Just east of Great Falls, Mont., Malmstrom Air Force Base is both a military installation and a construction zone. The base dates back to the beginning of World War II in 1939, but renovations continue to transform it into a 21st-century facility. As part of this ongoing process, the on-base family housing is being extensively updated, along with the underground infrastructure that supports it.
When Falls Construction was awarded the contract for all of the project’s utilities, president Guy Huestis knew there would be some unique challenges — especially with the aging sanitary sewer line. There had been some failures in the line over the years, and although several different liners had helped prolong the life of the pipe, they also complicated its replacement. That was only one of the issues the contractor needed to address.
Pipe Bursting Required
The existing sewer line ran directly under a concrete duct bank, which would have been prohibitively expensive to tear up and replace. Housing construction also needed to continue aboveground while the sewer line was being replaced below.
Open-trenching was not an option. In fact, pipe bursting was a contract requirement. However, bursting concrete pipe lined with high-density polyethylene (HDPE) can be difficult. The liner material tends to accordion-fold back on itself, bunching up until it creates a major blockage that has to be excavated. It couldn’t be pulled out on this site, either, because the old concrete pipe would have collapsed.
Because of these issues, Huestis knew his crew would need to slit the poly liner and burst the concrete at the same time. That’s when he started looking for the right equipment for the job.
“We talked to several different people about bursting and some of them had real reservations about the HDPE liner,” Huestis recalls. “Then we got in touch with the people at HammerHead Trenchless Equipment. They put us in touch with one of their techs who had done this kind of work all over the world. He explained the risks and gave us confidence that this project was doable.”
A Custom Solution
Based on the issues and specifications of the job, Falls Construction decided on a HammerHead pipe bursting system — a Hydroburst HB100 100-ton static pipe bursting system — designed to replace potable water, sewer and gas lines from 3 to 16 in.
To ensure project success, the underground engineers at HammerHead Trenchless Equipment reviewed the project specifications with Huestis free-of-charge and recommended that his HB100 be outfitted with a longer-than-normal pilot and specially designed slitters to cut through the HDPE liner. This custom-built setup assured Huestis that his HB100 could provide the muscle needed specifically to handle the Malmstrom Air Force Base project.
“We were impressed with the pulling force and small footprint of the unit,” says Huestis. According to Huestis, the system worked much like a box cutter. “As we pulled the head ahead, it cut the top and bottom of the HDPE, so it would split apart instead of bunching up,” he says.
HammerHead Trenchless Equipment’s engineers were onsite throughout the entire project, providing training on the one-of-a-kind HB100, assisting with the project startup and offering on-target solutions for situations that occurred during the job.
Overall, the pipe replacement job required eight bursts averaging 400 ft, over a total of approximately 2,500 ft. Different sections of the existing 8-in. concrete pipe were lined with different materials, including HDPE and fold-and-form liner — all of which Falls Construction had to burst through before pulling in the heavy new 8-in. HDPE SDR17.
Preparing the Site
The company was able to begin work in January, thanks to a relatively mild winter without much frost. Huestis estimates the trenchless pipe bursting system only called for about 15 percent of the excavation normally involved in open-trenching, but the job still demanded a substantial amount of preparation.
“In some places, we were working at depths of 14 to 16 ft in solid brown clay,” he explains.
Once the crews set up the site with trench shields and shoring, the burst itself went quickly. A 400-ft pull took under two hours, and the operating pressure never peaked more than 1,000 psi (24 tons).
“When that machine was running, it looked so easy,” Huestis says.
Occasionally, the crew ran into a coupling that had been used to repair the pipe, and the coupling would start to push through with the burst. “Fortunately,” Huestis says, “those instances were at the end of the runs and didn’t cause significant problems”.
Because there were housing developments on both sides of the site, the sewer line also needed to remain in service throughout the project. Falls Construction was able to use bypass pumping to keep the system up and running for those customers. In addition, extensive fencing was required due to the residential setting.
Maintaining Infrastructure Is Key
As a third-generation family business, Falls Construction has been working in the area for more than 60 years — handling open-trench projects, cable plowing, directional drilling, sliplining and boring, in addition to pipe bursting jobs like this one at Malmstrom Air Force Base. That experience gives Huestis excellent perspective on how local systems have been maintained.
At a time when the nation’s failing infrastructure is getting a lot of press, Huestis gives Great Falls credit for doing a great job of investing in the system and updating it one piece at a time.
“There are aging water, sewer and storm drain lines across the country,” he observes. “For many communities, it’s out of sight, out of mind — but there will come a day when they have a massive problem.”
Camille Wolfe is a technical writer with Performance Marketing, based in West Des Moines, Iowa.