At Lake Mathews in Southern California, JF Shea is preparing to construct a 169ft (51.5m) high x 32ft (9.75m) i.d. concrete tower as part of the new reservoir outlet facilities. The new tower, built within a 112ft (34m) deep excavation, protected from the waters of the lake by a soldier and sheet piled cofferdam, is connected at the bottom to a new outlet tunnel.
The new works are designed to supplement the existing outlet facilities which were determined under recent studies to be incapable of withstanding a maximum credible earthquake from either the Elsinore or San Jacinto Faults.
Owned and operated by the Metropolitan Water District of Southern California (MWD), Lake Mathews is the terminus of the Colorado River Aqueduct and is the principal source of potable water to some 8.4 million people in the Los Angeles, Orange and Riverside counties. As the original outlet facilities could not be decommissioned to carry out lengthy rehabilitation to meet the necessary seismic requirements, the decision was to build new facilities.
MWD, under project manager Lilly Shraibati, managed the design of the new outlet facilities and engaged Jacobs Associates to design the underground and surface excavations. The underground works include 541ft (165m) of 17ft 10in (5.4m) i.d. tunnel to connect the new tower, with its six tiers of five intake butterfly valves, to a new 48ft (14.6m) i.d. x 165ft (50m) deep land-based junction shaft. From the shaft two short 40ft (12m) x 14ft (4.3m) i.d. tunnels connect to the existing 2,400ft (730m) long x 14ft (4.3m) i.d. steel lined outlet tunnel that leads to the reservoir’s forebay. Once the new facilities are competed the original outlet tower will be modified by removing its top 125ft (38m). The new tower will draw from higher levels of the reservoir with the lower ports of the original tower drawing from deeper levels. Water from the two towers will combine in the junction shaft and flow through the existing tunnel to the reservoir’s forebay. A bulkhead in the existing outlet tunnel between two short connecting tunnels will divert deep water draw into the junction shaft and direct water outflow into the existing tunnel. A third gated opening in the junction shaft is the stub of a tunnel of up to perhaps 10 miles (16km) long to be excavated as part of MWD’s future water management plans.
All excavation for the new facilities in the granitic rock mass, was by drill+blast and with strict controls to avoid any damage of the adjacent man-made dyke, holding one side of the reservoir, and the existing outlet facilities, that had to remain operating at all times.
Work on the $45 million contract by Shea started in September 2000 with drill+blast of the junction shaft. At the same time the level of the reservoir was drawn down to allow construction of the sheet piled cofferdam in dry conditions. A grout curtain was injected beneath the perimeter of the cofferdam to treat weathered granite and to help seal off the foundation. With the reservoir level restored, a head of some 18ft (5.5m) is acting against the cofferdam. Excavation for the tower within the cofferdam was also preceded by installation of a 100ft (30m) deep grout curtain.
During a T&TNA visit to the job in early April, all involved agreed that the project has been a major success to date. “There were situations of changed conditions during the tunnel and tower excavations,” said project manager Steve Cox for Shea, “but these were negotiated effectively and resolved without claims.”
The most significant change was the more severely crushed nature of weathered granite than expected in a set of identified shear zones. “The faces had very low stand up times,” said Steve Dubnewych, for the Jacobs design team. “These first appeared in the tower excavation and then in the tunnel alignment. But for the fast acting support of fiber reinforced shotcrete applied in flash layers as quickly after excavation as possible, the tower excavation would have become extremely difficult and much more complex. In the tunnel, steel arches supplied on site by Shea were installed systematically through the majority of the tunnel excavation. The time and cost implications of dealing with the changed conditions were settled at the time and long before the end of the contract.”
The land-based junction shaft was excavated using one of the few existing Gardner Denver three-boom shaft drilling jumbos. The rig is suspended from a tower crane and held within the shaft by lateral grippers.
In the tunnels and tower foundations Shea used a two-boom Tamrock drill rig. The shotcrete used on the project was a wet mix from a local batching plant and dosed at the plant with 66lb/yd³ of Dramix steel fiber. Shotcrete was applied using hand held nozzles in 4in thick layers and support also comprised 15-35ft (4.5-10.6m) long rockbolts as required.
When T&TNA visited the site, all major excavations were complete and the in-situ concrete lining in the new outlet tunnel was also complete. Over the coming months the new concrete outlet tower will be constructed and the in-situ concrete lining in the junction shaft will be cast.
To complete the project, the water of the reservoir will be drawn down again in February 2003 and the existing facilities shut down for two-months to excavate the final tie-in to the existing outlet tunnel. Liquidated damages of $30,000/day or a bonus of $5,000/day will apply if the work is or is not completed within the two-month window. Success during this critical round-the-clock activity will close out what has been as a satisfactory project for all.
