Published July 2007

Underground Innovation

By Ken Wysocky (page 42)

A wide range of trenchless methods provide cost-effective alternatives for community water main rehabilitation programs.

Operators of the nation’s 54,000 community drinking water systems are well aware of an emerging crisis: the need to repair hundreds of thousands of miles of aging water mains, some of which date back to the late 1800s.

While estimates vary widely, the American Water Works Association (AWWA) estimates the cost of main repairs at more than $250 billion over the next 30 years. And the bill is coming due at a time when municipalities and utilities are under pressure to limit rate hikes and reduce spending.

In this climate, many officials are looking to trenchless technology for affordable solutions. Trenchless repairs often cost less and minimize disruption to neighborhoods and traffic. And they often can be completed faster. Here is an overview of trenchless main repair options.

Giving the slip

Slip-lining is one of the oldest and most common rehabilitation methods. It involves pulling or pushing a slightly smaller pipe — usually lengths of high-density polyethylene (HDPE) butt-fused together — into an existing main, explains John Hemphill, executive director of the North American Society for Trenchless Technology (NASTT), a not-for-profit organization.

Grout is then injected into the annular space between the new pipe and the host pipe. The method works well with mains that need higher structural strength. “Slip-lining is also well suited for straight shots with few turns and few service connections,” Hemphill says.

The smaller pipe does reduce capacity to varying degrees, depending on its diameter. But in some instances, reduced surface friction on the HDPE pipe’s smooth interior somewhat compensates for the loss. Generally speaking, larger-diameter mains are the best candidates for slip-lining, Hemphill says.

Because host pipe irregularities hinder installation, CCTV inspection is required before lining. Slip-lining is not completely trenchless, as it requires excavation of entry and exit pits and separate excavations for all service connections. But the same is true for many other rehabilitation methods, Hemphill notes.

In a modified method of slip-lining, called fold-and-form, lengths of pipe — again, usually HDPE — are folded into C or U shapes, which fit more easily into the host pipe. The pipe is wound onto reels, then winched through the host main. Hot water or steam is used to expand it. “The advantage here is a tighter fit or less annular space to fill,” Hemphill notes.

In another variation, known as the tight-fit method, the liner pipe is reduced slightly in diameter by pulling it through rollers or reducing dies, then expanded inside a thoroughly cleaned host main until it fits snugly. This eliminates the need to fill the annular space with grout.

Cured-in-place

Another common repair method, cured-in-place-pipe (CIPP), uses a seamless, flexible tubular membrane that is saturated with a thermosetting resin on the outside and coated with an impermeable, elastomeric membrane on the inside.

The walls of the existing main must be thoroughly cleaned for the CIPP to adhere properly. After that, the membrane is inserted into the host pipe using water, steam or compressed air. The resin-filled side of the tube gets flattened against the host pipe wall. Then steam, hot water or ultraviolet light is applied to cure the resin.

Another system employs a winch that pulls into the host pipe a collapsed membrane that is then inflated and cured. And in yet another method, a water-filled polyvinyl chloride (PVC) pipe is inserted into the membrane, keeping the lining firmly pressed against the host pipe. When the membrane is cured, the pipe is removed.

Whatever the method, the result is a tight-fitting, impermeable, corrosion-resistant barrier. Some membranes can even be designed to add structural support. “Manufacturers can use a thicker membrane or fiberglass to reinforce the ‘sock’ and increase its structural integrity,” Hemphill says. “Also, different resins can provide varying structural strength.”

Spray-on applications

One of the most common methods of main lining, especially where the host pipe does not need added strength, is spraying of the interior wall of the main with cement mortar or epoxy.

“Cement mortar lining is best suited for old cast-iron and steel mains,” explains Bruce Britton, vice president of W. Walsh Company Inc., a pipeline rehabilitation company in Attleboro, Mass. “It’s not a structural improvement, just a corrosion inhibitor. Once it’s finished, the pipe is good for another 50 to 100 years.”

Just as with CIPP, clean pipe walls are critical. The pipe must be cleaned down to bare metal and thoroughly inspected with CCTV. The surface also must be completely dry. Cement mortar is applied with a centrifugal sprayer centered inside the pipe and either skid- or tractor-mounted. The thickness of the coating is controlled by the flow rate and the speed at which the sprayer travels.

The larger the pipe, the thicker the mixture that must be applied. The thickness standards developed by the AWWA range from 3/16 inch for 4- to 10-inch pipes to 5/16 inch for pipes 24 inches or larger.

“To smooth out the cement mortar in smaller pipes, a drag trowel follows the sprayer,” Britton explains. “In larger pipes, we use a rotating trowel.” The coating must be inspected to ensure the correct thickness. A depth gauge is inserted at intervals. To ensure uniformity, each interval measurement is taken at the 3, 6, 9 and 12 o’clock positions. All appurtenances, including ball valves, must be inspected to ensure they retain full range of motion.

Epoxy coatings may be more appealing in some applications because they dry faster than cement mortar — 8 to 9 hours versus 24 to 48. “We can usually return to water service within 24 hours,” observes K.C. Bindel, sales director of Innerline Technologies in Mansfield, Texas. “Epoxy also is typically less expensive and lasts longer.”

Moreover, an epoxy coating is typically thinner than cement mortar. “Epoxy results in significantly less reduction in carrying capacity,” Bindel says. Even though the lined pipe bore is slightly smaller, water flows more easily through it because the smoother surface offers less friction.

Give pipes a break

When mains are in such bad condition that lining won’t work, when additional carrying capacity is required, or when open-cut replacement is too expensive or not feasible, pipe bursting is a sound alternative, notes John Rafferty, who heads up technical development at TRIC Tools Inc., based in Alameda, Calif.

Pipe bursting requires entry and exit pits. Then a steel cable is threaded through the pipe. When it reaches the other end, a steel bursting head is attached. The new pipe, typically HDPE, attaches to the back of the bursting head. HDPE pipe is used because it’s corrosion-resistant, strong, flexible, and should last 200 years, Rafferty says.

A hydraulic system pulls the bursting head through the old pipe section, shattering it and pushing the fragments into the surrounding soil, while the new pipe is drawn into place. “Pipe bursting works best if the pipe being replaced is fractural,” Rafferty points out. Concrete and cast-iron pipes make the best candidates. Galvanized pipe tends to split like a banana being peeled, exposing sharp edges that can shred the HDPE pipe.

“If it’s 2-inch diameter galvanized pipe or less, and the repair section is short and buried fairly shallow, it’s conceivable that pipe bursting could work with galvanized pipe,” Rafferty says.

“Long runs of wet, sandy soil can also make pipe bursting difficult, because it’s hard to shore up. It also creates a lot more drag on the pipe you’re pulling through, and that can actually cause the pipe to stretch. You also have to be careful about other nearby utility lines.”

More to consider

Of course, there are a host of other factors to consider. Slip-lining and pipe bursting, for instance, require larger excavations than other methods. Sections of pipe with many service connections and valves may be poor candidates for trenchless methods. And all the methods require bypass piping to maintain water service for customers while the work is performed.

But when conditions allow, there’s little doubt that trenchless technology is changing the landscape of water main rehabilitation.