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.”
To minimize disruption, the public works department wrote a 13-page “Horizontal Directional Drilling Guidelines Handbook” that specifies permitting, design, construction, stormwater pollution prevention, and drilling fluid requirements for contractors working for private utilities in the public right of way. Morehead then taught employees from other departments how to inspect construction sites, and began shutting down uncooperative contractors.
“They got with the program real quick after that,” Morehead says.
Require the project designer to conduct a geotechnical analysis. Soil conditions influence what construction method to use—pipe ramming, for example, works better for loose or unstable soil, such as sand, than in dense material like clay—and bid prices.
Most contract claims on trenchless installations arise from “changed ground conditions.” Owners can minimize potential liability by involving contractors as early as possible in geotechnical investigations.
The extent of the investigation depends on the project-delivery method, says Naresh Koirala, senior technical engineer with Golder Associates in Burnaby, British Columbia, a geotechnical and environmental consulting firm that works worldwide.
For conventional design-bid-build projects, the investigation should be carried out in phases: a preliminary investigation to determine what trenchless construction method would work best given the soil conditions and a more detailed investigation later during design.
With a design-build contract, the owner or its geotechnical adviser conducts only the preliminary investigation because the contractor is responsible for the more detailed investigation. Specialized Services Co., whose clients include Phoenix and Scottsdale, has been retained under design-build and contract-manager-at-risk contracts, not under design-bid-build.
With a “no product /no-pay contract,” the owner doesn't pay if the job isn't installed properly. Of course, owners pay handsomely for offloading all potential liability onto the contractor.
Write a competitive request for proposals. This is easier than you may think, says Jesse Van DeCreek, PE, an associate with underground consulting firm Hubbell, Roth & Clark Inc. in Bloomfield Hills, Mich.
Simply prepare a bid document with all the elements—such as excavation, traffic control, and restoration—of traditional cut-and-cover construction.
Then, assuming you've determined the appropriate trenchless method that will be used, prepare an alternative bid proposal in the same contract that includes all the elements of work unique to trenchless construction for the same length of pipe. In many cases, the per-foot cost to complete the work will be lower for the trenchless method.
For horizontal directional drilling projects, you can also require the contractor to submit a mud-mix design and drill plan that shows how the drill rig and bore path will be set up.
For pipe bursting, microtunneling, and in-situ lining, prequalify contractors or use a quality-based selection process to ensure all elements of the bid—not just price—are satisfactory. Low bid doesn't mean lowest project cost if the contractor isn't capable of the work.
While there's no zero-impact pipe installation method, trenchless construction methods are saving communities millions of dollars on pipe rehabilitation by minimizing disruption.
Related Articles & Links
To learn more about products that keep water flowing, click here
- WEB EXTRAS: To view Overland Park's guidelines for horizontal directional drilling, and to develop specifications for “subsurface utility engineering” services, visit the "Article Links" page.
- Read the article "Eenie, meenie, miney, or mo?" to learn about four rules of thumb for selecting the right application.