Under the right conditions, the City of Wausau, Wis., uses slip-lining as an alternative to cured-in-place pipe (CIPP) lining to rehabilitate aging sewer lines. It’s a double- barreled approach that lets this northern city slice the cost of specific sewer replacements in half.
“Obviously, slip-lining isn’t feasible for every sewer in the system,” explains Joe Gehin, director of public works and utilities. “But where we are able to use it, we’ve found that we can save money.”
Slip-lining uses high density polyethylene (HDPE) pipe, which the city purchases at about $3 per foot. A thermal bonding process joins the 40-foot lengths together, and the liner pipe is then pushed into the existing sewer. Even including installation costs, Gehin calculates the procedure costs about 50 percent less than CIPP, although he says the cost for cured-in-place technology has been coming down as well.
Following a pattern
Wausau has about 250 miles of sanitary sewer and 200 miles of storm sewer, 99 percent separated. Some of the pipes are old enough to have been constructed of hand-laid brick. The sewer rehabilitation program follows a regular pattern:
The city rebuilds streets using a pavement management program, which rates streets according to their condition, and then recommends reconstruction projects to the city’s Capital Improvement and Street Maintenance committee.
As a street is targeted for reconstruction, the sewer department inspects the lines beneath using a pan-and-tilt CCTV system from Aries Industries Inc. If the line needs replacement, Gehin’s depart-ment takes advantage of the street reconstruction to accomplish both jobs at once, using a single contractor, and avoiding extra excavation.
“Coordination between street and sewer rehabilitation is really a key to success,” says Gehin. “We have one contractor and one point of responsibility. It’s essentially a single, all-inclusive project.” The department also replaces sewers in case of pipe collapses or other emergencies.
Years of experience
Wausau has been using the dual approach to sewer replacement for about 20 years. In 2007, the city rehabilitated about 2,500 feet of sewer with slip-lining, and more than 20,000 feet with CIPP. The city bids out both types of jobs.
For CIPP lining, the city uses Insituform Technologies Inc., based in Chesterfield, Mo., and Visu-Sewer Clean & Seal Inc., based in Pewaukee, Wis. According to city records, costs for CIPP have ranged from $22 a foot for 8-inch pipe to $32 a foot for 12-inch pipe.
For slip-lining, the city secures proposals from several area manufacturers and provides much of the labor itself, working with the street contractor when sewer replacement accompanies street rebuilding. Gehin estimates that the all-in cost for slip-lining can run as low as $6.50 per foot for the smaller diameters, and up to $10 per foot for larger diameters. Aside from cost, several other factors affect the decision to slip-line or use CIPP:
• Depth of sewer to be rehabilitated
• Changes in grade or direction of sewer to be rehabilitated
• Size of the host pipe
• Condition of laterals
If the line needing replacement is deep underground, or on a steep grade, the city will likely choose CIPP, explains Gus Strehlo, wastewater superintendent. That’s because CIPP liners are flexible enough to be inserted through manholes, whereas slip-lining requires an excavated trench for insertion. If the host pipe makes abrupt changes in direction or grade, flexible CIPP lining is better suited to the twists and turns.
Size does matter
Wausau has found CIPP to be the better choice as well, if the existing pipe is 8 inches in diameter or smaller. “If the host pipe is 8 inches or less in diameter, we’ll use CIPP because it’s not practical to slip-line sewers that small,” says Strehlo. “We tried with a 7.25-inch liner pipe once but it’s not something we’d do again.”
The condition of existing sewer laterals can also be a factor. With CIPP, a robotic cutter makes holes in the liner to connect with existing laterals, while slip-lining requires an excavation at each lateral connection. However, explains Strehlo, if the city plans to replace existing laterals, as well as the sewer line, then slip-lining makes sense because the excavations must be made anyway.
When the conditions are favorable, and when the failing sewer line qualifies for slip-lining, Wausau has developed a reliable program for installation. First, the city purchases the pipe and has it delivered to the site. A thermal fusing machine ties the lengths together. City workers cut slits in the leading end of the pipe and push the end sections together to form a point, or cone, that is secured with a bolt. The cone aids in the insertion of the liner into the host pipe. The crews can also use bentonite lubricant to promote the slipping action.
The HDPE pipe is not as flexible as CIPP, so the crew excavates a small entry trench to provide access to the host pipe, or to a manhole if the insertion will begin there.
Push, not pull
Wausau crews push the liner pipe into the host pipe, rather than pulling it. “We feel that pulling the pipe could elongate it or stretch it out of shape,” Strehlo says. Crews use a backhoe with a sling attached to the bucket. With the pipe positioned in the sling, the bucket arm extends, pushing 10 to 12 feet of pipe at a time into place. Workers have pushed lengths of 300 to 600 feet of liner pipe using this method.
The HDPE pipe slides easily, and since it is in sections, it can be installed manhole-to-manhole. Once the liner pipe is in place, city crews grout the new pipe to the old to form a seamless connection, and connect the laterals with saddles and banding.
After installing a large amount of slip-lining over the years as it has, Wausau has learned valuable lessons. Besides avoiding small-diameter pipe, it’s important not to put too small a liner into a host pipe. “The liner pipe needs to stay on the floor of the existing sewer,” says Strehlo, “so if there’s a large space around the liner, you’re better off to fill that space with mud so the liner won’t float.
“We’ve found that if the host pipe has I&I, that could float the liner pipe and interrupt the gradient. One solution is to weigh the liner down with mudding; another is to cut holes to drain the water out of the host pipe.”
Flow rate through the sewer is important, too. “We’re actually downsizing the sewer diameter by about one inch,” says Strehlo. While the smoother, more uniform surface of the liner pipe can actually increase hydraulic capacity, the smaller diameter could impede flow in the case of extremely high volumes.
In either slip-lining or CIPP, the walls of the host pipe need to be smooth and sound, and pipe offsets need to be as clean as possible.
Gehin and Strehlo are confident that with experience, and with resolution of key issues like groundwater infiltration, slip-lining provides a city with a cost-effective tool for rehabilitation of aging sewer lines.
