The Las Vegas valley depends on Lake Mead, the largest reservoir in the US, for 90 per cent of its drinking water supply. But a severe drought in the past 10 years has caused water levels to drastically recede, straining the system that supplies water to 25 million people in the Southwest.

With two intakes already in operation, Southern Nevada Water Authority (SNWA) began planning to construct Intake Number Three. Once the intake is complete, it will be able to draw from deep in Lake Mead and keep water flowing to Las Vegas even if the reservoir drops below the two existing intake pipes.

Contractor Vegas Tunnel Constructors (VTC), a joint venture of Impregilo and its subsidiary SA Healy, has a USD 488M design-build contract for a 2.8-mile (4.5km) tunnel and the intake structure. The tunnel is lined with a gasketed precast concrete segment lining with an inner diameter of 20ft (6m).

Renda Pacific, a joint venture of Oscar Renda Contracting and Southland Contracting, is excavating the connector tunnel of around 2,820ft (860m), a contract worth USD 45M. In spring 2010, Barnard of Nevada completed a small 570ft (174m) tunnel that will connect the new Intake Number Three with the existing Intake Number Two, which was worth USD 32M. Engineering services for Intake Number Three include program manager Parsons Corporation, and a joint venture of MWH and CH2M-Hill is the designer.

One of the main challenges of the project is dealing with varying ground conditions of solid to weak and highly fractured rock, with the potential for high water pressure up to 17 bar. In 2010, the team excavating the starter tunnel for the TBM hit a fault zone, causing water and debris to flood the work area. The area flooded twice more that year.

"Since work began the most significant challenge is the conditions that we found in the original starter tunnel, an ancient fault in the rock," says Jim Nickerson, deputy project manager of VTC. "With exploratory work that we carried out from the surface and from underground we had an interpretation of where that fault was going. This facilitated a new alignment and this time the geology was what was anticipated and excavation went accordingly."

In January 2011, VTC carried out three horizontal cores and 15 surface holes before selecting an alignment 23 degrees offset from the original. In addition, the launch chamber needed to be widened another 20ft (6m) to get a face to start a new tunnel.

Launching the TBM
The change order extended the timeline for the project to the summer of 2014. "As far as the TBM tunnel is concerned, we don’t have huge progress to report," says Nickerson. "We finished the starter tunnel at the end of July 2011 and then the TBM assembly could begin. We had a strand jack system specifically with the capacity to lower the TBM down the 600ft (183m) access shaft. We lowered the front section of the TBM, weighing 1,500t, and then we had to install eight gantries of the TBM, about half of the gantries.

"Once we completed the assembly of the front portion and the first eight gantries, which took us until the end of December 2011, we mined around 300ft (91m) and then stopped and put the rest of the gantries in, taking us into February 2012. We mined forward again and stopped roughly around 500ft (152m). Right now we are assembling the TBM for open mode."

The USD 25M Herreknecht TBM was built in Germany specifically for the third intake project. The 23.5ft (7.16m) diameter machine is a slurry and hard rock hybrid, roughly 623ft (190m) long. There is a continuous conveyor throughout the length of the tunnel, as well as a full slurry system. It is estimated that 70 per cent of the tunnel will be done in open mode, with the last stretch, at least 609m, in closed mode.

It takes a crew of about a dozen people to run the TBM. Mining operations are 24- hour, and overall average progress rates of 35 to 40ft (10 to 12m) per day are expected, but have not yet been achieved.

"All the mining that has been done so far, has been in closed mode, which means all the excavated material is being removed by a 14in (356mm) slurry line," Nickerson explains. "Our higher advance rates are going to be achieved in our open mode condition where we’re using our tunnel conveyors. However, the TBM has acted accordingly and I’d say that 35 – 40ft (10 to 12m) per day is not unrealistic."

Nickerson adds that only minor tweaks have been made to the TBM. "Our most significant change to the TBM was done prior to the assembly of the machine," says Nickerson. "We reinforced our cutterhead with more wear plates and more protection. We haven’t made any major changes to the TBM because we’re not fully commissioned with Herreknecht yet; right now I’d say we’re 90 per cent commissioned. The last bit is to commission the machine with the conveyor system and the screw conveyor that’s on the TBM."

The majority of the tunnel drive will be located in the Muddy Creek formation, which consists of late Tertiary sedimentary and volcanic bedrock. As the TBM approaches the area in which the intake structure will be built, it will pass through about 1,213ft (370m) of red sandstone and about 295ft (90m) of Callville Mesa basalt.

After completing the tunnel, the TBM will drive into the bottom section of the concrete intake riser at the bottom of the lake. This structure has steel reinforcement, overall, with fibreglass where the TBM will mine and dock. Once the connection is sealed and watertight, the TBM’s shield will be welded into a 30ft (9m) steel ring in the structure and left in place.

Lake excavations
The other component of VTC’s contract is marine operations to excavate the hole for the intake riser by drill and blast some 300ft (91.44m) below the surface of the water. These lake excavations have recently been completed.

"Our biggest achievement to date is this marine work," says Nickerson. "We blasted this shaft under 394ft (120m) of water using shaped charges, an explosive charge designed to focus the effect of the explosive’s energy.

We finished that excavation in late December 2011.

We had pre-built a 1,200t intake structure and the top portion of the structure was made out of stainless steel. The overall structure is roughly 98ft (30m) tall. In January 2012, we lowered a steel foundation, a guiding frame, into that excavation as that was what the intake structure was going to sit on."

The intake structure was constructed on a docking barge anchored near the shoreline. Upon completion of the underwater blasting and excavation, the intake structure was transported to the proper location by tugboat pushing the docking barge. It was lowered to the lake bottom in place with roughly 12,000 cubic yards of concrete.

"We had to pour concrete to lock it in place," explains Nickerson. "The pour had to be continuous, and it was continuous for 11 days. All of the concrete trucks, of which there were about 1,200 of them, were parked up two miles away and floated out on these barges until the pouring was complete. This has never been done in fresh water before."

In addition, Nickerson adds that the lake excavations were completed without divers. "All of this work has been done by remote operated vehicles, which is pretty difficult considering we were 300ft (91m) deep underwater," says Nickerson.

Back on land, VTC’s tunnel will connect with those being built by the Renda Pacific joint venture. Renda Pacific has excavated a 26ft (7.9m) diameter shaft by drill and blast and lined it with block concrete to a depth of 450ft (137m).

This is the depth at which the JV will build a 320ft (97.5m) tunnel to VTC. In the other direction, it is excavating a 2,500ft (762m) tunnel by drill and blast to the existing pump station.

Both tunnels are supported with 8ft (2.4m) long CT bolts at 5ft (1.5m) centres, and lined with 4in (100mm) of fibre-reinforced shotcrete. There is a 6in (152mm) concrete slab at the bottom. The shorter tunnel to VTC is 20ft by 20ft (6m) and for the most part flat, while the longer tunnel to the pump station is 16ft tall by 14ft wide (4.9 by 4.2m) with a 3.3 per cent grade. There are also two curves, each with a 160ft (48.7m) radius.

Toward the end of its contract, the JV will install a temporary steel bulkhead between the VTC tunnel and Renda’s shorter tunnel to allow the completed connection to be flooded. Separately, a bulkhead will be installed at the bottom of the shaft to all water to be conveyed in the tunnel during future construction of a pumping station for Intake Number Three that had previously been put on hold.