|Behind the Cutter. Crews operate TBMS in tough ground with high water pressure. (Photo by John J. Kosowatz for ENR)|
Stymied once by tremendous groundwater pressures and difficult geology, tunnelers in southern California are trying again to push two critical links to completion in a $1.2-billion project that will provide water for fast-growing and increasingly thirsty cities and towns spreading east from Los Angeles. Two years into their second attempt, Metropolitan Water District of Southern California officials are keeping their fingers crossed as work crews startand stopthe Arrowhead tunnels penetrating the San Bernardino Mountains.
The tunnels mark the beginning of the Inland Feeder project, a 44-mile conveyance system that will deliver up to 646 million gallons per day of water from the State Water Project when supplies in northern California are plentiful. Its final destination is local groundwater basins and Diamond Lake in Riverside County, which will provide a supply of water capable of meeting the regions need for up to six months in an emergency and up to five years in a drought. Click here to view map
The district is pinning its hopes on two 19-ft-dia, advanced tunnel boring machines and a redesigned tunnel and lining scheme. Because the project undercuts the San Bernardino National Forest, it is governed by a strict environmental permit issued by the National Forest Service that limits groundwater drawdown. Those limits have plagued MWD since 1997, when it began its first effort to build the tunnels.
The original tunneling contract was terminated in 2000 because water infiltration exceeded permit levels and a grouting program proved tedious and impractical. The route is close to the San Andreas fault and moves through a number of fault zones and a variety of hard rock consisting mostly of granite and gneiss, with a good deal of sandstone and some marble.
"The geology is extremely variable, fractural and extremely difficult," says Dan Tempelis, MWD program manager.
Arrowhead West tunnel will stretch 19,800 ft, taking flows from the Devil Canyon powerplant and delivering them to a pipeline and the 30,466-ft-long Arrowhead East tunnel. Both are being lined with thick precast concrete segments, but the design calls for erecting the segments with bolted gaskets to accommodate pressures up to 25 bar, or 360 lb per sq in.
About 8,000 ft of the east tunnel was mined under the original contract with the joint venture contractor Shank/Balfour Beatty. Now, crews working a downhill heading from Strawberry Creek portal toward the reach have progressed 7,000 ft since the present contractor, a joint venture of J.F. Shea Co. and Kenney Construction Co., began work in late summer of 2003 under a $242-million contract. "Things have been going well enough for the last couple of months," says Tempelis.
Progress in the west tunnel, from Waterman Canyon portal uphill toward Devil Canyon portal, is slower. Some 2,500 ft of Arrowhead Wests 19,880 ft have been excavated.
"Its been slow but steady," says Tempelis. "Theres been more startup problems. Its been a difficult combination of groundwater control and TBM performance."
The contractor declines comment but has cited differing site conditions at the Waterman Canyon portal that called for major grouting to consolidate material ahead of the TBM. To handle groundwater volume and pressure, Shea/Kenney purchased two TBMs from Germanys Herrenknecht AG. They are designed to work in hard rock but are fitted with a shield similar to soft ground machines. It allows them to...