Cured-In-Place Pipe Renews Ailing Sewer

By Mike Larson, Editor | September 28, 2010

Cured-in-place pipe (CIPP) provided a smooth solution for renewing 12,000 feet of sanitary sewer running along Anderson Street in north-central Madison, Wis., as well as other locations in the Truax area.

Owned by the city of Madison, this section of sewer runs under a parking lot, crosses under the street, and then follows the ditch on the south side of the road. Along the way, it runs under a small river.

Terra Wins Bid

Terra Engineering and Construction Corp., Madison, won the bid to renew the aging section of sewer.

Dan Shiley, who has 15 years of experience in the sewer business, was Terra's manager on this two-month-long, half-million-dollar project.

Says Shiley, "Replacing this sewer by the traditional dig-up-and-replace method would have been more disruptive and expensive, so the city specified using cured-in-place pipe. Terra's experience with CIPP enabled us to deliver the low bid."

Eliminates Digging And Restoring

The cured-in-place method forms a durable resin pipe, just a fraction of an inch thick, that conforms to the inside of the existing, or "host" pipe.

Shiley says CIPP pipe works inside a host pipe of any shape and made of any material, from clay to concrete or cast iron. And it offers a rated working life of 50 years, although some less-conservative contractors claim it will last up to twice that long.

The cost, he says, is considerably less than open-cut replacement, particularly in populated areas where the open-cut method requires digging up and restoring streets, parking lots and private yards.

CIPP also presents an advantage, he says, when a sewer runs under wetlands, rivers or other ecologically sensitive areas.

"On this project," he says, "our largest pipe is 24 inches in diameter, but I've been involved in projects on pipes up to 84 inches in diameter — and I've heard of others using the process on pipes in the 110-inch-diameter range."

Felt-And-Plastic Tube Delivers Resin For Pipe

The material for CIPP includes a continuous tube of polyurethane and felt, along with a polyester resin that cures into the new pipe when heated.

The plastic-and-felt tube is used solely in placing and curing the resin that hardens into the finished pipe.

Shiley explains, "The felt is there to absorb and hold the uncured resin for delivery to the host pipe, and the tube's plastic layer is there to make the tube airtight and waterproof during application. The felt and the polyurethane are not adding to, nor do they take away from, the design and integrity of the new pipe."

Resin Applied In Controlled Process

The tube of felt-and-plastic material comes from the manufacturer with the felt facing in and the plastic facing out. Terra has built and equipped a special section of its shop just to apply the resin to the felt side of the tube, a process called "wetting out."

Wetting out consists of applying vacuum to the tube, filling it with a specific amount of the catalyzed resin, and then squeezing the tube between two rollers to spread the resin evenly along the felt.

Immediately after wetting, the resin-impregnated tube is stacked and folded into a refrigerated trailer for delivery to the job. The low temperature in the refrigerated trailer keeps the resin from curing before it's installed on the site.

Proper Pipe Prep Imperative

Says Shiley, "Just like the old adage about painting, one key to successful CIPP installation is proper surface preparation."

In CIPP, that means making sure the inside of the host pipe is clean, as the liner will mimic the host pipe's condition.

On the Anderson Street project, Terra diverted sewer flow through 1,500 feet of 8-inch-diameter plastic pipe that carried the sewer contents around each section as it was being worked on.

Since the temporary shunt sewer was laid on the ground, a 6-inch pump replaced gravity to make sure the contents kept flowing the way they should.

Shiley says that CIPP is most efficient when you can work on about 1,000 feet of pipe at a time. "The length of pipe you can fix varies with site conditions and the location of manholes for access," he says.

Once the sewer flow had been redirected around a section of sewer line, Terra sent in its remotely controlled robotic sewer rover equipped with a video camera to help see exactly what the pipe looked like.

Terra inspects and records video before work begins, after the pipe is cleaned, and after the CIPP application has been completed. It then provides its customer with copies of he videos.

A high-pressure water nozzle was then sent through the pipe to remove dirt, roots and mineral deposits that had built up over time.

A final video inspection then confirmed that the pipe was ready for installation of the CIPP.

Installation Like Turning Balloon Inside Out

To install the CIPP, Terra pulled the leading edge of the material off the truck with the resin-coated felt still to the inside.

The leading edge of the tube was then fastened to a frame above the starting manhole and the rest of the tube was fed through the hole, turning the tube inside out as it went.

That inversion put the resin and felt to the outside, so it would form up against the host pipe's walls.

In a process that lasted a couple of hours, the remaining tube slid down into the pipe, pushed along and expanded by hydrostatic pressure from the water being added to it.

A Sky Trak 6042 telehandler equipped with slings helped lift and feed the snaking tube up and into the hole.

The process looked similar to stretching the mouth of a balloon over the open end of a hollow tube, then holding the balloon's closed end between your fingers and lowering it as you pour water over the balloon to force it down into the tube.

When the process was complete, what was originally the outside of the tube was facing inside and filled with water, while what was originally the inside of the tube was facing outward and pressed tightly against the inside of the pipe.

Says Shiley, "You can use either water pressure or air pressure to invert and install CIPP. The choice depends on the jobsite conditions and the diameter and length of the installation."

"On this job," he continued, "we chose to use water partly because of the long length of CIPP we were installing. But the major factor was the amount of ground water infiltrating the host pipe. Using water pressure to install the CIPP counteracts the pressure of the ground water to provide a better fit."

Heat Makes The Cure

Curing the resin requires about eight hours of continuous 185-degree Fahrenheit heat.

On this section of the Anderson Street job, that meant heating the nearly 25,000 gallons of water that filled the pipe to 185 degrees, then holding that temperature for eight hours.

To do it, Terra relied on its 8-million-BTU boiler truck to heat up and circulate the water. Even with that powerful heater, it took 14 hours to reach the 185-degree mark and 22 hours overall to complete the curing process.

Smooth Finish

When curing was complete, Terra let the water cool, then emptied it into a sanitary sewer. Crew members then used handheld reciprocating saws to cut out entry holes in the manholes.

There were no laterals connecting to this section of sewer. If there had been, the remote-controlled robot would have been equipped with a router bit to cut holes where they connected.

After a final robotic video inspection, the section was put back into service, and Terra's crew moved down the line to start over again on the next length.

Although the CIPP narrows the overall diameter of the pipe by a fraction of an inch, Shiley says the CIPP surface is so smooth that flow through the sewer actually increases from the installation.

Says Shiley, "I've worked with CIPP for more than 15 years now, and the techniques keep advancing, the materials keep getting better, and the practice is becoming more accepted all the time. It's not the answer for every job, but in the right situations it delivers a great result while really saving time and cost over traditional methods."