The city of Knoxville, Tenn., is somewhat of a sports mecca. In addition to the University of Tennessee, Knoxville is also home to the Knoxville Ice Bears SPHL professional hockey team and the NCAA Women’s Basketball Hall of Fame, located in downtown Knoxville on Hall of Fame Drive.
There are plenty of spectator sports in Knoxville, but a recent sewer main replacement, while certainly a hall of fame candidate, was completed mostly out of sight like any good trenchless project.
The highly traveled Hall of Fame Drive runs perpendicular to the Tennessee River, just minutes from the university. Recently, Hurst Excavating of Knoxville was contracted to replace a sanitary sewer main underneath the road as part of a sanitary sewer main replacement program established by the Knoxville Utilities Board (KUB). The existing 8-in. diameter vitrified clay pipe (VCP) main was failing, and access was extremely difficult for establishing both launch and exit pits.
According to Hurst Excavating president David Hurst, a creative design approach was needed for this project. “We choose not to use a standard pneumatic pipe bursting because of the nominal thickness of the pipe and the fact that the access was so deep and difficult,” he said. “We thought that static pipe bursting might be a better approach. So after acquiring some specialized static bursting equipment, we came up with a plan to access the pipe and burst it.”
For the project Hurst used a Grundoburst Mini 400G static pipe bursting system from trenchless equipment manufacturer TT Technologies, Aurora, Ill.
“This static bursting unit is compact, but provides a lot of power, approximately forty five tons of pulling power. So for a small unit, it really delivers where it counts. And for this project, David needed the small footprint and power to make it feasible,” said TT Technologies pipe bursting specialist Eddie Ward.
The Knoxville Utilities Board was created by an amendment to the Knoxville City Charter in 1939. According to the KUB website, as an independent agency of the City of Knoxville, the KUB’s goal is to provide safe, reliable and affordable utility services. Those services include electric, natural gas, water, and wastewater services to more than 445,000 customers in Knoxville and parts of seven surrounding counties.
KUB, like many other utilities throughout the country, faces the challenge of replacing aging infrastructure. Launched in May 2007, Century II is a long-range infrastructure management program designed to improve and maintain the electric, natural gas, water, and wastewater systems for KUB customers. Replacing the deteriorating VCP line under Hall of Fame Drive was part of that program.
KUB project manager Dwayne Frye explained why a trenchless approach to the main replacement was the best choice. “The use of trenchless technology was vital to the success of our Downtown Century II Utilities Upgrade Project,” he said. “During the project, we encountered a wastewater pipe, which could not be replaced or rehabilitated using standard methods due to its depth, its poor condition, and its proximity to other nearby utilities, including a 48-in. water main.
“Open-cut replacement would have required a 60-ft wide excavation on a main arterial roadway in downtown Knoxville and been a threat to the integrity of the water transmission main. By using the trenchless technology, KUB was able to replace the main with little excavation and using manhole access, thus providing continued reliable wastewater services to its customers without interrupting their commute.”
The trenchless pipe bursting project was difficult and took an all-star contractor like Hurst Excavating to pull it off.
Hurst Excavating was founded in 1994. Initially, the company’s work consisted of sub-division, condo, and commercial and residential site preparation. After some time, Hurst, who had previous utility experience, began focusing on serving municipalities and other utility groups. Today, Hurst Excavating specializes in pipe bursting for the rehabilitation of existing utilities, while still providing commercial and residential site preparation, infrastructure and underground utility construction.
Hurst has a sizable pneumatic pipe bursting equipment arsenal including an assortment of pneumatic bursting tools, as well as several constant-tension pipe bursting winches, one of which assisted in the hall of fame burst.
Planning was key to the success of the project. The burst would essentially run from a manhole near a medical center on one side of Hall of Fame Drive to another manhole located on the other side of the street. Access on the medical center side of the run was tight with the 29-ft deep manhole located approximately 10 ft from the center itself. That prohibited the use of that manhole as the launch area.
So a plan was devised to create a launch pit approximately 30 ft behind the center and access the manhole and ultimately the VCP pipe through a 16-in. ductile iron pipe that also fed into the same manhole from the south.
Hurst crews excavated the launch pit to expose the ductile iron, gravity fed line. Then a 12-in. wide by 10-ft long top section of the ductile iron pipe was removed to serve as an access point to the manhole 35 ft away.
“The location where we decided to enter the ductile iron pipe was about 22 ft deep and well behind the medical center,” Hurst said. “From there it was 30 ft into the manhole to the point where we started bursting the 8-in. clay pipe. We had other utility lines to contend with as well, including a 48-in. water main and a 48-in. diameter storm drain close by, so we built the launch pit using a slide rail system.
“While we were building the pit, we also bypassed the 8-in. main. The ductile iron pipe we were using as an access was fed by another 12-in. pipe. But the fact that we just took off the top, meant that we didn’t have to interrupt flow during bursting operations.”
Game Winning Shot
At the other side of the bursting run, the exit or machine pit was a 33-ft deep manhole 351 ft from the launch pit, under Hall of Fame Drive and in front of the civic center. The manhole had a drop invert coming from the arena where the Knoxville Bears minor league hockey team plays.
To set the machine in the manhole, the invert needed to be cut out. “When you have a 33-ft deep brick manhole, the invert is extremely thick to support weight of all those bricks,” Ward said. “A wooden radius was created to match the curvature of the manhole. From that a steel support was built and it matched up perfectly. Crews also cut a 2-ft deep window directly behind the machine to allow for the bursting rods, this also allowed the expander and pipe into the manhole upon completion of burst. It took almost 12 hours to cut out the invert and prep the manhole.”
While work was being completed on the machine pit, crews fused the new 8-in. HDPE and attached it to the bursting head. A plan was then devised to speed up the rod loading process. A 12-ton Grundowinch was used to pull the QuickLock bursting rod string through the existing clay sewer main and to the machine pit. Once the rods were in place, bursting was ready to begin.
Bursting began on a Thursday. A boom truck with a bucket was used to bring the bursting rods out of the manhole. The first 200 ft was installed relatively quickly, but the remaining 150 ft proved difficult. Ultimately it was successfully completed later that evening. After the bursting was complete, Hurst crews bypassed the 12- and 16-in. pipes then cut out the section of the ductile iron pipe they had used as a feeding point for the burst and put in a point repair.
As to the success of the difficult project, Hurst said it’s all about planning. “You go in with a good plan, you execute that plan and it can work out well. Sometimes you have to think outside the box,” he said. “You have to study the project and come up with good ideas. And sometimes the person with the best ideas gets the project.”
“This was probably one of the more difficult projects we’ve seen in a while. There was a lot of planning that took place and great execution by Hurst Excavating. They were exceptional,” Ward said.