Due to expansion of the soil around the hole, there is a risk of heaving at shallow depths. “Say there is an existing 10-in. sewer line that’s 4 ft. deep and the project owner wants to install a 16-in. line. You’re not going to be able to pipe burst at that depth because the expansion that takes place would actually heave whatever is on top of that,” Goodman explains. “If there is a road, other utilities or a water line running across that, it would actually fracture and crack that water line, gas line or road.”
“Depth of cover is an important issue,” agrees Orton. “It is critical not to burst pipe that is too shallow in the ground. Measurements should be taken to evaluate the depth of pipe.”
As a rule of thumb, Goodman recommends having 10 in. of cover for every inch you plan to upsize. But even size-on-size installations produce a certain amount of expansion. For example, a 12 1/2-in. expander may be used to replace a 10-in. pipe with new pipe of the same diameter. “So really, we’re expanding from a 10-in. line to a 12 1/2-in. hole,” says Goodman. Round that up to 3 in. for easy figuring, and it would require the pipe to be at least 30 in. below the surface.
Yet, he cautions against trying to pipe burst too deep. “Depth can also work against you,” Goodman indicates. “Let’s say you want to do a 10-in. upsize to 12 in. If you’re 25 ft. deep, that ground is extremely compact and it’s very, very difficult to expand...
“So you can have jobs that may not be a pipe burst candidate because they’re too shallow and you’re concerned about heaving or breaking another utility,” he states, “or you can have a project that requires replacement of a line that is really deep and may be too difficult to expand, depending on your requirements for installation and new pipe diameter size.”
A more friendly solution
Pipe bursting can offer both environmental and social benefits.
According to Matt Timberlake, vice president and senior project manager, Ted Berry Company Inc., Livermore, ME, you can reduce carbon emissions by as much as 95% with trenchless construction. “The cost of trenchless technologies, and primarily pipe bursting, is coming down in North America,” he states. “Contrary to that, the costs of open cut construction are becoming more expensive due to the increased cost of fuel, disposal, environmental and social impacts.”
Timberlake cites a project his company performed in York Beach, ME, involving total replacement of 1,000 linear ft. of sewer line. To replace 12-in. VCP with 12-in. HDPE, the open cut method would require a 1,777-sq.-yd. excavation, including replacement of 666 sq. yds. of pavement. Estimated total cost for the application, equipment, personnel, etc. was $275,000. In contrast, pipe bursting would require 99 sq. yds. of excavation and 43 sq. yds. of pavement removal/replacement, at an estimated total cost of $125,000.
“On a pipe bursting process, there is much less support equipment needed,” Goodman comments. “When you’re open cutting, you have an excavator digging the entire line. There are support trucks to [haul material]. So from a ‘green’ perspective, there is much more fuel consumed, there is much more exhaust... Whereas, with pipe bursting, you’re only excavating one or two pits along with your services, if you have them.”
Just as telling is the estimated number of business disruptions anticipated in Timberlake’s example — five for pipe bursting vs. 15 for open cut.
“We are seeing more ‘downtown’ type applications for pipe bursting,” says Orton. “Project designers are beginning to recognize the advantages of pipe bursting in difficult locations.”
The benefits are readily apparent in urban installations involving multiple utilities. “When you’re dealing with congested areas, you wouldn’t want to start digging due to concerns about breaking other utilities,” Goodman states. “That’s an area where [the ability] to minimize disruption would be beneficial.”
Installations along easements can also benefit from bursting. “Easement areas are usually troublesome for all but pipe bursting because of poor access and the need to maintain existing hardscape, landscape and buildings,” says Orton. “Many pipebursting jobsites may be in difficult access easement areas behind homes or down steep slopes. Open cut is virtually impossible in many situations.”