SAK/Quest Completes Austin’s Downtown Wastewater Tunnel

January 2013, Vol. 68, No. 1

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.”