Last summer, Matt Carter and his crews were like kids with a new toy.
He'd rented an 88-ton TT Technologies static pipe-bursting machine to replace 1200 feet of 6-inch and 650 feet of 8-inch cast-iron water line beneath two- and four-lane roads with HDPE. They were just two of the 30,000 feet of repairs and replacements Water District No. 1 of Johnson County, Kansas, makes each year to 2600 miles of ductile-iron and cast-iron pipes serving 35,000 customers in 16 cities.
He also rented a slip-lining winch. But Carter, who's the district's project engineer and infrastructure planner, chose a difficult job: a 1400-foot segment with 12 service connections. Though the slip lining itself took only four hours, coordinating the job, he says, “was killer.”
Today, Carter's trying to decide whether to buy the TT Technologies machine or a 100-ton Vermeer HammerHead unit.
“It's not inexpensive,” he acknowledges. “But that pipe-bursting project cost 20% less than traditional cut-and-cover. At least half our pipes are good candidates for pipe-bursting, so we're figuring pay-back within one year.”
Like every infrastructure manager trying to capitalize on the benefits of trenchless construction, Carter went through a process of elimination to determine what method works best for his agency's unique situation.
It's All About The Soil
Unlike the Water District No. 1 of Johnson County, though, most public works departments contract out the work. And here's where things get tricky.
Trenchless construction is multidisciplinary and mistakes are expensive. In addition to using specialized equipment, it requires contractors to design and manage the drilling slurry—called “mud”—that supports the drill hole and through which earth is brought to the surface during horizontal directional drilling. Once limited to HDPE pipe, trenchless projects can incorporate ductile iron and PVC.
When “working blind,” the quality of the installation depends on the quality of geotechnical and utility-locate information available to the designer and contractor. The steps required to gather this information, though, can increase design costs 10% to 30%, says Arvid Veidmark III, senior estimator for Specialized Services Co. in Phoenix, an underground construction consulting firm. But you'll pay up to four times the original estimate to adjust design and construction later.
To ensure long-lasting, cost-effective installation, consider the following:
Require the contractor to visually verify the location of all existing utilities before excavating. Don't be satisfied with marking paint and locate flags placed based on as-built drawings, which often don't reflect updates; or on information provided by one-call centers that often gather and share the same inaccurate information.
The Water District No. 1 of Johnson County in Kansas uses vacuum excavators to find utilities by “pot holing.” These utility trucks use pressurized water or air to break up soil and suck it into an onboard tank debris tank. They're extremely fast: In Overland Park, Kansas, crews dug a 30-inch hole in 26 seconds to find a cable television line without harming it.
Overland Park, by the way, has come far in controlling work in the right of way. Between 2000 and the end of last year, contractors from 20 states broke gas lines and water mains almost daily while laying conduit and both coaxial and fiber-optic cable for incoming Internet, phone, and cable TV providers, otherwise known as “fiber to the node” or “fiber to the premises.”
It was, says right of way coordinator Murv Morehead, “barely controlled chaos.”