Slip lining is a trenchless method for repairing damaged underground pipes by sliding a new, smaller pipe inside the existing one. Instead of digging up an entire pipeline to replace it, crews insert a “pipe within a pipe” that restores structural integrity and stops leaks. The technique works on both gravity-fed sewer lines and pressurized water mains, with pipe diameters ranging from 4 inches to over 12 feet.
How Slip Lining Works
The core concept is straightforward. A new carrier pipe, slightly smaller than the original, gets pulled or pushed through the deteriorated host pipe. Once in place, the gap between the two pipes is filled with grout to lock everything together. The result is a fully reinforced pipeline that can last 50 years or longer when installed correctly.
Because the new pipe fits inside the old one, there’s an unavoidable tradeoff: some internal diameter is lost. For most applications this reduction is minor and doesn’t significantly affect flow capacity, especially since the smooth interior of the new pipe creates less friction than the corroded original. But for systems already operating near maximum capacity, engineers need to calculate whether the smaller diameter will still meet demand.
The Installation Process, Step by Step
A typical slip lining job follows seven stages, starting well before any pipe goes into the ground.
First, crews inspect the existing pipe to assess how badly it’s deteriorated and whether slip lining is feasible. This often involves sending remote-controlled CCTV cameras through the line to map cracks, collapses, or obstructions. If the pipe is too deformed or partially collapsed, slip lining may not be an option.
Next, two small excavations are dug at either end of the section being repaired. These entry and exit pits are the only digging required, which is the main advantage over traditional open-cut replacement. The host pipe is then cleaned using water jets, mechanical plugs, or specialized cleaning tools to remove debris, roots, and buildup that could block the liner.
The carrier pipe sections arrive on site and are joined together to match the installation length. How they’re joined depends on the material. High-density polyethylene (HDPE) sections are typically fused together by heating the pipe ends until the plastic melts, then pressing them firmly together to create a seamless bond as the material cools. PVC and fiberglass pipes use bell-and-spigot joints, where one end slots into the next.
The assembled liner is then either pulled or pushed through the host pipe. Fusion-welded HDPE liners are usually pulled into place from the exit pit. Bell-and-spigot pipes must be pushed from the entry side to keep the joints from separating. After installation, the liner sits for about 24 hours to relax and settle into position.
Finally, any lateral connections (branch lines feeding into the main pipe) are reopened by cutting through the liner and fitting wrap-around saddles at each junction. Terminal connections at either end are restored using pressure-rated fittings or flanges.
Filling the Gap Between Pipes
The space between the old pipe and the new liner, called the annular space, doesn’t stay empty. Filling it with grout is one of the most important parts of the job. The grout creates a solid barrier that supports the liner, prevents ground movement from shifting the new pipe, and stops the old pipe from deteriorating further.
Two types of grout are commonly used. Cement grout, made from Portland cement or a cementitious mix, provides high strength and durability. Cellular grout, a lightweight concrete, flows more easily and can be pumped over longer distances at lower pressures, making it practical for large-diameter or extended installations. The choice depends on the pipe size, soil conditions, and how far the grout needs to travel.
Continuous vs. Segmental Slip Lining
There are two main variations of the technique, and the difference comes down to how the liner is assembled.
Continuous slip lining uses long strings of pipe, typically HDPE or PVC, that are fused together on site before being pulled through the host pipe in one piece. Sections usually arrive in 5-meter lengths and are welded together using specialized rigs. This method creates a seamless liner with no joints inside the host pipe, which reduces the risk of leaks. It handles diameters from about 4 inches up to 63 inches.
Segmental slip lining uses shorter pipe sections that are pushed into the host pipe one at a time and joined inside. This approach accommodates a much wider range of diameters, from 4 inches all the way up to about 158 inches (over 13 feet), making it the go-to method for large culverts and trunk sewers. Materials include HDPE, PVC, and fiberglass-reinforced plastic. Both methods can handle installation lengths up to roughly 1,000 feet.
Why It’s Preferred Over Open-Cut Replacement
Traditional pipe repair means digging a trench along the entire length of the damaged section. In urban areas, that translates to torn-up roads, closed lanes, rerouted traffic, and weeks of disruption. Slip lining avoids most of that. The only excavation happens at the entry and exit points, so streets stay open, landscaping stays intact, and the project wraps up faster.
The cost savings are significant. Less digging means less labor, less equipment time, less pavement restoration, and fewer permits. Environmental impact drops too, since there’s minimal soil disturbance and far less construction waste. For municipalities repairing aging infrastructure under busy roads or near sensitive areas, those advantages often make slip lining the default choice.
Common Applications and Materials
Slip lining is used across water, sewer, stormwater, and industrial piping systems. Sanitary sewers are one of the most common applications, particularly lines in the 6- to 12-inch range where HDPE liners are standard. Larger stormwater culverts and drainage structures, typically 36 inches and above, may use fiberglass-reinforced pipe or segmental concrete liners.
HDPE is the most widely used liner material because it’s flexible, corrosion-resistant, and easy to fuse into continuous lengths. PVC is another common option, especially for smaller gravity sewers. Fiberglass-reinforced plastic handles larger diameters and offers high strength-to-weight ratios for pressurized systems. The material choice depends on pipe size, whether the system operates under pressure or gravity, and the chemical environment inside the pipe.
How Long Slip-Lined Pipes Last
Independent testing supports a minimum service life of 50 years for quality trenchless liners installed correctly. That number comes from accelerated aging tests and long-term field data, and many engineers expect well-installed liners to outlast that estimate. Since HDPE and PVC resist corrosion, root intrusion, and chemical degradation far better than the aging clay, concrete, or cast iron pipes they’re replacing, the rehabilitated pipeline often performs better than the original did when it was new.