State of Technology Report for Force Main Rehabilitation, Final ...

Figure 3-21. Summary of Replacement Technologies
3.5.1 Online Replacement. The following sections will cover the online replacement methods in
greater detail and highlight some of the recent advances in the trenchless replacement technologies.
3.5.1.1 Sliplining. The second most common method of replacement has traditionally been
sliplining, particularly for large diameter pipes. Sliplining involves the insertion of a new pipe with a
smaller OD than the inner diameter (ID) of the pipe to be rehabilitated. Typically, the OD of the new pipe
is a minimum of 2 inches less than the ID of the existing pipe. This can be relaxed for smaller diameters,
straight runs, and when there are no offset joints that could interfere with the movement of the new pipe.
The most common pipe material used for pressure sliplining has been PE. The PE can be in the form of a
coil, a long welded string, or discrete lengths. Fusible PVC is now also gaining popularity as a slipliner.
In large diameters, steel, DI, and GRP/FRP pipes have all been used as pressure slipliners. The new pipe
can either be pulled or pushed into the existing pipe. The annulus between the two pipes is often filled
with a low density grout to anchor the new pipe while increasing its buckling resistance.
Sliplining is a relatively low cost, easy method of installation that is not disruptive to adjacent utilities or
structures. All connections and fittings require excavation for reinstatement. The main disadvantage is
the reduction in flow capacity from downsizing the inside diameter. This can be offset to a certain degree
by the improved flow properties (lower frictional resistance) that many new plastic sliplining pipes offer.
Another disadvantage is the difficulty in finding any leaks that may develop in the new slipliner. Any
water or sewerage could migrate long distances between the slipliner and the host pipe before exiting at a
weak point in the host pipe. The external visible evidence of a leak could be far removed from the actual
leak in the liner.
Fused Pipe Lengths
Polyethylene, with butt-welded joints, has been used extensively for replacement using the sliplining
method. The axial strength of a butt-welded HDPE joint is equal to the material, so this makes HDPE
ideal for pulling into an existing pipe. Polyethylene pipe is normally sized with the OD equal to the
nominal diameter. However, on large projects, some PE vendors will tool up and custom manufacture a
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