Deep Shafts Will Maintain Las Vegas Water Supply
On the shores of a shrinking Lake Mead in southeastern Nevada, the Southern Nevada Water Authority is four years into a $650-million low-level pumping station that’s been blasted and excavated and fitted with specially designed pumps to serve a thirsty Las Vegas.
The pumping station—called L3PS by the Southern Nevada Water Authority—is the final piece to a $1.3-billion plan developed more than a decade ago in response to declining lake levels. From 2008 to 2015, a joint venture of S.A. Healy Co. and Italy’s Impregilo S.p.A completed a third intake for Lake Mead that is able to draw water at 860 ft, the lowest possible level.
Through a drill-blast-excavation approach, a 600-ft-deep, 30-ft-wide access shaft was carved for the tunnel-boring machine launch chamber, stub tunnels and mole starter tunnel. A nearly three-mile-long tunnel was connected to the preconstructed intake deposited on the bottom of Lake Mead.
While the new intake was connected to two decades-old pumping stations, they both would become useless at the lowest lake levels. Consequently, in 2015, the Southern Nevada Water Authority (SNWA) Board of Directors approved the low-level pumping station. The contractor is Barnard of Nevada. Parsons has served as construction manager for both the third intake and L3PS.
Initially, both the third intake and L3PS were to be built concurrently beginning in 2008, but Erika Moonin, project manager, SNWA, says that while the deferred start delayed completion, it benefited the end result.
“The delay was good because it allowed us to take a hard look at the station and redesign. So we made it deeper and able to operate at a higher capacity,” says Moonin.
Blast, Drill, Finish
The main components of the new pumping station include 34 well shafts more than 500 ft deep and 6 ft in diameter to accommodate pumps, a 26-ft-dia access shaft, a similarly sized riser shaft and an underground forebay 36 ft high by 33 ft wide by 377 ft long. The flow rate of the pumping station will be more than 900 million gallons per day when it enters operation in 2020.
The submersible pumps are in shafts and can be removed by a 220-ton gantry crane. The pumps are being installed with the crane. An adjacent support building contains power and pump controls. A 69/13.8-kv substation was also built to serve the pumping station.
The project’s first step was to create the forebay, access shaft and riser shaft through drill and blast excavation. Subsurface crews removed more than 58,000 cu yd of blasted rock typically called muck to make the underground openings, according to Bronson Mack, SNWA spokesman. To complete the underground excavation process, crews used 271 controlled blast rounds and more than 98,000 lb of package blasting explosive materials. Barnard Construction self-performed the excavation. The forebay was then finished by applying shotcrete to the surrounding walls.
Two bulkheads were installed at the access and riser shafts, Moonin says. One connects Intake No. 3 to the L3PS. The other, she says, allows for a possible future addition to the pumping station—it could be removed once another forebay was excavated and finished.
The forebay was completed in summer 2018.
The next step in the construction sequence was to drill the 34 pump shafts. For this task, which relied on accurately drilling approximately 500 ft to reach a precise location inside the forebay, Barnard relied on specialty contractor North American Drillers. Crews started the drilling of each shaft with an 8-in. directionally guided drill bit, says Jordan Hoover, project director at Barnard. The first drilling was followed by a 17.5-in. ream and a final ream of 91 in. with a 4-in. tolerance, he says.
The shafts were then finished by a tremie concrete pour, he says. Tremie concrete placement utilizes a vertical or nearly vertical pipe, and concrete is placed by gravity feed below water level. The lower end of the pipe is immersed in un-set concrete so concrete rising from the bottom displaces the water. Hoover says the tremie cementitious grout was used to fill the annular space between the 74-in.-dia well casing and the 91-in. reamed shaft.
In order to meet the requirements of the pumping station, construction manager Parsons and owner SNWA needed to acquire pumps that exceeded the pumping parameters of any pump on the market, Moonin says. As a result, three pump manufacturers were contracted to build working models of the low-lift pumps: Indar of Spain, Ebara of Japan and Andritz AG from Austria.
She says after design and performance tests—including running for 150 hours at the factory—the contract for the remaining 19 low-lift pumps was awarded to Indar. Each low-lift pump is 23 ft long and can pump 30 million gallons of water per day, Moonin says. They are cooled by the water in which they are submerged. Each high-lift pump is 28 ft long and can also pump 30 million gallons of water per day. As of early April, four pumps had been delivered and are currently being installed.
Integral to the submerged pump approach on L3PS was the gantry crane, says Kevin Ulrey, construction manager at Parsons. The 220-ton capacity of the crane was driven by the weight of the pumps, he says. Each low-lift pump weighs about 68 tons and each high-lift pump weighs about 79 tons.
A specially designed collar has been created for the ground-level access point to the pump shafts that assist operators when installing and removing the pumps. The collar is moved to each shaft by the crane when needed. When opened, it allows access to the pump shaft, and when closed, the collar can hold the pump at several heights.
SNWA expects to begin testing the plant’s control system and the pumps by the end of the year and anticipates the L3PS will come online in early 2020.