Tunnel boring and fiberglass-reinforced pipe eliminate sanitary sewer overflows with minimal disturbance.
Little Walnut Creek Tunnel Interceptor, Austin, Texas
Brown and Caldwell, Austin, Texas
Austin Water Utility uses tunnel boring technology and HOBAS pipe to address EPA sanitary sewer overflow mandate.
For more than 15 years, residents in an Austin, Texas, neighborhood complained about wastewater discharges from the Austin Water Utility’s (AWU) Little Walnut Creek interceptor. Infiltration and inflow caused the problems with the 42-inch pipeline that runs beneath a stream and has manholes rising out of the water every 100 yards. When it rained, the sanitary sewer overflowed into the creek.
During the late 1980s, the city proposed to replace the interceptor, but residents blocked the project over concern that the proposed open-cut construction would disrupt nearby neighborhoods and cause environmental harm to the creek. Unhappy residents also blocked a redesigned project.
However, the U.S. Environmental Protection Agency (EPA) Region 6 forced the issue when it presented the AWU with an Administrative Order requiring the Central Texas utility to eliminate sanitary sewer overflows by December 2007.
Austin faced a tight schedule and had to work quickly. Tasks included land acquisition, permitting, design, and construction of numerous projects for its five-plant, 2,316-mile collection system. However, the Little Walnut Creek Tunnel Interceptor Project One was the most challenging and critical.
Under Austin’s Clean Water Program (CWP), this third and final design was assigned to national engineering and consulting firm Brown and Caldwell, headquartered in Walnut Creek, Calif., with an office in Austin. Brown and Caldwell is one of several engineering firms involved in the many mandated projects. According to Brown and Caldwell, the design phase, including field investigations, was performed within six months and completed $100,000 under budget.
Combining engineering with community relations, Brown and Caldwell’s design used tunnel boring machine (TBM) technology to construct a new, 10,000-foot-long, 96-inch-diameter primary tunnel in one continuous run with no intermediate shafts. The $12.7-million project design called for a 60-inch fiberglass carrier pipe, which was specified to increase the pipeline’s useful life compared with other materials. City officials wanted a product with a proven track record and a leak-free joint system.
HOBAS centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe exceeded the specifications and was selected. The HOBAS pipe’s inherent corrosion and abrasion resistance contribute to its longevity. Austin representatives completed an extensive audit of HOBAS’ manufacturing facility in nearby Houston—from raw material through finished product and testing— that resulted in an approval by the city’s Standard Products List without any reservations.
The CCFRPM pipe’s 72 stiffness and flush bell-spigot couplings increased installation efficiency. Its high strength allows for a thinner wall than many competing products. This was of great benefit on the Little Walnut project, since it was a perfect fit to the available pipe carrier and provided extra room for alterations in pipe alignment within the primary tunnel, which was constructed of steel ring beam and wooden lag.
"Our initial bore was right at 99 inches, that’s the machine we had available," said Lee DuPont, project manager for KM&M JV of Solon, Ohio, the installation contractor. "The ribs were about 4 inches, so the primary tunnel provided plenty of working space for the 60-inch liner. We placed the primary support right behind the [TBM] cutting head."
Use of two insertion shafts expedited the time to place each carrier pipe within the tunnel. The first pipe—a bell-by-bell CCFRPM pipe specially manufactured by HOBAS—was placed at the midsection of the tunnel. Subsequent CCFRPM pipes were carried into the tunnel from the two insertion points, one at each end. Pipes were brought in with the bell trailing and blocked in place. Blocking was straightforward and rapid because of the flush exterior of the coupling. The simple push-together assembly of the couplings sped insertion.
Much of the tunnel is bounded within right-of-way limits of existing surface streets, minimizing land acquisitions and impacts to the neighborhood, traffic, and the creek. Also, construction shafts at each end of the tunnel are located on undeveloped property, further keeping the project out of the public eye.
"Things aren’t always as they seem from the surface," said Brown and Caldwell Project Manager Susan Kelly. "Following the surface streets is not only economical but also less risky."
Crispin Ruiz, who handles public information for the Austin CWP, said, "The public appreciated the fact that it was a tunnel and didn’t disrupt the neighborhood. The residents were also concerned about any disruption to the sensitive environment around Little Walnut Creek. So, from the neighborhood’s perspective, it’s been a very successful project."
Shoal Creek Tunnel Project
Another Austin CWP installation was part of the overall plan. The Shoal Creek Tunnel Project reached its first objective with the finishing of the Mainline Tunnel excavation. The tunnel lining is a 66-inch CCFRPM pipe, with 72 stiffness and FWC couplings. It was used to replace a 54-inch reinforced concrete pipe (RCP) wastewater line that was exposed within the banks of Shoal Creek. The creek was vulnerable to flooding and the existing line had a potential breech, so the project was included in the Austin CWP and installed by W.L. Hailey of Nashville, Tenn.
The 3,200-foot run was completed after the crew negotiated two tight radius curves of 400 feet and 600 feet that made up about half of the drive. They also had to reconfigure the cutter head in place, with 500 feet remaining in the drive, because of a change in geology from 500-psi clay to 10,000-psi limestone.
The Crosstown Shaft was excavated at the same time the 66-inch HOBAS pipe was being installed. The main part of the job was completed when the mainline tunnel was connected to the Crosstown Shaft, which carried flow to the existing Crosstown Tunnel. Other elements of the project included junction boxes, directionally drilled lateral connections, several short open-cut runs, and rehabilitation of several existing lines and manholes.
Barton Creek Lift Station relief tunnel
Another project, the Barton Creek Lift Station Relief Tunnel, was awarded to Dibco Underground of Ontario, Canada. The contractor set up the work area in Zilker Park, just north of Barton Springs Road. The project was planned to avoid disrupting activities in the park and on nearby Toomey Road.
It provides a 33-inch diameter HOBAS CCFRPM pipe installed in two sections of a tunnel. A shallow tunnel in Zilker Park is about 1,700 feet long, and the 33-inch pipe extends into the existing interceptor with the annular space filled with grout.
A drop shaft joins this shallow tunnel to a deeper tunnel that extends for 1,600 feet under Barton Creek to the shaft site off Toomey Road. The main shaft at Toomey is about 70 feet deep, and has a temporary lift station built within the shaft to lift the wastewater to the adjacent South Austin Outfall. The temporary lift station will be underground, and will be operated until mid-2010 when it will be taken offline by a deeper tunnel system.
Stan Evans, project manager, Austin CWP, said, "The Austin Water Utility established a policy long ago to have all new pipe that they install, at least large-diameter pipe, to be a fiberglass-type pipe so that they wouldn’t have to deal with the constant problem of corrosion. That is something they are experiencing now with concrete pipe and the maintenance effort that it generates, especially when it comes to having to reline those pipes—or as we are in the process of doing for some of them—having to replace them.
"And another factor is the tightness of the joint that the HOBAS pipe provides. The old joint installations with RCP, over decades, are eventually going to allow leakage. And that means we’re either releasing wastewater to the environment or we’re taking in groundwater, which increases the flow in our lines, can cause overflows, and creates quite a bit of additional load to treat at our plant. During an unusually rainy event, it can actually overwhelm our plants."
"The cost to install the infrastructure of these larger wastewater lines is horrific. It’s just crazy to put something in the ground knowing that natural forces are going to eat away at it, and you have to come back in 50 years or even sooner."
Kimberly H. Paggioli, P.E., is the marketing manager for Hobas Pipe USA. She can be contacted at email@example.com.
Sidebar: Creek restoration
The Austin Water Utility (AWU) and the Watershed Protection & Development Review Department are collaborating on the Austin Clean Water Program (ACWP) to restore creeks impacted by the city’s wastewater collection system. Previously, sanitary sewer lines were buried beneath creek beds because the corridors provided a natural slope for the gravity-flow systems. In 1986, the city of Austin passed the Comprehensive Watersheds Ordinance, which prohibits installation of new wastewater collection system lines in creeks. According to the AWU, many of the lines installed in creeks incurred serious damage from flowing water and occasional debris that washed down the creeks. In addition, many of the creeks experienced accelerated down-cutting, widening, and meandering caused by exposed pipes that altered stream mechanics. The ACWP is removing at least six sanitary sewer lines from creeks, and dozens of other lines currently installed in creeks will be abandoned or relieved by parallel lines installed outside of the creeks. ACPW projects include creek restoration where needed, including channel stabilization using native limestone rocks and plants.