Underground innovation
Southern Nevada is already dealing with the issues of water scarcity and is investing in underground infrastructure to prepare for the future. Erika Moonin engineering project manager with the Southern Nevada Water Authority explains the capital improvement projects underway at Lake Mead
THOSE WHO reside east of the Mississippi River driving west for the first time are often struck by the dramatic change in landscape as they motor along. Most travellers enter the desert southwest states of New Mexico, Utah, Arizona or Nevada after passing through hundreds of rain-blessed miles of green vegetation, rivers, and lakes.
 
A shaft at Lake Mead, Nevada
Following the setting sun west, bountiful waters become scarce and the land becomes more barren. The lack of verdant landscapes is unmistakably reflected in a stark terrestrial transformation caused by the West's naturally dry climate. Many visitors have struggled to see the beauty of the desert regions, but many others have found the raw exposure of the land to have a more sublime beauty and have been willing to adjust living practices and conserve the limited water resources in order to enjoy the benefits of living in the arid West.
Make no mistake; there are many water resources in the West, but compared to the East, water in the West is much more scarce and becoming more so, and that means communities need to make some difficult decisions. Answering the need
At first glance, Lake Mead, just on the outskirts of Las Vegas, appears to be the picture of water abundance. The 248-square-mile (642-sqaure-km) body of water is the nation's largest man-made reservoir capable of storing more than 28 million acre-feet (about 9 trillion gallons or 34 trillion litres) of water, which is used to satisfy the thirst of municipal and agricultural water users in Arizona, California, Nevada and Mexico. In fact, every drop of water in Lake Mead is highly regulated, each gallon allotted and every stored acre-foot bagged and tagged to meet water demands all the way to Mexico.
 
Workers at the various sites for Lake Mead's Intake No. 3 project.
More than 40 million people depend on the Colorado River, which flows into Lake Mead; however, severe drought conditions over the past 16 years have significantly reduced the river's flows and decreased Lake Mead's water levels. In response to the drought, water managers throughout the desert southwest are implementing progressive actions to maintain sustainable balances between water supplies and water demands.
Few water wholesalers have had to face as many droughtdriven challenges as the Southern Nevada Water Authority (SNWA). In a community that receives only four inches of rain per year, Southern Nevada relies upon Lake Mead for 90 per cent of its water supply. Ensuring reliable drinking water for two million residents and more than 40 million annual visitors, SNWA employs a comprehensive 50-year water resource plan that balances water supplies with the region's current and future water demands. While drought conditions heavily influence water resource planning efforts, capital investment in new underground water infrastructure is helping secure the community's access to its critical water supplies in Lake Mead.
An Underground solution
Long-term forecasts for Lake Mead raised enough concern about the future operability of SNWA's existing intake system that the board of directors in 2005 directed staff to design and construct a third water intake, and more recently, a new pumping station capable of pumping water from extremely low lake levels.
The primary objective for the Lake Mead Intake No. 3 project was to maintain access to Southern Nevada's existing water supply should the lake level continue to decline. It's not about investing in infrastructure to get more water - it's about investing just to protect access to the existing supply. Through forward-thinking leadership and an abundance of planning, engineering and construction ingenuity, the solution to securing access to Southern Nevada's Lake Mead water supplies was found deep underground, and deep underneath Lake Mead. By the time sustained drought-conditions reduced Lake Mead's storage to 43 per cent of its capacity in 2009, SNWA had already begun construction on the Lake Mead Intake No. 3 project - the most ambitious project and single largest public works contract ever undertaken by SNWA.
The main components of the project include an intake structure and underground tunnel that draws and conveys water from a deeper location in the lake. Even if the lake drops too low for water to pass through Hoover Dam to meet downstream water demands, Southern Nevada will still have access via the new intake. The project's earlier contracts also include a connecting tunnel from the new intake to the SNWA's existing intake system, as well as a number of underground connections and modifications to accommodate the flow of water from Intake No. 3 to existing intake pumping stations.
Though supported by a large above-ground presence, the heartbeat of construction for the Intake No. 3 tunnelling project was staged and progressed 600ft (183m) underground. There, contractors drilled and blasted first a 600ft-deep access shaft, then a large cavern where they assembled a 1,600-ton TBM. They then sustained the advancing TBM underneath Lake Mead for several years in challenging geologic conditions and extreme hydrostatic pressure due to the direct underground connectivity to lake water, setting the world record with a sustained operating pressure as high as 15 bar.
The skilled underground crews were reinforced by a robust team of engineers, managers, technicians and construction personnel who worked as a team to complete 3 miles (4.8km) of 20ft (6m)-diameter segmental-lined tunnel underneath the lake to connect to an awaiting 100ft (30m)-tall intake structure. The intake structure, constructed concurrently with the tunnel excavation, took nearly five years to design, construct and secure to the bottom of Lake Mead using nearly 12,000 cubic yards of tremie concrete. Work on the intake structure was completed in March 2012.
The new intake facilities were designed to convey 900 million gallons (3.4 billion litres) of water per day. Since its commissioning in September 2015, billions of gallons of water have already flowed through the intake system's series of tunnels, underground caverns, shafts and forebays before finally reaching customer taps throughout the valley.
Low Levels
By December 2014, with Intake No. 3 nearing completion, drought conditions in the Colorado River Basin were persisting, causing record low water levels since the filling of Lake Mead after the construction of Hoover Dam. Projections for Lake Mead's elevation indicated continuing declines in lake levels, absent significant relief from the drought, putting the operability of both of SNWA's existing intake pumping stations in jeopardy. The pursuit of a third pumping station, an original but deferred component of the Lake Mead Intake No. 3 project, was quickly sanctioned by the board of directors. Both the board and a citizen advisory committee strongly recommended the new pumping station be developed in the swiftest feasible timeframe in order to keep ahead of a dropping lake.
The aptly named new Low Lake Level Pumping Station (L3PS) will replace the capacity of the current intake pumping facilities and operate at very low lake levels.
Once again, planners and engineers looked underground for the solution. The new pumping station includes a 525ft (160m) deep, 20ft (6m) diameter vertical shaft, an underground forebay tunnel and connection, and 34 deep well shafts for submersible pumps.
The contractor and the SNWA have formed an alliance under a Construction Manager at-Risk contract approved by the board of directors in May 2015. The L3PS team, comprised of SNWA project representatives, consulting engineers, and the Construction Manager at-Risk and the program management firm, is engineering the project to include design features that are both economical and compatible with efficient construction methods. Construction of the pumping station is under way and the project is expected to reach completion in 2020.
Conclusion
At a time when drought and water scarcity are increasingly affecting communities worldwide, Lake Mead Intake No. 3 demonstrates how Southern Nevada has remained ahead of the curve when it comes to the drought and declining water levels in Lake Mead.
Using innovative tunnelling technology and progressive engineering solutions, SNWA has incorporated underground space - including a 3-mile tunnel underneath Lake Mead - to secure a reliable water supply for the long-term future of Southern Nevada. The cutting edge, innovative use of underground space for the purpose of protecting the community's water supply ensures that water will continue to flow despite the persistent signs of scarcity ringing the shores of Lake Mead

3 May 2016
http://www.tunnelsonline.info/features/underground-innovation-4881661/