Before going straight to excavation, consider other methods of fixing a sag post lining
“I shot a 268-foot liner and it sagged. I think I understand the buckling issue. Yes, we had rain a few days before I lined the pipe and there was groundwater coming into the pipe, but I assumed that the water would be about the same temperature as the ground. Anyway, is there any chance I can 'round out' the liner and if so, how?”
Before we address the “rounding out”, let’s look at the reason for the sag because it may help prevent future sags.
Rain falling from the sky starts out as ice in the upper atmosphere. As it falls, it heats up, and by the time it melts and hits the ground, it may have ambient temperatures in the upper 30s to mid 40s — or 10 to 15 degrees colder than the ground. This water, as it infiltrates, cools the surrounding ground. If that cooling is near a pipe you’ve just lined, you may see a colder ground temperature so it cures at a much slower rate. If the rest of the liner achieves a cure state above the buckling point, you are good to go for those sections. But if a section of liner is below that, you can experience buckling. This buckling can cause a sag, a shark fin, or other similar condition.
What are the options?
You can attempt to “round out” the sag by inserting a calibration tube, heating the liner to a temperature above the point to which it had been cured, and holding it there until it is fully rounded out to the wall of the host pipe. Each resin has a different heat deflection temperature (HDT) after it has been fully cured. This is the temperature at which the liner will soften. But before it is fully cured, the HDT is less than what is advertised. If your resin’s HDT is advertised as 197 degrees F, then any temperature you achieve in excess of that may allow you to remold it to a condition that fits the host pipe.
This would be my first approach to take, and the sooner you can attempt the repair after finding the sag, the better your chances of success. Just remember that this will require you to slowly cool down the remolded liner so you don’t stress crack it with cold water after you’ve finished rounding it out. The older the liner, the more curing it has achieved, so timing is important here.
The next approach would be to remove the sagged portion of the liner. You can employ wire brushes, sandpaper, or carbide cutting tools to mill out the portion of the liner that has sagged. Once this has been completed, a point repair patch of liner and resin may be inserted to cover the removed portion. This a pretty straightforward repair you can complete with a thermoplastic polyurethane leader welded onto a section of liner tube, wetted out, and inverted into place followed with a regular calibration tube to cure it. Or you can wrap the section of liner tube turned inside out over a pipe packer, wet it out, and pull or push it into place. Fiberglass may be used in place of scrap liner tube for this application.
The final approach is to dig it up. When all else fails an excavator may be your only option, although this option is used more often than necessary out of panic. I can’t tell you how many times I’ve seen the excavator brought in because of panic and it costing everyone more money.
Those are three ways to solve an issue that you may encounter in your lining experiences. Hopefully learning why these occur can help you make sure your liner is cured enough before you pull your calibration tube. Just 15 minutes of added cure time may save you days of fixing the result of a sag in the liner.
About the author
John Heisler is the owner of Pipe Lining Supply and Quik-Lining Systems Inc. He has 20 years of experience in the CIPP lining industry and over 40 years in the underground construction industry.