The history of Austin is pock-marked by devastating floods that have destroyed property and killed residents. For some 40 years the city has mulled over solutions and prevention methods that will keep downtown safe and functional.

Waller Creek has a 100-year flood plain that encompasses one million square feet, which inundates 12 roadway crossings (a majority of flood-related deaths are caused by people attempting to drive through moving water) and 42 buildings.

To protect the area the City of Austin is building a mile-long tunnel. This USD 146M project also includes an inlet structure and an outlet at Lady Bird Lake. And finally, there will be new park trails, restoration of the creek bank with native landscaping and other beautification efforts.

Joe Pantalion, deputy director of the Watershed Protection Department for the City of Austin describes this project as "recreating a stream that you might see in the hill country of Texas, right here in the heart of downtown Austin."

As of this summer, the city announced, the project is on time and on budget. Three of the four construction contracts have been awarded. Tunnel excavations started in January. And while there has been flood activity along the creek, nothing has caused problems for construction.

Design for decades
Flood protection for the creek is not a new idea, with the tunnel solution dating back to the 1970s. The city began organizing funding and preliminary design for the project in the late 1990s. Due to budgetary issues, it wasn’t until 2007 that the joint venture of Kellogg, Brown and Root (KBR), and Espey Consultants returned to do design, bid and construction management services for the project.

Divided into four contracts, the project includes a 5,600ft (1,707m) tunnel contract awarded to S.J. Louis Construction in April 2011. Zachry Construction Corporation was announced the lowest bidder for the outlet contract this spring at USD 13.16M and started construction on July 9. Oscar Renda Contracting secured the contract for the inlet in November 2011. The final contract, for the 8th Street Inlet will likely be awarded next year (see Figure 1).

The tunnel portion varies in diameter from 20.5ft at the inlet, increasing to 22.5ft and finally 26.5ft at Lady Bird Lake (5.2m, 6.9m and 8m). There is a 0.6 per cent grade and depths range from 60ft to 70ft (18m to 21m). Most of the excavations are taking place in limestone, with some shale (see Figure 2).

Two roadheaders are mining the tunnel, both starting from a 70ft (21m) deep entrance shaft between 4th and 5th Streets, one going northward to the inlet at Waterloo Park and one going southward to the outlet at Lady Bird lake.

As of the beginning of the summer, S.J. Lewis has tunneled laterally more than a 1,000ft (305m) northward and 300ft (91m) southward. On average, the tunneling crew progresses 14ft (4.3m) per day.

"They started the first of this year and they are about 33 per cent complete with the excavation in total volume," says Gary Jackson, project manager with the City of Austin’s Department of Public Works project management group.

"The shaft between 4th and 5th Streets isn’t in the exact center of the tunnel, but it is volumetrically," he explains. "Both the south heading and the north heading have to remove the same amount of rock, volumetrically. But it is a longer distance going north, and a shorter distance going south, but the south has a larger diameter."

So far so good
Earlier this summer the city announced the Waller Creek project on time and on budget. Jackson and Lizan Gilbert, chief engineer for S.J. Louis, attribute this success, at least in part, to good planning.

"Along with a good characterization of the ground so that no surprises have been encountered," Gilbert says. "And perhaps better than anticipated ground conditions."

Austin is on the edge of the Balcones Fault system and the Edwards Plateau, which is mostly limestone rock — not great for flash floods, but good for tunneling.

"The GTR [geotechnical reports] that we did are very accurate," Jackson says. "We identified a few minor faults right where predicted. There has been very little groundwater, which has been beneficial. The consistency of the rock has been very positive for production so we’re making very good headway into it."

The most challenging aspect of the project, says Gilbert, has been equipment selection and maintaining productivity. "Our objective is to implement the most cost-effective solution, incorporating flexibility in the operations to minimize downtime and optimize utilization within the tunnel," she says. "For example, when equipment goes down, we have the ability to maintain operations and space to stockpile material in the tunnel and not interfere with production."

She explains, "If there was a bad day, it generally has to do with equipment. Equipment can sometimes just be challenging. There is a lot of wear and tear that is a part of all the equipment in this kind of operation. So a bad day is a number of pieces going down simultaneously. That ultimately affects production."

S.J. Louis does regular maintenance every Wednesday. "We are regularly working to maintain equipment," Gilbert says. "Everyday there is something that is being done. We change teeth in general every couple of days or so. Teeth wear is pretty good. They are wearing evenly on the tips instead of off-to-the-side. We’re doing a nice job of making sure our teeth are oriented and matching the materials we are excavating.

"With all the equiment we spend quite a bit of our time maintaining it and just to make sure we keep everything going.

During the excavations, S.J. Louis is installing temporary safety systems — rock bolts and gunite on a 5ft (1.5m) grid, depending on the geological conditions. The secondary lining of a 12in (305mm) thick, cast-in-place concrete with rebar reinforcement will be installed after the tunnel is excavated.

And not only the Waller Creek project is benefiting from this rock. Spoil from the excavation is being repurposed for a new Formula One track being built in Austin, Circuit of the Americas. "A lot of the material we’re bringing out of the tunnel, the limestone, is being used there for the base under the race track and the secondary roads associated with the construction," Jackson explains. "It’s very consistent; we bring out, basically, crushed limestone."

An example of preliminary planning being a boon to the project’s progress, is site selection. Interstate 35 runs along the site, and in this highly-urban area, hauling trucks don’t interfere with local traffic. For Jackson, surface elements have been the most challenging aspect of the excavations — getting material, such as concrete, steel, etc., in and out — but also coordinating with public interest. The project has done press tours and a media day, and has seen a very positive response, he says.

Weather woes
In general, water ingress hasn’t been a problem. "There has been almost no ground water in the tunnel itself," Jackson says. "There is some coming down the shaft. In between the space between the alluvial soils and the rock there has been a little seepage, but all in all there has been almost no water in the excavations."

However, this project has its roots in severe weather and stormwater management will be critical once construction moves above ground to the inlet site. Fortunately, no catastrophes have occurred during construction. "We’ve had, what I consider, two flood events along the creek so far. But right now with the construction phase we are in, it’s not a problem," Jackson says.

Gilbert adds, "The surface work and excavations won’t start until next year at which time we will have to be much more focused on weather and construction staging to accommodate potential floods."

By the end of 2012, the Waller Creek project hopes to be completing one heading, and by the end of summer, the expectation is to mark the halfway point.