Slip lining is perhaps the oldest of all trenchless techniques. It involves the insertion of a new pipe into an existing pipe. Under the right conditions, slip lining is also the simplest trenchless technique. A new pipe with an outside dimension smaller than the inside dimension of the host pipe is either pulled or pushed into the host pipe. The ideal host pipe for slip lining are straight with no deformities, that is pipes with no or modest bends, no severe protrusions into the pipe, and only modest offset joints. Slip lining may be continuous or segmental.
Polyurethane (HDPE and PE) and polyvinyl chloride (PVC) pipes are most commonly used for continuous slip lining circular, non-man entry pipes. The new pipe is laid out above ground and pulled through an excavated pit into the host pipe. The new pipe is then winched through the host pipe to an exit pit or manhole. In situations where space for layout of the pipe above ground is limited, the new PE or PVC pipe can be butt-fused during the installation process.
After the new pipe has been installed, the annular space between the new and host pipe is grouted. Grout may serve only to restrain the new pipe and transfer load from the existing pipe. The grout may cause the new and host pipe to act as a composite, increasing the pipe’s ring stiffness and its resistance to external hydrostatic loads. The proper selection and application of grout is often the most difficult part of a slip lining job. Grouts that serve only as a filler to restrain the new pipe are relatively low strength grouts with low viscosities. Structural grouts that serve to link liner to the host pipe have higher compressive strengths than grout used only to restrain the liner.
Forces on the liner during grouting may be greater than what the liner will encounter during normal service. Excessive grouting pressure could damage or collapse the liner. In addition, floatation forces on the liner need to be taken into account when grouting and means taken to avoid floatation especially in large pipes, such as filling the pipe with water and grouting in stages.
Segmental slip lining is typically used to insert Glass Reinforced Plastic (GRP) pipes into circular and non-circular pipes. The size of the host pipe can range from small non-man entry to larger man entry pipes. Segmental liners may be pushed into the host pipe by hydraulic power or winched in place. Concrete and steel pipe with higher compressive strength and stiffness may also be installed as slip liners and present fewer engineering challenges than plastic pipes during grouting.