SNWA’s 20-ft. (6.09 m) diameter, 3-mi. (4.8 km) long Intake No.3 tunnel is nearly complete. The yellow pipe on top of the tunnel is part of the airflow system to support construction activities, and it will be removed before the new intake system is
A milestone was reached for the Southern Nevada Water Authority’s Lake Mead Intake No. 3 on Sept. 10, when the first 11 million gal. of an expected 41 million gal. of water entered the new 3-mi. (4.8 km) deepwater intake underneath the Lake Mead reservoir in Nevada.
Furthermore, the cap for the new intake was removed on Sept. 23, permitting water from the Colorado River reservoir to directly flow into two existing SNWA pumping stations located at Lake Mead. The $817 million design-build contract to construct the intake was awarded to Vegas Tunnel Constructors (VGC), a joint venture consisting of Salini Impregilo/S.A. Healy Company.
The project had its challenges and setbacks and had crews working aboveground, underground and on the water. It also required the technical skills of engineers and the latest scientific knowledge on mining rock and soil conditions, and the physics of operating in high-pressure environments.
The underground tunneling and excavation began Dec. 27, 2011 via tunnel boring machine (TBM) manufactured by Herrenknecht AG in Germany. The machine is approximately 620 ft. (189 m) long (including 16 gantries); weighs 1,659 tons (597 t); can operate in hard, sedimentary and gravel rock with high water inflow; has a 23.6-ft. (7.19 m) diameter with 44 disc cutters and 23 knives; estimated to cover an average of 35 ft. (10.6 m) per day; and has an operating crew of approximately 14 persons. The TBM’s best day had it drill 108 ft. (33 m) of tunnel.
The intake will be fully operational and have an expected lifespan of more than 100 years.
The Las Vegas Review Journal recently reported that crews initiated the first part of the lid removal process, which required a second watertight lid to be lowered over the mouth of one of the existing intakes, to supply water to the surrounding area. The water, in the sealed connector tunnel, was drained via valves in the temporary bulkhead and directed into the new intake tunnel.
The bulkhead prevents water in the connector tunnel from seeping into the intake during construction. In order to remove the bulkhead, the tunnel has to be sealed off from the lake and drained so that crews from VTC, the contractor, could start on the inside work. When the bulkhead was removed, the SNWA let in another 30 million gal. at a controlled rate to fill the rest of the tunnel.
The project started in 2008. Some of the key elements are: it opens at elevation 860 ft. (262 m) below sea level, has a 3-mi. long tunnel that draws and conveys water from a deeper location in Lake Mead, and has a connecting tunnel from the new intake to the existing intake pumping stations.
The tunnel has a finished diameter of 20 ft. (6 m) and a gasketed concrete segmental lining that forms a ring — six segments per-ring, with 2,429 rings in total. The intake structure and riser are secured to the bottom of Lake Mead by a large concrete mass.
In addition, significant modifications to the existing intake system to accommodate interconnection with the new third intake were undertaken, including emergency connections made to tie in SNWA Pumping Station No. 1 to the new Intake No. 3 system, providing additional operating range for the existing pumping station and providing greater flexibility in operating the system.
This project is necessary to ensure water for the greater Las Vegas area.
“An ongoing, severe drought in the Colorado River Basin — and its impact on Lake Mead’s water level — are key factors defining the purpose and need for the SNWA’s Lake Mead Intake No. 3,” said Bronson L. Mack of SNWA public outreach and media relations. “Lake Mead is responsible for 90 percent of the Las Vegas Valley’s drinking water supply, and the lake is currently at less than 40 percent of its storage capacity, evident by a 130-foot-high ’bathtub ring’ visible around the lake.
“The declining lake levels raised enough concern about the future operability of one of the existing two intakes,” he said, “prompting the SNWA Board of Directors in 2005 to approve design and construction of a third intake into Lake Mead. The new intake’s primary objective is to maintain southern Nevada’s access to its primary water supply. Additionally, in 2015, the board approved the SNWA to begin developing a low-lake-level pumping station that will further ensure that southern Nevada can draw upon its Colorado River water supply, even if Lake Mead’s levels drop too low for water to be released to downstream users in Arizona and California.”
The total project cost covers construction, engineering, construction management and materials. The engineering firm involved in the project, brought in by the J-V, was ARUP, which subcontracted Brierley Associates. SNWA and Parsons Corporation are handling construction management.
“We did a concept-level design, which was very preliminary and we set the criteria for them,” said Erika Moonin, SNWA’s project manager, “and they took it from there — it was their team that developed the design. We looked at the Arrowhead Tunnel project in southern California, the Brightwater project near Seattle; and the Hallandsås Ridge Tunnel in Sweden — a high pressure tunnel project, not as high as what we encountered, but it was the highest pressure one in the world at the time. We also looked at several others for similar conditions and how they approached the work.”
While not involved in the TBM selection, the SNWA placed operating requirements into the contract, including minimums and the ability to operate at a maximum pressure of 17 Bar units — a metric unit of pressure exactly equal to 100000 PA, which is nearly equal to the atmospheric pressure on Earth at sea level. Also included was the need for backup equipment.
The SNWA took about 52 geological core samples during the pre-design phase in looking at the alignment section and when the J-V arrived, it took additional core samples.
“We knew that it was highly faulted and that we were going to have some very difficult conditions,” said Moonin. “We ran into a large fault — inactive. We knew the area was faulted — we just didn’t understand the situation at that point when we put the contract together how complicated it was and the magnitude of it until we encountered it.”
The project was followed by several organizations, including the California Water Fix Team; a group from Hawaii which had started construction on a tunnel; and the New York Department of Environmental Protection, which is planning to build a tunnel underneath the Hudson River to provide water and is anticipating some high water pressure, potentially higher than the conditions encountered in Nevada.
“We’re trying to accommodate one another as owners, because that is how we learned and put things in our contracts,” sais Moonin. “If we have something to share that can benefit others, we’re willing to do that. We’re also asked about our delivery method — how we put together the procurement for the design-build and the construction manager-at-risk contracts, an alternative delivery method.”
Moonin and her team held regular progress meetings with the contractor to discuss various issues, which was essentially a daily dialogue to ensure that problems were rapidly resolved and potential problems were addressed early on.
“It’s probably the reason why we were successful with the project,” she said. “We had commitments with upper management from the very beginning. We wanted to have open communications and develop a true working partnership.”
This attitude was crucial when trouble arose with the setting up of the starter tunnel that needed to be created by traditional drill and blast methods — it had to be created before the TBM could be assembled.
“When they encountered that large complicated fault zone, the material flowed into the workspace and it flooded the TBM assembly chamber and the starter tunnel,” she said. “We had to work together to overcome it and a lot of experts were brought in to develop plans to go back in there and start tunneling again. We had two subsequent inflows on that same alignment. This challenge brought us all together and it was the contractor who proposed we re-align it as the ground was difficult to get through — they felt the re-alignment would be faster and less costly than the original one.”
It was at this point that the SNWA and J-V made an agreement to quickly wrap up everything into one change order and share the risk going forward.
“We asked the contractor to take on some risk that they don’t normally do,” said Moonin, “and via the change order we paid for the work on the original starter tunnel and paid for the work going forward. It really worked out. We hadn’t made any payments to the contractor for the tunnel recovery work and we had an insurance claim, so it was a very complicated situation. Everyone realized that for the sake of the project that we had to move forward and resolve this major challenge quickly.”
The cap removal operation went smoothly, but it required a massive amount of coordination as the SNWA had to shut down two major pumping stations and two intakes.
“This was done one at a time,” said Moonin, “which ultimately meant the shut down of a water treatment plant and the pumping stations. All the parties were pleased with bringing the intake on-line.”
Vegas Tunnel Contractors crews began drilling the access shaft for the TBM in 2008 and between 2008 and the end of 2011, successfully excavated the access shaft, starter tunnel, TBM assembly chamber, and a tail tunnel.
“There were contractual delays, of which there was a time extension for the grouting in the shaft,” said Jim Nickerson, the J-V’s project manager, who has been with the project since 2009, “then here was the contractual delay of the original starter tunnel, which delayed the project for about a year. The entire project is headed to finishing early with the entire contractual conditions.”
Only one TBM manufacturer expressed an interest in the project — Herrenknecht AG and the machine was purchased in the spring of 2008 and arrived on site in September 2009. The TBM operations started at the end of December 2011, and were completed on Dec. 10, 2014.
“It was one of the most complicated conditions that a TBM has ever operated in because we had to mine up to 15 Bar pressure,” said Nickerson, “which is the first time it’s ever been done. We encountered a lot of volcanic rock, basalts, and sedimentary rock. A lot of it was fractured and broken, but the real problem was the hydrostatic pressure of the lake above it that caused most of the grief.
“We had up to 15 Bar pressure and when you have water with rock, it makes for very unstable ground condition,” he added. “We had water in the shaft, cavern and tail tunnels, but those were conventional excavations — drill and blast. After grouting, we had anywhere between 800 to 1,000 GPM of water after the grouting programs.”
Nearly 360,000 cu. yds. (275,239 cu m) of rock were brought to the surface and due to environmental permit requirements, the material remained on site and was used to create a view-shed berm, which when the lake level is high, prevents boaters and those in the Lake Mead National Recreation Area from seeing the water treatment plant.
“It could have been used elsewhere,” said Nickerson, “but nothing was wasted.”
The TBM drilled down to 600 ft. (183 m) and then Nickerson had his crews create an underground cavern system. The width of the cavern is 30 ft. (9.1 m) and as it was expanded, equipment and vehicles were lowered down.
Crews accessed the cavern via personnel hoists made by Alimak and baskets attached to Cimolai gantry cranes. To lower the equipment and vehicles, mine hoists were used.
“All the oils and diesel had to be drained and everything was turned vertically,” said Nickerson, “and everything went down a doorway, through a platform opening on top of the shaft.”
Some of the equipment brought down for the drill and blast operations included a large Tamriock Jumbo, a 5 cu. yd. (3.8 cu m) rubber-tired mine mucker, a 939 track loader, forklifts, and at one point, a concrete mixing truck.
Nickerson, with 32 years experience in underground work, not only put his knowledge to the test, but also his team’s as they applied some cutting-edge techniques and brought in innovations via the TBM, such as mining in closed mode up to 15 Bar.
“The other aspect was connection with the TBM and marine intake structure that we had to perform — fabricating and installing an intake structure that weighed 1,200 tons,” he explained. “We had to blast a shaft in Lake Mead and then we had to set the 1,200 ton structure within that excavation and then we poured 12,000 cubic yards of concrete around that intake structure. After that was done, we mined our TBM into it and completed the connection to the inlet to the lake. We did all of that work without divers, using remote-operated vehicles.”
In fact, the J-V had a small fleet of barges, tugboats, and crew boats made to specifications of the project, as well as the remote operated submersibles.
There was plenty of room for temporary field offices, materials storage and equipment and vehicles. The job site was accessed via park road that was bolstered by building a paved road that has been given to the park.
The work went on 24 hours a day, nearly 365 days a year via three shifts — day, swing and graveyard.
“Our safety record was very good and we ended up with about a 4.0 recordable incident rate — standard in our industry, and a lost time incident rate of .89,” said Nickerson. “We nailed it and the water for the whole region is coming from the tunnel we built. We already had a history of being able to work on a slurry/closed mode TBM job and we’re leaving with the experience of mining in 15 Bar conditions.
“At the height of the work we had about 150 hourly employees for most of the critical path work, we were down to 75 once the marine work was done and we started to finish up the tunnel after the boring ended,” he added. “This has been a very difficult and challenging job. It was the dedication and perseverance of all our people. There were times when you wondered if you could build it — to complete it.”
J-V crews did most of the work. Subcontractors included BMT for the riser section of intake structure, Precision Aggregate for the purchase of the trimming concrete.
To provide air for the crews in the cavern/underground work area, a forced air ventilation system was set up, one that was constantly monitored. Safety training also included how to avoid being bitten by rattlesnakes and poisonous spiders. The job site infringed on the habitat of a species of endangered desert tortoise and crews were instructed on how to avoid incidents with them.
Concrete was produced on site via a Liebherr portable concrete plant.
“We designed the concrete mix. It took us 12 months to find the right formula,” said Nickerson.
Other equipment used for the project included excavators, dozers, backhoes, and dump trucks.
“We had a lot of on site mechanics for all of the equipment, whether it was the TBM or underground and surface equipment,” said Nickerson. “Maintenance on the TBM was done daily. Herrenknecht AG provided a staff for the commissioning of the TBM and they had a person on site for about a year. They also provided training for the operators and the mechanics.”
The daily progress of the TBM varied between closed and open mode operations.
“On good days in closed mode we did 50 feet and saw over 100 feet with open mode,” said Nickerson.
Moonin arranged for SNWA personnel to visit the site — aboveground and below, and the project experience has yielded many lessons learned for future initiatives such as partnerships and risk management approaches.
“The alternative delivery method worked for us. We used qualification-based selection of contractor teams and we’re using it again for the new pumping station project,” she said. “We also know a little bit more about the ground and sited the pumping station where we believe there is less of a chance to encounter that large fault zone. We know that we will encounter some smaller faults, but we’ve shared information with the contractor, Barnard Inc. It’s really about being open and transparent and making sure that everyone understands the risks.
“We also had some unique contract provisions for different site condition claims and had allowance items,” she added. “We put those same provisions in the new contract and we have shared that with other owners interested in how we developed them.”
The pumping station will be built about 500 ft. (152.4 m) from the new intake tunnel.