Ashland, Neb., located between Omaha and Lincoln, is a rapidly growingcommunity. Growth and flow problems in an existing section of pipe led the Cityto install a new trunk sewer earlier this year.
The City chose to use pilot tube microtunneling (PTMT) for portions of theproject with much success. In addition to a clean, on-grade installation thatwill serve the community well into the future, the project team also set arecord of 400 ft for the longest drive using 12-in. clay jacking pipe in theUnited States.
Segments of new 12- and 18-in. trunk line were needed for several reasons.The project allowed the City to divert sewage to alleviate a problem area, servea new development, allow for future growth to the north and west and eliminatethe need for a lift station.
While part of the project was completed using traditional open-cut methods,Ashland city administrator Wes Blecke said conditions in two areas were idealfor PTMT. A three-block section of 30th Street was chosen because its 24-ftdepth would have been expensive to dig and full of safety issues. Another sixblock area, along Furnas Street, was selected because of existing infrastructureand traffic concerns.
“We knew it was a little more costly, but for safety and timeliness PTMT wasthe way we decided to go,” Blecke said. “Residents’ driveways were saved, and weavoided a gas line and other utilities by going trenchless.”
“Traffic was also an issue. We were working near a school and the City didn’twant to close streets while school was in session,” said Jeff Forney, designengineer with Olsson Associates.
PTMT was selected because it combines elements of conventionalmicrotunneling, directional drilling and augering to produce a cleaninstallation, accurate on both line and grade.
Crews from Horizontal Boring & Tunneling Co. pilot tube microtunneledapproximately 3,200 lf of 12- to 18-in. pipe. Rob Tumbleson, sales and marketingdirector for Akkerman Trenchless Equipment Specialists, said the team used a GBM240A pilot tube microtunneling system. The GBM 240A was used alone on somedrives and in conjunction with a Barbco auger boring system on other drives.Drive lengths varied between 75 and 400 ft. Horizontal Boring & TunnelingCo. worked in tandem with Judds Bros. Construction Co., which installed theshafts and performed the open-cut portion of the project.
Brent Moore of Horizontal Boring & Tunneling Co. said the crew used athree-pass method of PTMT throughout except for a two-pass system used on onedrive where it had to exit from inside an existing manhole.
The three-pass method involves the guided installation of the pilot tubestring as the first pass, followed by boring equipment to install steel pilottube pipe casings through the entire run. Once the steel pilot tube pipe casingsare in place, a pipe adaptor makes the transition from the pipe casing to theNo-Dig pipe. In the third pass, the crew installs the No-Dig pipe as they removethe steel pilot tube pipe casings from the reception pit.
Crews installed No-Dig vitrified clay pipe, a specialized jacking pipemanufactured in Pittsburg, Kan., by Mission Clay Products. Its high compressivestrength holds up to the jacking force of microtunneling and the pipe’slow-profile joints make jacking possible. The clay material’s abrasionresistance prevents damage from the surrounding ground while chemical resistancegives it longevity. Gasketed joints compressed with series 316 stainless steelsleeves provide an air-tight seal.
Despite quality materials and equipment, the project wasn’t without itschallenges. During construction, crews discovered an undocumented high-pressuregas line along Furnas Street. The line was outside of its easement and broughtconcerns of possible contamination.
The team obtained the necessary permission and adjusted the line into thestreet. This unforeseen change also shifted several manholes further into thestreet, something they had hoped to avoid.
The different soils throughout the project and the lengths of the drives wereadditional challenges. The project site contained soils from lean clays to cleansands and of the 11 runs, six were between 300 and 400 lf.
“To keep from exceeding the jacking loads on the 12-in. No-Dig pipe, we hadto maintain a good bentonite slurry around the No-Dig pipe all the way throughthe pipe runs,” Moore said. “We were successful and never went over 50 percentof the designed loads, including safety factors.”
Crews also encountered one high point in the project, setting a record forthe longest bore with 12-in. clay pipe from manhole to manhole.
This record will likely be broken as use of PTMT increases, somethingindustry experts agree is coming. “This technology is still in its infancy, butwe’re doing more and more work,” said David Gill of Mission Clay Products.“We’re supplying pipe from coast to coast.” Mission Clay Products is one of onlytwo suppliers of clay jacking pipe in the United States.
According to Jeff Boschert of the National Clay Pipe Institute, the PTMTmethod first came to the United States in 1995 with one contractor. Currentlymany contractors in the United States own the equipment, and he expects themarket to continue to grow.
“In the past 10 years, this technology has evolved from 8- and 10-in.diameters with 200-lf drives to 24- and 27-in. diameters with drives up to 300lf,” he said. Boschert expects larger diameters and longer drives using PTMT inthe near future.
Gill agrees, “Pilot tube microtunneling is becoming a very competitive andefficient process for installing sewers.”
Although the technology is still catching on, using PTMT for portions of theAshland Furnas Street project made sense. Leaving trees, landscaping anddriveways preserved meant less disruption for property owners. And accurate,problem-free sewer lines mean the city is well-prepared to service existing andnew areas of the community. Social implications were kept to a minimum, but thetrue value of this installation technology to a community is immeasurable.
“This project has helped us get ready for expanded growth,” Blecke said.Although citizens were not calling the city thanking it for choosing PTMT, headded, “you don’t hear the good things.”
Since completion, the sections of sewer laid via pilot tube microtunnelinghave functioned smoothly, and Blecke said the City would consider using thetechnology on future projects.
The project team was pleased with the results as well. “This was some of thebest installed pipe I’ve seen. It was almost like a rifle bore,” Forneysaid.