling/Case-setting
The crews alternated between the QL and Terranox hammers, using one to back up the other when one was being cleaned or serviced. The crew noted that performance was pretty much the same, explaining that the Terranox required more air (about 300 psi), and seemed to hit more softly.The average rate of penetration was about 1.3 feet to 1.5 feet per minute while drilling the 180-foot holes. They set each casing 10 feet to 15 feet into the sandstone beyond the shale, as stipulated in the design plan.
Each hole required nine 20-foot casings that were added and welded together during the drilling process. The casings are perforated at 1 meter intervals to permit grout to fill the annulus outside the casing. Each 20-foot section of drill steel and casing pipe took about 13 minutes to 15 minutes to drill. The total process of completing a hole generally took eight hours to nine hours. This included drill string makeup, as well as joining nine 20-foot casings together, capping the casing, grouting the pipe, and then setting up on the next hole.
Andrus says it took him about 10 days to get comfortable with the drilling and case-setting system. “You have to watch your rotation, watch the hoses. Listen to your drill; it will tell you what to do.” He adds that they were impressed by the maintenance crews from Atlas Copco. Commenting on their response time, he says, “It’s right away. They’ve been available 24/7.”
Drilling in Stages
Drilling on the lateral, the Casagrande C-8 rig’s reach is about 8 feet. To drill higher holes for the arched canopy required that they build up the rock and earthen pad three times. Each time, the crew had to remove all equipment from the drill pad, build it up with rock, and then reset all the equipment on the pad. This took about 24 hours to 36 hours (two to three shifts) to complete each time.Because these holes are drilled so closely together, they did not want to drill them in a progressive series. Instead, they let the grout of one hole set up well before coming back later to drill next to it. That way, should there be any interference between holes, the grout from the one hole would be unaffected by pressurization during grouting of its adjacent hole.
FoxFire is on schedule at this point to finish the excavation by fall. Projected completion of the bore for public use is set for late 2013.
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