Digging Deeper to Assure Las Vegas Water Supply
When a nearly 3-mile-long tunnel reached a preconstructed intake into Lake Mead in December, it was a decisive moment for a vast team of planners and contractors as it marked the beginning of the final stage of an $800-million project that began more than six years ago.
In the course of the project, which intends to add a third intake to Lake Mead, contractors set a world tunneling record. Perhaps more importantly for residents and business owners in Las Vegas, though, is that their drinking water will continue to flow even if water levels continue to decrease in the lake.
Under the direction of owner Southern Nevada Water Authority, a vast majority of the work was performed through a design-build contract with Vegas Tunnel Constructors, a joint venture of S.A. Healy Co. and Impregilo S.p.A. Additionally, Parsons is performing construction management for the owner.
The current drought in the West has turned the Intake No. 3 project from one primarily designed to increase water quality to one that is designed to save the Las Vegas Valley from drying out, says Erika Moonin, project manager at Southern Nevada Water Authority.
"In the early planning stages, we didn't anticipate that the lake level was going to drop so severely," Moonin says. "By 2005, with the drought persisting, it became more of a concern of capacity. So, when we took it to the board, it was primarily about capacity."
Lake levels in the reservoir have dropped to nearly 1,080 ft above sea level in fall 2014 from 1,200 ft above sea level in 2000. Intake No. 3 will allow for water to feed the Las Vegas area until it reaches about 875 ft.
The tunnel that feeds the third Lake Mead intake meets with the tunnels from the existing two, which then transport raw water to the Alfred Merritt Smith Water Treatment Facility and the River Mountains Water Treatment Facility. After approval from the Southern Nevada Water Authority board and the selection of Vegas Tunnel Constructors, work began on the intake structure in 2008 and included extensive barge and underwater work.
The project got off to a booming start. More than 23,000 shaped charges were attached to a remote operated frame and lowered by a crane barge to the lake bottom to blast much of the area where the intake's base was to be placed. After blasting and excavation, the intake structure was transported to the proper location by tugboat and barge. The intake structure, which included a reception eye for a tunnel boring machine (TBM), was built within a 95-ft-wide by 159-ft-long by 83-ft-deep pit at the bottom of the lake at a water depth of about 350 ft. Tremie concrete was then used to backfill the area. A 16-ft stainless steel riser sits on top of the base. About 12,000 cu yd of underwater concrete was used to anchor the intake's base.
The total volume of the excavation was 40,000 cu yd, of which 15,000 cu yd was overburden and 25,000 cu yd was rock. Underwater excavation lasted nearly two years.
The intake structure was completed in 2012 and sat waiting for the tunnel from the shoreline to be completed. The tunnel construction process was begun in early 2010 and included preparation work for the TBM that would drill out the tunnel. Vegas Tunnel Constructors began that process by drill, blast and excavation to create a 600-ft deep, 30-ft wide access shaft where a tunnel boring machine launch chamber, stub tunnels and a boring machine starter tunnel were excavated.