January 2013, Vol. 68, No. 1
Newsline
SAK/Quest Completes Austin’s Downtown Wastewater Tunnel
Far beneath the surface of Austin’s Lady Bird Lake ‒ a 416-acre water reservoir that provides recreational opportunities as well as flood control ‒ years of intense “dirty work” took place, largely unnoticed by local citizens, as crews labored to create a huge wastewater tunnel that ran as deep as 80 feet underground.
The technological and safety challenges of excavating a four mile, downward-sloping tunnel along a path that runs parallel to a river and underneath a lake at three different locations were considerable. But the need for a gravity-powered wastewater system to accommodate Austin’s growing population well into the future was apparent to city of Austin planners. To tackle the job, the city brought in the joint venture team of SAK/Quest, consisting of St. Louis area-based SAK Construction and Quest Civil Constructors Inc. of Phoenix, along with Parsons Engineering. The tunnel, which was recently completed, is expected to last with minimal maintenance for at least a century ‒ twice as long as the antiquated system it replaced.
Construction on the $40 million tunnel began in early 2010. The project included approximately 20,554 linear feet of tunnel and utilized 121-inch and 84-inch diameter tunnel boring machines. In addition, SAK/Quest lined the tunnel with pipeline using the materials designed for long life and low maintenance. The Downtown Wastewater Tunnel is managed by the City of Austin Public Works Department.
Challenges to project team
How to perform deep excavation on the tunnel ‒ which closely parallels the Colorado River for much of its length as well as crossing underneath the lake ‒ was the major challenge for the SAK/Quest team. To allow tunnel construction access from five surface locations for the initial segment of the tunnel, SAK/Quest crews constructed four shafts to depths of 75 to 80 feet. In addition, the team set up bypass sewerage pumping to allow an existing lift station to be cleaned and converted into a fifth shaft.
During the excavation process, the SAK/Quest crew encountered one particularly challenging area of low rock cover. “In an area with less than three feet of weathered and broken rock separating the tunnel from 50 feet of sand and an unlimited supply of groundwater above the rock, the risk of catastrophic failure that could endanger the tunnel workers, city property, a nearby railroad and the general public, was a major concern,” says James Byrd, senior project manager for SAK. “The project team went to work to find an answer. SAK/Quest offered a solution to lower the tunnel, yet still maintain a downward slope to keep the sewer flowing by gravity. The city ordered additional geotechnical drilling and Parsons engineered a risk-reducing design incorporating the SAK/Quest solution. Subsequently, the risk was greatly reduced, and the new design resulted in the tunnel being safely and successfully excavated through this area.”
Benefits to the city and its residents
The Downtown Wastewater Tunnel and the pipeline installed inside it dramatically expand the system’s capacity to a level that is expected to accommodate the area’s projected population growth for the next 60 years. The tunnel replaces a 50-year-old system, deteriorated concrete pipeline and the associated lift stations that were previously necessary to power wastewater through the sewer system. By installing a sloping tunnel that instead utilizes gravity, the city was able to eliminate the need for two lift stations that many residents considered a blemish on the downtown scene.
In addition, the tunnel houses glass-fiber-reinforced polymer mortar pipe that is expected to last at least 100 years. “SAK/Quest installed centrifugally cast fiberglass reinforced pipe, which, unlike concrete, is not susceptible to the corrosive effects of sewer gas,” says Byrd. “Special cement-free polymer mortar manholes ‒ the largest in diameter of their kind in the nation ‒ere also constructed at the city’s request. Through the use of these new corrosion-resistant materials, SAK/Quest dramatically reduced potential maintenance and repair costs, resulting in a potential savings of tens of millions of dollars over the lifetime of the wastewater system.”
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