According to the 2010 census, 790,390 people call Austin, Texas, home. Add to that a number of rare and endangered plant and animal species found nowhere else on earth, which reside in the Balcones Canyonlands Preserve.
In 1992 citizens of Austin approved bonds totaling USD 22M for the acquisition and improvement of the preserve where more than 13,000 acres of woodlands, wetlands and grasslands are owned and managed by the City of Austin. To build a 10km tunnel through this area is going to raise more than a few eyebrows.
The Jollyville Transmission Main (JTM) is part of the USD 508M Water Treatment Plant Number Four (WTP4) in Austin, Texas. The plant will consist of water intakes, a pump station and pipelines to convey water drawn from Lake Travis to the treatment plant. The JTM will carry treated water from the plant to the Jollyville reservoir. Phase one of the plant is scheduled to be operational by 2014, and will produce 50M gallons of water a day.
Austin has two operating water treatment plants, built in 1954 and 1969, that are capable of meeting the city’s current water needs. Population growth is the driving force behind this third facility. Projected growth for Austin shows a population increase of 500,000 people by 2040.
With such a sensitive area in play rigorous studies have been done on the 34,565ft (10.5km) alignment, taking into consideration all of the environmental issues, community impact, cost and operations. The client is the City of Austin. The Public Works Department is managing the design and construction, and the Austin Water Utility will operate the plant once completed. Black & Veatch is the lead designer of the JTM, and MWH Constructors are the construction managerat- risk (CMAR) for the whole of the WTP4. Bids were received from subcontractors on June 30, and the project should be awarded in July.
“The project is a great illustration of tunnelling’s use in an urban environment to preserve what is there—in this case, the Balcones Canyonlands Preserve,” says David Egger, director of Black & Veatch’s heavy civil group, within its global water business. “It plays to tunneling strengths.. A surface pipeline was considered—but the decision was taken to go deep underground because the benefits outweighed the costs.”
Hoping to accommodate all the shareholders’ interests, there has been a four-way impact analysis on various aspects of the project, which has gone through a one-and-a-half year process for selection of the alignment.
“The community was very influential in the selection, especially with the shaft locations,” says Faruk Oksuz, associate vice president with Black & Veatch’s water business. “We weren’t looking into anything that would make an impact on the preserve, so it eliminated any access into this pristine area of the project. That really helped in terms of addressing the community and environmental issues.”
Jollyville geology
There are three limestone units going down into the Glen Rose formation where the tunnel will be built. From youngest to oldest these are the Edwards, Walnut and Glen Rose (see Figure 1, left). Shafts will go through the Edwards, a karstic formation that is home to six endangered cave-dwelling invertebrates (arachnids and insects) and supplies most of the springs, which are a critical feature of the Preserve’s environment.
The JTM has been designed roughly 50ft (15.2m) deeper than originally anticipated to avoid any potential environmental impacts, which also means tunnelling will not experience any changing ground from the flat line set of carbonate rocks.
There are four circular shafts in total: two working shafts of approximately 40ft (12.2m) excavated diameter, and two TBM retrieval shafts of approximately 30ft (9.1m) diameter (see figure 2).
JTM will be excavated with a two-heading schedule to make sure the tunnel is operational by the time construction on the water treatment plant is completed.
The contractor will tunnel from the JR shaft, located at the Jollyville Reservoir, to the PARD shaft (Parks and Recreation Department). The PARD shaft and the FPA shaft (Four Points Area), straddle the Balcones Cayonlands Preserve—the area the design team had specifically agreed not to disturb—creating a TBM drive of more than 20,000ft (6km), going downgrade.
There is also a drive from the FPA shaft to the WTP4 shaft, from which construction activities were limited because of crowding while the plant is being built. The locations of the working shafts were also dictated by the impact that muck removal would have on the surrounding neighborhoods, and the chosen JR and FPA sites will not have an effect on anyone.
As for the springs, the three shafts that will be excavated through the Edwards formation should not cause an issue.
“We are taking great precautions to maintain the potential flow pathways around our shafts during construction,” says Ray Brainard, senior engineering geologist with Black & Veatch. He explains, “We have permeable rings built into the shaft to keep potential ground water pathways open in the karstic formation.”
The contractor is required to take steel liner plates down to a certain geological level of the shafts. Behind the liner will either be grout to seal it off or limestone gravel to allow water to pass around the shaft perimeter to reconnect with existing flow pathways. Below the liner plates shafts will be supported with dowels and mesh.
“Karstic formations are typically a nuisance in tunnel or shaft construction, and it’s always preferred to block or close them off as quickly as possible,” Oksuz says. “Whereas on this project those features are home to endangered species so we were doing totally the opposite, trying to protect those features and build a project around it.
“This is a unique application for how you build a tunnel or a shaft in a karstic formation without impacting those voids. When you hear ‘voids’ your first reaction is, ‘close them.’ Whereas this project you have to think, ‘protect them.’”
Protecting the pristine
Two TBMs, likely to be 10ft (3m) in diameter, will be used for the tunnel’s excavation, although the final diameter will be 84in (2.1m). The contractor needs to finish the JTM within 30 months. September 1 is tentatively the construction notice to proceed, and the TBM is expected to launch within six months of that notice.
This is the first project for the City of Austin to establish an environmental commissioning team, Brainard explains. “We have worked hand in hand with their biologists, hydro-geologists and environmental scientists through the design process,” he says. “We brought them up to speed on how tunnelling is done so that they can see how that potentially affects the environment.”
Beyond concerns for the endangered species and species of concern, and avoiding any impact to the preserve, on the surface of the alignment is Bull Creek, and its tributaries. “We follow mostly through roadways and the city’s own easement, so there isn’t anything major as far as a structurally intensive area,” Oksuz explains. “We cross a couple of expressways and highways. Other than that there aren’t major buildings.”
The design team has worked with the U.S. Fish and Wildlife Service, among other environmental agencies, to go through all of the potential impacts and mitigation strategies for the water bodies and their species. The alignment provides a minimum cover of 100ft (30.5m) beneath Bull Creek and its tributaries.
Because the preserve and creek are the primary surface features taking precautions is inevitable. Pre-excavation grouting is not an option on this project, for example.
“The environmental commissioning team from the city will not allow us to grout ahead of the machine because of the concern that if we would hit an open fracture we could get some grout into the creek,” Brainard says. “That was taken off the table completely.”
Instead, when there is an inflow the contractor will put up steel liner plates inside of the tunnel and grout the annulus, cutting off water inflows. In areas without inflow initial support will be rock dowels. The final lining is being bid as either a welded steel pipe or pre-stressed concrete cylinder pipe (PCCP). One section of the alignment that travels under a critical spring does require a welded steel pipe.
While the design team did look at various options for muck disposal, the decision has been left to the discretion of the contractor, as there are a number of alternatives for isposal sites.
The transmission main project aims to leave behind a very small footprint. The intermediate shafts will be backfilled and restored as if nothing had ever happened. “That was part of the agreement with the neighborhoods—to restore the project shaft sites to the original condition or better,” Brainard says.
As T&TNA went to press the City of Austin issued a notice to proceed for the Jollyville transmission main tunnel to a joint venture of Southland/Mole.
“There was a lot of interest from the industry, Oksuz says. “It’s one of those highly tunnelling-focused projects that rarely come up because a lot of water projects will have a pump station. The component of the water treatment plant that we’ve designed is purely tunnel.”
Figure 2 – Shafts, going west to east Figure 3 – Shaft mitigation to keep flow pathways open Figure 1 – The tunnel alignment and four shafts
