The City of Austin has seen devastating flood damage near Waller Creek too many times to list. Along with expensive repairs to property, hundreds of lives have been lost to flooding over the city’s history, the most recent in 2007.
Located in the flash flood alley of central Texas, the city sees heavy downpours due to moisture coming up from the south from the Gulf of Mexico, meeting colder air masses coming up from the north. That clash creates significant amounts of rainfall and heavy storms.
Contributing to that is the city’s geographic location, explains Joe Pantalion, deputy director of the Watershed Protection Department for the City of Austin. “We are at the edge of the Balcones Fault system where we have the Edwards Plateau creating some very steep slopes. And it’s a limestone rock formation as well, so we have a lot of thin soils and a lot of exposed rock outcroppings—that really doesn’t allow the rain to soak in.”
Run-off in these heavy rains becomes a big problem for Waller Creek, particularly the lower mile (1.6km), which is on the eastern boundary of Austin and in a highly urbanised area. The 100-year flood plain for Waller Creek encompasses an area of one million square feet (304,800sqm), and inundates 42 structures and 12 highway crossings. In some places the flood plain is as much as 800ft (244m) wide.
For decades the city has discussed flood protection for the Waller Creek area, deciding to build a 5,600ft (1,707m) tunnel and awarding the project’s first construction contract, worth USD 49.5M, last spring.
The project has been divided into four contracts to take advantage of the current bidding climate, says Pantalion, but also to coordinate each part finishing in and around the same time to be operational in 2014. SJ Lewis Construction had notice to proceed last spring for the tunnel construction. The first inlet project was awarded to Oscar Renda for a total cost not to exceed USD 28.5M. In a statement the council contracts department says, “Due to the potential for unknown conditions that could be encountered during underground tunnel construction, a 2.5 per cent contingency in funding has been included to allow for the expeditious processing of any change orders.”
Construction began late last year and should be completed in summer 2010. The other two contracts for the outlet and the 8th Street creek side inlet were expected as T&TI went to press. The total cost of the project so far, including engineering and real estate acquisition, is USD 146.5M.
Consulting on the project is a joint venture of Kellogg, Brown and Root (KBR) and Espey Consultants, which is supported by a multitude of local firms, particularly Brown & Gay Engineers, and Jenny Engineering of New Jersey.
Underground downtown
The tunnel idea dates back to the 1970s—the bicentennial of 1976 in particular, which triggered efforts to beautify parts of Austin, including the Waller Creek corridor. The city acknowledged that to be able to preserve the area, there needed to be a solution in place for the problematic flooding. Widening or channelling the creek had been considered, and deemed financially infeasible because of the commercial development alongside the creek.
In 1998 the city passed bonds for USD 25M to fund the tunnel project and the consulting joint venture started preliminary design in May 1999. At the end of preliminary design the price tag was around USD 60M and the project sat dormant until a funding source could be determined. The city brought the joint venture back on the project in late 2007 for design, bid and construction management services.
Trying to locate the tunnel within the city’s right-of-way, proved quite difficult in the downtown area. The final alignment has both the inlet and outlet located on city-owned parkland, and primarily follows Sabine Street, which closely parallels the creek.
“That really limited the amount of real estate we had to acquire,” Pantalion says. “But that also created a challenge in that we were really focused on designing a tunnel inlet and tunnel outlet that could be seen as amenities or blending into a parkland setting.”
The tunnel outlet structure is located at Waller Beach, where water will be discharged into a lagoon on Lady Bird Lake. The tunnel’s inlet structure is at Waterloo Park, about one mile north of the lake (see Figure 1). Two smaller creek side inlets at 4th and 8th Streets will convey additional flows into the tunnel. Designed as an inverted siphon, the diameter of the tunnel will start at 20.5ft and increase to 22.5ft and then 26.5ft (6.2m, 6.9m and 8m), with a 0.6 per cent Grade (see Figure 2).
Tunnel depth will range from 60 to 70ft (18 to 21m) putting the alignment primarily in limestone, with some of the excavations in shale. “The geology, starting from oldest to the youngest is Del Rio clay shale, Buda limestone, the Eagle Ford shale and then we’ve got the lovely Austin limestone, which is generally where the tunnel horizon is,” says KBR’s Matthew Crow, project manager for the consulting joint venture.
The contract didn’t specify means and methods for excavation, but leaned toward conventional tunnelling, either roadheader or drill and blast. SJ Lewis decided to use two refurbished roadheaders and is in negotiations for securing them. Excavations will be done in a modified horseshoe shape, and the contractor expects rates of 8 to 14ft (2.4-4.3m) per eight hour shift, as the tunnel varies in diameter.
Detailed design did provide the primary support and secondary lining. Because most of the alignment is in good rock, the support system will consist of rock dowels and shotcrete. “In the poorer limestone we’re using lattice arches and shotcrete and then in the Eagle Ford shale there is a system of steel arches, which enable the lining to squeeze in on itself with the potential for swelling,” Crow explains.
For the secondary lining SJ Lewis will install a 12in (305mm) thick cast in-place concrete lining with a conventional rebar reinforcement on 12in (305mm) centres. The lining doesn’t have a waterproof membrane so grouting will be imperative.
“We’re next to a pretty sizeable body of water, Lady Bird Lake, so we’ve got forward probing, and then if need be tunnel grouting to control ground water ingress,” Crow says. “We’ve also got grouting to minimize water inflows in the completed lining.”
Probe holes will be 80ft (24m) long, with a minimum overlap of 20ft (6.1m).
Pantalion adds, “We are currently constructing wastewater tunnels here in the city in the same formation. These are down into the rock structure, and groundwater has not been a significant issue for them. We’re hoping that will be the same for us as well.”
Above ground
Either of the two creek side inlets on 4th and 8th Streets could be used as a drive shaft. The 4th Street inlet has advantages in the grand scheme of the project’s schedule and contract packaging, as well as the availability of a suitable and level site for staging construction. Another benefit of working out of a 4th Street shaft is its close proximity to Interstate-35 for muck hauling, especially with the project’s urban location.
The Waller Creek project held a ceremonial grounding breaking in April 2011 and in the summer was working on setting up the site. The shaft is 70ft (21m) deep with about 35ft (11m) in soil. Dimensions are 31ft (9.4m) in the soil burden and then 30ft (9m) in rock. Excavations in the soil will be supported with ring beams and steel liner plates. In the rock the contractor will use rock bolts with fiber-reinforced shotcrete, as needed.
While the impetus for the Waller Creek tunnel was flood protection, Austin does have concerns for drier seasons. At times in the summer, the creek can become stagnant and experience low or no flows. Designed as an inverted siphon there will be water in the tunnel at all times, allowing the city to pump out the tunnel at the inlet facility to allow lake water to flow down the creek. At the same time this will prevent water in the tunnel from becoming septic in the dry seasons.
“By having this base flow we are really able to promote an environmentally-stable aquatic habitat year-round. There is just a tremendous benefit for the environment,” Pantalion says.
The USD 32M inlet contract being bid this summer includes underground work to build the 20ft (6m) diameter inlet shaft and a stub tunnel, among the 20,000sqf (6,096sqm) surface level facility, rakes, screens and mechanical and electrical work associated with those features.
A massive, reinforced concrete morning glory, or spillway, will be built to bring water down the shaft into the tunnel. Groundwater is not expected to be an issue on the inlet contract as this aspect of the Waller Creek project is the furthest north of the lake.
It goes without saying it’s a completely different story at the other end of the alignment. Construction for the outlet facility will include a stub tunnel, outlet shaft, and an outlet lagoon and spillway to convey water into the lake.
The average velocity of stormwater in the tunnel will be 20ft/s (6m/s), and the outlet structure, and in particular the 40ft (12m) diameter outlet shaft, is designed to dissipate this energy to minimise lake shoreline erosion. From an aesthetic point of view, the city and design team heard many concerns about the appearance of outlet facilities. One of the initial proposals included a lot of exposed concrete with an amphitheatre and other structures.
“As the project went through the public input stage we just heard an overwhelming response from the public that anything along Lady Bird Lake should really respect the character of the natural shoreline,” Pantalion says. “The outlet lagoon really tries to take on, as much as possible, the characteristics of the natural shoreline that exists there now.”
To construct the outlet, a shaft will need to be sunk within a cofferdam in the lake. In his presentation at RETC last June, Dorian French, senior vice president at Brown & Gay, explained the complex geologic formation at the outlet site. Even with grouting it will be a challenge to keep water out, he says.
Excavation through the alluvium and overburden soils consisting of clay, rubble and typical river deposits, between 4.5 to 28ft (1.4 to 8.5m), shouldn’t experience significant difficulty. Going deeper into the limestone and shale strata will be more challenging. At this point in the alignment, tunnel excavations are in the Austin limestone, 13 to 28ft (3.9 to 8.5m) deep, and the Eagleford shale between 34 to 74ft (10 to 23m) deep.
According to the paper French authored with Brian Reis, vice president of Espey Consultants, a floating dike system is planned to serve as a cofferdam on Lady Bird Lake to impound at least 16ft (4.9m) of water. On top of that, “5 to 25ft [1.5 to 7.6m] or more of saturated cohesionless alluvium will require seepage cut-off (particularly beneath the cofferdam impounding the lake), dewatering and stablisation.”
Significant groundwater is expected in the lower portions of the alluvium, which has higher permeability. Meanwhile a groundwater head of at least 16ft (4.9m) in fractures in the bedrock strata is expected to be problematic, the paper says. The design team developed presumptive field permeability values (Table 1) for cost-estimating purposes.
Outlet construction is scheduled to finish in early 2014. SJ Louis will install bulkheads in the tunnel at the inlet and outlet (and the 8th Street inlet), which will be removed in April 2012 by the contractors who build those structures.
“The biggest challenge is really timing the other projects,” says Lizan Gilbert chief engineer with SJ Louis. “We’ve got schedule constraints—they are aggressive—but we’ll meet them. The challenge is how the other two projects are coordinated with ours.”
For example, part of SJ Louis’ contract is to build an ecological weir in Waller Creek upstream of the 4th and 8th Street inlets, and at the main inlet structure.
These strategically-placed limestone boulders will create channels that replicate the natural currents and pools that can occur in bedrock stream habitats of the Texas Hill Country.
“That weir will essentially back up water behind it and the result of that will be a flooded tunnel. We can’t construct that until the tunnel is operational,” Gilbert says. “So that’s where we’ve got this one last, pretty big component of the job in terms of how it works, how it is presented, that we can’t begin on in any meaningful way until those other projects are connected.”
For the city, one of the biggest influences on the schedule is financing. Funding for the tunnel will be generated from an increase in property values in the area adjacent to the creek using a tax increment reinvestment zone, established in 2007.
“We would like the tunnel to be completed as soon as possible to provide a higher level of flood protection, which we hope would lead to significant redevelopment within this corridor,” Pantalion says.
And for the citizens of Austin, who will no longer need to worry about their safety, nor property damage, in the event of a flood, there is the added bonus of beautifying their city. Among the new facilities to keep trash out of the creek, there will be new park trails as part of the project.
“We’re going to have a cleaner creek, one with water year-round, and we’re going to be able to restore the banks with native landscaping,” Pantalion explains.
“We’re really excited about recreating a stream that you might see in the hill country of Texas, right here in the heart of downtown Austin.”
Downtown Austin, Texas, and Lady Bird Lake Figure 1, a map of Waller Creek and the tunnel alignment going through downtown Austin Figure 2, the alignment Waller Creek today, courtesy of Daniel Herrera Flood damage at Waller Creek circa 1915 Table 1: Soil permeability Figure 3, the inlet structure in Waterloo Park where water will enter the tunnel Figure 4, the outlet to Lady Bird Lake
