In this replacement application a new pipe of smaller outside diameter than the inside diameter of the existing pipe is inserted (pushed or pulled) within the existing pipe and provides a complete replacement of the structural and flow functions of the existing pipe. This is usually referred to as slip lining. In order to facilitate the insertion of the replacement pipe, the reduction of the inside diameter of the new pipe with respect to the existing pipe is often substantial. The potential reduction in flow capacity may be offset by a lower roughness coefficient for the replacement pipe or the ability to operate the new pipeline at a higher pressure. The slip-lining pipe typically is grouted in place within the host pipe depending on the application requirements.
The ideal host pipes 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.
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 the 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, flotation forces on the liner need to be taken into account when grouting and means taken to avoid flotation-related displacements especially in large pipes. These measures can include filling the pipe with water and grouting in stages.