Problem: Texas capital experiences frequent sanitary sewer overflows.
Solution: Use tunnel-boring technology to replace problem pipeline.
For more than 15 years, residents of Austin, Texas, had complained about wastewater discharges from the Little Walnut Creek interceptor. Infiltration and inflow were problematic for the 1050-mm (42-in.) pipeline that runs beneath a stream, with manholes rising out of the water every 91.4 m (100 yd). When it rained, the sanitary sewer would overflow into the creek. Located in the Highland Lakes area of Texas, Austin is home to Barton Creek and the Barton Springs Edwards Aquifer, a sole-source drinking water aquifer and one of the most prolific artesian aquifers in the world.
The city originally proposed to replace the interceptor during the late 1980s, 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 redesign project.
But all that changed when U.S. Environmental Protection Agency Region 6 presented the Austin Water Utility with an administrative order requiring the central Texas utility to eliminate sanitary sewer overflows by December 2007 to protect its water supply.
Little Walnut Creek Tunnel Interceptor
Austin faced a tight schedule and had to work quickly in order to meet the order’s deadline. The tasks included land acquisition, permitting, design, and construction of numerous projects for its five-plant, 3726-km (2316-mi) collection system. The Little Walnut Creek Tunnel Interceptor Project One was the most challenging and critical. City officials wanted a product with a proven track record and a leak-free joint system. They conducted an extensive evaluation and chose centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) from HOBAS Pipe USA (Houston).
Under Austin’s Clean Water Program (CWP), the third and final design was assigned to national engineering and consulting firm Brown and Caldwell (Walnut Creek, Calif.), one of several engineering firms involved in the many mandated projects.
Combining engineering with community relations, Brown and Caldwell’s design used tunnel-boring-machine (TBM) technology to construct a new 3048-m-long (10,000-ft-long), 2.4-m-diameter (96-in.-diameter) primary tunnel in one continuous run with no intermediate shafts. The $12.7 million project design called for a 1.5-m (60-in) fiberglass carrier pipe that was specified in order to increase the pipeline’s useful life, compared with other materials.
The pipe was selected along with 72 stiffness and flush bell-spigot couplings. To expedite the time to place each carrier pipe within the tunnel, Austin used two insertion shafts. The first pipe, a special bell-by-bell CCFRPM pipe, was placed at the tunnel’s midsection. Subsequent CCFRPM pipes were carried into the tunnel from two insertion points, one at each end. Pipes were brought in with the bell trailing and blocked in place. Blocking was straightforward and rapid due to the flush exterior of the coupling. The simple push-together assembly of the couplings sped insertion.
A long pipe life was of primary importance to the City of Austin. Representatives completed a thorough audit of the system from raw material through finished product and testing. This resulted in an approval by the City of Austin’s Standard Products List without any reservations. Installation was assigned to KM&M JV (Solon, Ohio).
Lee DuPont, project manager, was pleased with the selection of pipe.
“Steel pipe is a pain in tunnels because of the working conditions, having to weld joints and so forth,” DuPont said. “HOBAS pipe is just a clean operation.”
The mining crews worked 43 m (140 ft) below the surface in the Austin hill country during the hot Texas summer, pushing a TBM along the pipeline route. The thin wall of the CCFRPM pipe was ideal for 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 [2.5 m], that’s the machine we had available,” DuPont said. “The ribs were about 4 inches [100 mm], so the primary tunnel provided plenty of working space for the 60-inch [1.5 m] liner. Our TBM was 31 feet [9.4 m] long. With a trailing sled to accommodate the conveyor and muck [spoil-removal] cars, this totaled 90 to 100 feet [27 to 30 m]. We were placing the primary support right behind the cutting head. We encountered all solid rock the whole way; the conditions were pretty consistent.”
Some of the tunnel was located within 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, who handles public information for the Austin CWP. “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 just because it didn’t disrupt their day-to-day lives.”
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 1.7-m (66-in.) CCFRPM pipe with 72 stiffness. It was used to replace the 1.4-m (54-in.) reinforced concrete pipe 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 (Nashville, Tenn.).
The 975-m (3200-ft) run was completed after the crew negotiated two tight radius curves of 122 m (400 ft) and 183 m (600 ft) that made up about half of the drive. They also had to reconfigure the cutter head in place, with 152 m (500 ft) remaining in the drive because of a change in geology from 3448 kPa (500 lb/in2) clay to 68,950 kPa (10,000 lb/in2) limestone.
The Crosstown Shaft was excavated at the same time the 1676-mm (66-in) 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.
For more information, contact HOBAS Pipe USA (Houston) at (800) 856-7473, (281) 821-2200, or email@example.com. Facts also are available at www.hobaspipe.com.