For more than 15 years, residents in an Austin, Texas, neighborhood had complained about wastewater discharges from the Little Walnut Creek interceptor. The problem was with the 42-in. pipeline that runs beneath a stream with manholes every 100 yards. When it rained, the sanitary sewer would overflow into the creek.
During the late 1980s, residents blocked a proposed project to replace the interceptor because of concerns that the proposed open cut construction would disrupt nearby neighborhoods and cause environmental harm to the creek.
But all that changed when the U.S. EPA presented the Austin Water Utility with an administrative order to eliminate sanitary sewer overflows by December 2007 to protect its water supply.
Tight schedule, quick solution
Austin faced a tight schedule and had to work very quickly. This included land acquisition, permitting, design and construction of numerous projects for its
five-plant, 2,316-mile collection system. The Little Walnut Creek Tunnel Interceptor Project One was the most challenging and critical.
After an extensive evaluation, the utility selected Hobas centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM). Combining engineering with community relations, engineering and consulting firm Brown and Caldwell's design used tunnel boring machine (TBM) technology to construct a new 10,000-ft, 96-in.-diameter primary tunnel in one continuous run.
The $12.7-million project called for a 60-in. fiberglass carrier pipe to increase the pipeline's useful life, a primary consideration for the city of Austin. To expedite installation in the tunnel, two insertion shafts were utilized, with pipes carried into the tunnel from each end. The simple push-together assembly of the couplings sped insertion.
Installation was assigned to KM&M JV of Solon, Ohio. The company's mining crews worked 140 ft below the surface in the Austin hill country during the hot Texas summer, pushing a TBM along the pipeline route. The high-strength pipe's thinner wall made it a perfect fit to the pipe carrier, and provided extra room for pipe alignment.
Some of the tunnel was located within the right-of-way of existing streets.
"The public appreciated the fact that it was a tunnel and didn't disrupt the neighborhood," said Crispin Ruiz of the Austin Clean Water Program. "So, from their perspective, it's been a very successful project."
The pipe's inherent corrosion and abrasion resistance contribute to its longevity.
"We are also concerned about the design life," said Stan Evans, project manager for Austin's Clean Water Program. "Hobas tells us it could last 100 years, which is great because we have a hard time getting concrete pipe to last 50 years."
Austin has plans to use the pipe on more projects because of its strength, long life and ease of installation.
Evans added that he is pleased with the Hobas installations and would specify the product again. "Well, we pretty much require it in the city of Austin," he said.