BC Water News Exclusive

AUG. 22, 2005

Big Change in the Pipeline
for Little Walnut Creek

Tunnel Boring Machine technology to eliminate
sanitary sewer overflows in Austin

by Dan Foscalina | BC WATER NEWS

Tunnel Boring Machine technology is being used to replace the Little Walnut Creek Interceptor with a new 10,000-linear-foot, 96-inch-diameter tunnel in one continuous run with no intermediate shafts. Photo by Jim Doyle

CLICK PHOTO FOR SLIDESHOW

AUSTIN—When the Environmental Protection Agency’s Region 6 came knocking on the Austin Water Utility’s door, it didn’t come empty-handed. The regulatory agency brought an Administrative Order (AO) requiring the central Texas utility to eliminate sanitary sewer overflows by December 2007.

Austin was under the gun: It needed to quickly implement land acquisition, permitting, design and construction of a large number of projects across its five-plant, 2,316-mile collection system. The city put together a rotation list of engineering firms to work on several fast-track collection system projects, one of the most challenging and critical of which was the Little Walnut Creek Tunnel Interceptor Project.

Infiltration and inflow had been a problem on the Little Walnut Creek interceptor, with residents and neighbors complaining about wastewater discharges for more than 15 years. The existing 42-inch pipeline runs beneath a streambed, with manholes rising out of the water at 100-yard intervals. During wet weather events, wastewater overflows into the creek.

But in the late 1980s when the city originally floated the idea to replace the interceptor, residents blocked the project, concerned that the proposed open cut construction would disrupt nearby neighborhoods and harm the environmental integrity of the creek. The city went back to the drawing board and redesigned the improvement project, only to have it blocked again by dissatisfied neighbors.

Fast-forward to July 2005: Under the City of Austin’s Clean Water Program, this third—and current—design is being led by national engineering and consulting firm Brown and Caldwell. To gain public buy-in, BC produced a design that uses Tunnel Boring Machine (TBM) technology to construct a new 10,000-linear-foot, 96-inch-diameter tunnel in one continuous run with no intermediate shafts. The $12.7-million project design also calls for a 60-inch fiberglass carrier pipe, which will Significantly increase the pipeline’s useful life over more traditional reinforced concrete pipe.

Mining crews are 140 feet below the Austin hill country and stifling Texas heat, pushing a 300-foot-long TBM along pipeline route. The best part? The tunnel is bounded within right-of-way limits of existing surface streets, minimizing land acquisitions and ensuring zero impact to the neighborhood, traffic or the creek. Also, construction shafts at each end of the tunnel are on undeveloped property, further keeping the project out of the public eye.

"Things aren't always as they seem from the surface."

“Things aren’t always as they seem from the surface,” says Brown and Caldwell Project Manager Susan Kelly. “Following the surface streets is not only economical but also less risky.”

The project also includes open cut construction of 3,700 linear feet of 60-inch wastewater interceptor, allowing AWU to abandon a similar length of deteriorated pipeline and lay the groundwork for a city park near one of the construction shafts.

“We listened to the community and decided to invest in the more expensive tunnel design,” says Reynaldo Cantu, AWU assistant director. “The design successfully addresses residents’ concerns regarding the impact of construction activities on the neighborhood, traffic and the creek, thereby preserving their quality of life."

The design phase, including field investigations, was performed within six months and completed $100,000 under budget. The tunnel construction is well under way to meet the EPA’s AO deadline.