The Mansfield Dam was built between the years of 1937 and 1941 across a deep canyon at Marshall Ford on the Colorado River in Texas. With its completion, Lake Travis formed on the edge of Austin.
Ever since 1942 Lake Travis has provided the region with recreational activities and flood protection. For the city of Austin, in 2014, potable water will be added to that list when it completes the construction of the new Water Treatment Plant Four (WTP4).
Considering the city’s ever increasing population, without a new plant, the current treatment capacity will be stretched thin, creating restrictions on swimming pools, car washes and landscaping—not to mention losing adequate water pressure from fire hydrants.
Austin currently has two water treatment plants, both of which draw from Lake Austin. By building a new plant, not only will the city have enough water to meet the growing demand, but it will also diversify the source of its water.
Another advantage to drawing water from Lake Travis, rather than Lake Austin, is its higher elevation, which will allow the plant to rely mostly on gravity to distribute the water, not electric pumps. Energy savings of some 20,000MWh annually are expected, and that translates into enough electricity for a year for more than 2,000 homes.
For WTP4, the client, the Austin Water Utility, is working with the City of Austin, which is managing the design and construction. The Austin Water Utility will operate the plant once completed. This first phase of WTP4 will produce 50 million gallons (227.3 million litres) per day. With all phases eventually completed, the plant will be able to produce 300 million gallons (1.4 billion litres) per day.
There are separate design and construction contracts for the underground construction on WTP4, including its raw water system and the Jollyville Transmission Main, which will convey water from the plant to the reservoir (See page 14). Packaging the transmission main tunnel on its own helps to ensure it is completed around the same time as the other aspects of the project, providing a functional plant as efficiently as possible.
The raw water system mainly comprises an intake on Lake Travis, a tunnel from the intake to the pump station and a transmission main tunnel from the pump station to the water treatment plant (Figure 1).
MWH Constructors is the construction-manager- at-risk (CMAR), and Carollo Engineers is the prime designer of the treatment facilities for the city. AECOM, with subcontractor Brierley Associates, is the designer for the raw water system. The contractor for underground work on the raw water system is Austin Hill Country Constructors, a joint venture of Obayashi USA and Manson Construction Co., with the former working on land and the latter completing all marine work.
Location
The City of Austin has been working on developing a new treatment plant since the 1980s, and the previously-designed project had been put on hold. Its recent reincarnation, as of 2002, has introduced a change in the WTP4 design and in the tunnel alignments. In the earlier design, circa 1986, the site of the water treatment plant, to which one of the raw water tunnels connects, was in an environmentally sensitive location.
“In certain locations, we’re located on the Edwards formation, which is known to have a karstic environment,” says Jason Bybel, project manager for the City of Austin. “There is a potential to open up one of these karst features, or voids, and find endangered species.”
There are karst invertebrates, migratory birds and salamander species either classified as endangered or soon to be. For example, there are several karst invertebrates that are not yet on the protected list but they are of concern to the U.S. Fish and Wildlife Service.
To avoid these species the city purchased other sites for the water treatment plant and pump station to relocate the facilities in less critical areas. The tunnels were also relocated to avoid environmentally sensitive areas.
As with the Jollyville Transmission Main, excavations will be through three different formations: the Edwards, Walnut and Glen Rose—a dolomitic limestone. Most of the tunnel excavations will be through the Glen Rose.
Above ground the area is mostly greenfields, and there are no issues expected with subsidence. However the tunnels go under critical environmental features that can’t be disturbed. “One issue throughout this project, and this area of Texas, is that it’s significantly environmentally sensitive,” says Shelby Eckols, project manager with AECOM.
The 9ft (2.7m)-diameter raw water tunnel from the pump station to the intake at Lake Travis is 4,386ft (1,337m) long and will be excavated through the Glen Rose limestone, and below the karst formations. “We have put that tunnel as deep as we could to get the water from Lake Travis and minimize any environmental impact from that tunnel going to the lake,” Eckols says.
From the pump station to the water treatment plant there will be the 7ft (2.1m)-diameter raw water transmission main, which is 3,873ft (1,180m) long. He explains, “The tunnel was routed to avoid any environmental features. We came close to one critical environmental feature, and we added facilities in the tunnel—we put in concrete retards to prevent any water migration down the tunnel—so we did not impact that critical environmental feature.”
Factoring these concerns into the design of the raw water system requires not only locating facilities and alignments in areas with minimal environmental impact, but also taking preventative measures for critical areas above ground. Once a suitable location had been arranged for the pump station the site was excavated last year by Austin-based Ranger Excavating.
“We designed the pump station site to prevent storm water from leaving the site and impacting adjacent properties, both during construction and for the completed facilities,” Eckols says. “During construction, we required a water treatment plant to treat storm water that enters the site and filter it through a 30-micron filter, prior to discharge from the site. The storm water leaving the site is clean and doesn’t impact anything downstream.”
Excavating at the pump station and keeping storm water on site is important because the site is surrounded by preserve land, or land slated for future preservation, Bybel adds.
“Those areas are significant to some of the endangered species we have in the area. So it is critical that we limit the area of disturbance to the limits of construction.”
Construction
Austin Hill Country Constructors received notice to proceed on January 24. Whether designing or building the raw water system, the consensus is that the two biggest challenges to the project are making the connection from the raw water tunnel to the intake, and the 450ft (137m) deep access shaft to the raw water tunnel.
The 25.5ft (7.8m) finished diameter access shaft is located at the pump station site and is currently being excavated by a modified roadheader configured for shaft sinking (Figure 2).
This deep shaft includes a 9ft by 9ft horseshoe-shaped tunnel about 20ft (6.1m) long for connecting a future transmission tunnel about 110ft (33.53m) long that will serve as a pump suction cavity. Five 60in (1.5m) inside diameter pump suction wells will be excavated from the pump station site at grade, into the pump suction chamber, a vertical distance of 370ft (112.8m). Obayashi will start excavating the pump suction chamber and the pump suction wells (using a raised bore technique) this fall.
One of the main features of the construction contract is the chamber and series of pump wells connected to the access shaft—they must be completed before construction can start on the pump station, explains Jim Brennan, project manager with MWH. “The critical milestone for Obayashi is to get those shafts done so we can bring a contractor in to install the pumps and complete the pump station.”
Currently, 179ft (54.6m) have been mined on the access shaft, and Obayashi is installing a cast in place concrete liner for160ft (48.8m). Initially the design called for a shotcrete lining, but at MWH and Obayashi’s request the design is adjusted to accommodate a concrete lining. Eckols explains, the constructions plans defined specifically how and where the project would be constructed with variables available to the contractor.
“To effectively perform our design and provide the flexibility that the contractor required, we used the global resources of AECOM,” he says. “We had peer review from tunnel experts, with Verya Nasri from New York and Robert Frew from Hong Kong participating in the project, to identify the different construction technologies that would be desired by the contractor.”
Obayashi has chosen to excavate both tunnels by roadheader and has started the raw water transmission main. Construction is being done out of a portal at the site of the raw water pump station. The raw water main is above the water table and ground water probing is not anticipated for this drive. Currently, the contractor is using one Antraquip AMQ 100 and has mined 330ft (100.6m) using pattern rock bolts for support.
Obayashi has set up trailing gear similar to that of a TBM to remove spoils with muck cars and a small train. Material is being hauled to a city-approved recycling facility. As a requirement from the city, all material removed from a site such as this one must be disposed of or reused at a permitted site or facility. It could be used on other projects for roads or site development, Bybel explains.
“Typically this material that you’re removing from the Glen Rose and Walnut formations is pretty good, especially for backfill, for roadways. I have no doubt that from 90 to 100 per cent of this material is being reused.”
For the raw water transmission tunnel, Obayashi will complete the full length of the excavation before installing the 84in diameter lap welded steel pipe liner, which will be backfilled with low-density grout.
The pump station will be pumping water about 160ft (48.8m) to 200ft (61m) up hill in this tunnel, requiring a steel pipe lining to take the pressure from the pumps transferring the water to the water treatment plant.
MWH evaluated numerous types of steel pipe during the bid process. The types of pipe included full penetration butt-welded steel pipe, welded lap joint steel pipe and gasketed joint steel pipe. These alternatives were evaluated in the bidding process to get the most cost effective pipe for the City of Austin.”
Unlike the transmission tunnel, the raw water tunnel from Lake Travis to the pump station is surcharged.
“It has the head of the lake on it and must convey a wide range of flows: 50 million gallons per day to 300 million gallons per day. A consistent tunnel diameter was required at the initial flow of 50 million gallons per day in order to maintain desired velocities,” he explains. “We required it to be concrete lined so we could achieve a specific diameter in the tunnel. But beyond that it did not have specific requirements—it’s in stable ground. If we could have excavated clean enough, we could have probably just used a finished rock surface, but we needed a specified diameter.”
The raw water tunnel excavation will be supported with rock bolts and excavated the full length before installing the concrete lining. There is a specified probing program of 60ft lengths, which will determine whether pre-excavation grouting will be needed.
Obayashi expects to start working on the raw water tunnel in April 2012. Its joint venture partner, Manson, will start marine work for the intake this fall, hopefully finishing in December or January, says Darrell Liebno, project manager with Obayashi.
There will be three intake screen levels in Lake Travis the deepest of which will be attached to the intake structure. Joint venture contractor Manson will excavate a shaft at the bottom of Lake Travis, roughly 12ft (3.7m) in diameter and 80ft (24.4m) deep.
An intake riser pipe will be embedded in that excavation and grouted in place. This will include a series of bulkheads, already installed. Obayashi will begin excavating the raw water tunnel at the pump station and will bore to the lake. Once the riser is in place and grouting completed, the excavations can intersect that pipe to make the connection. Welded steel pipe will be installed on the riser to form a ‘T’ connection with the tunnel.
Within the lake, the intake riser pipe will support the lower intake screen and a 9ft diameter pipe manifold to connect to the other two intake screens, the middle and upper screens. Each intake screen structure is approximately 40ft (12.2m) high and consists of a steel support frame and a 30ft diameter screening structure. The middle and upper intake screen structures will be supported on pile foundations. Manson will be responsible for construction of these facilities within the lake.
“The lake is 150ft [45.7m] deep, so the intake riser pipe in the bottom of the lake is being constructed in a water depth of 100 to 150ft (30.5 to 45.7m). Then we have mechanical facilities, pipe and intake screens on the lake bottom that will need to be constructed, and that’s challenging,” Eckols says.
Another challenge on the horizon is the Austin City Council, which approved a resolution, as T&T went to press, directing the city manager to determine the cost of delaying the WTP4 construction for both five and 10 years. This vote halts all further issuances of Notice to Proceed until the estimates are available.
The access shaft to the raw water tunnel Excavations on the raw water transmission main Figure 1; The raw water tunnel and transmission main tunnel Figure 2; The raw water system The raw water transmission tunnel is being excavated from a portal
