Casinos, fantastic hotels, stone for stone copies of famous European landmarks, star studded cabarets and spur of the moment marriages - images that spring to mind when the name Las Vegas is mentioned. Las Vegas is also the location for major conferences and exhibitions - all comfortably accommodated in the desert city's extensive hotels.
All this activity, currently running at 34 million business and leisure visitors a year, requires a substantial resident community to cater to its needs. Southern Nevada is the fastest-growing metropolitan area in the whole of the US. Its desert location means that it is also the driest.
The result is a $2bn capital improvement programme by Southern Nevada Water Authority. Work began in 1997 and is due for completion in 2017. Projects include a massive second intake tunnel, a new treatment works and an upgrade of the existing Alfred Merritt Smith water treatment facility.
Las Vegas draws 85% of its potable water from Lake Mead, created by the building of the Hoover Dam. Eyebrows have been raised, since the upgrade gives the capacity to deal with more than the area's current allocation of the lake's resources and there are also concerns about the quality of the water being taken into the system.
Cornerstone of the project is the new intake which was completed earlier this year at a cost of $80M. Contractor for the intake and its associated tunnel was Lake Mead Constructors, a joint venture of two regional subsidiaries of the Nebraskabased contractor Peter Kiewit Sons. This group also won the contract for the new River Mountains treatment plant, plus a number of others, bringing the value of its work to $427m. Construction manager for the entire program is Parsons Infrastructure & Technology.
The new intake is on Saddle Island, site of the first intake, and geologically the best choice, being a block of old basement rock surrounded by younger volcanic flows and pyroclastic deposits.
The shaft for the 3.6m wide steel intake was drilled and broken out working from a floating barge in the lake. A 15m square bench was cut in the lower side of the island, about 69m below the surface of the lake. The intake leads into a 488m long intake tunnel and from there into a chamber, intersected by no fewer than 22 wells, each 116m deep - all contained in an area measuring 4.5m by 45m.
The rock through which the tunnel, chamber and wells were driven proved to be highly fractured during the drill and blast drive calling on a complex grouting operation using microfine cement grout. Grout was injected prior to blasting to keep water infiltration to a minimum and the team attempted to reach even the smallest cracks. Nonetheless, at times 2,800 litre/min was entering the tunnel at pressures of around 100psi. This meant consistently wet and muddy conditions for the tunnelling crew - a sharp contrast with the dry desert air above ground.
The 4.8m high tunnel runs into the 49m long, 9m wide and 10.6m high chamber from where vertical pumps in the wells will draw water into the pipelines which run to the new treatment plant.
The original plan was that these 1.8m diameter shafts would be raise bored, but investigation showed the rock to be more fractured and groundwater more extensive than thought, making that technique unsuitable.
One solution would have been an extensive grouting operation, but this was rejected in favour of blind boring, which involves downhole drilling following a directionally drilled pilot bore.
Accuracy was important because the shafts have to accept steel linings. A programme of rock bolting and shotcreting on the arched roof of the chamber avoided rock loss.
Grouting of the 76mm annulus between the well shaft wall and the steel lining is carried out in stages to be certain of filling the void. Once this is complete the transition between the arch of the chamber and the casing will be smoothed with mortar. The array of pumps in the 22 shafts will have a capacity of 22.7M litre/day.
Designer of the Lake Mead intake and raw water pumping system was a joint venture of Montgomery Watson and CH2M Hill, also responsible for the River Mountains treatment plant.
Upgrading of the existing Alfred Merritt Smith works was designed by a joint venture of Carollo Engineers and Black & Veatch and incorporates an ozone disinfection system. Contractor JR Jacks, a subsidiary of Ogden Energy Group, successfully completed what is believed to be the world's largest hot tap when two holes were cut into the 3m diameter, 280mm thick concrete pipe which delivers lake water to the plant, to prepare for the diversion of water to the newly added ozone system.
Finding a site for the new treatment plant proved difficult and the one selected is just a few hundred metres from a residential community. Following concern about the transport of chemicals through the area, the plant is being designed to use sodium hypochlorite which will be produced on site. Initial capacity of the plant will be 5.7M litre/day but construction allows for eventual doubling of capacity, a change order for which Kiewit will be paid an extra $31m on top of its original $147m bid.
The project has attracted controversy over the authority's labour agreement which specifies that union workers be used.
But work is currently on time and will almost certainly come in at less than the $2.2bn originally envisaged.
One group of Nevada residents has certainly had its interests protected. The Nevada desert tortoise is an endangered species so contract documents specified that specialists be present on site to monitor and relocate the creatures. So far more than 50 tortoises have been moved to protected areas. The workers who found them now sport T-shirts bearing the legend 'preserving Nevada's desert' as well as recognition stickers on their hard hats.