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Rehabilitation Method
Perhaps the largest share of the trenchless market
is represented by the requirement to rehabilitate defective pipelines with
some residual structural and physical life, which can be used as a structure
for the new line. Examples of rehabilitation techniques include
Cured-in-Place Lining (CIPP), Close-Fit Lining, Slip-lining, and Spray
Lining, all with their own-patented variations, as well as various other localised repair techniques. Variations relate to the
material used, wall thickness provided to offset structural or physical
defects, the rate of rehabilitation, and the minimum time of shut-down for
the existing service. The rehabilitation of small diameter
underground pipes is a new area where the cost competitiveness of Trenchless
Technologies is well recognised. Many utility
pipelines, sewage in particular, become defective due to the corrosiveness of
modern effluents. They also suffer from overloading and loss of capacity. One
of the advantages of rehabilitation is that the new lining materials have a
much lower surface friction coefficient, thus it is often possible to
increase the capacity of the refurbished pipe without increasing its
diameter. In CIPP, a fabric impregnated with polyester
or epoxy resin is inserted into the defective pipe and inflated to fit
against the pipe wall. It is then cured by hot water, steam or ultraviolet
light. The system has many variants and can be designed to provide different
wall thicknesses to meet particular needs. One advantage is that the lining
adjusts to variations in the size of the pipe. It is widely used for the
rehabilitation of gravity sewers, including laterals, and usually results in
no loss of capacity. Close-fit linings take many forms. The lining
is deformed through a swage (a metal die) or manufactured in a folded state
so that it can be pulled into the host pipeline. Various methods can then be
used to allow the lining to revert to its full size or to the shape of the
host pipe. Spirally wound liners are a form of close fit
in which a PVC strip is fed though a small access into the defective pipe.
The PVC strip is then helically wound into place against the pipe wall using
a winding machine operated from within the pipe. This technique is
particularly useful for emergency repairs and for adding strength to
pipelines that have been weakened. Slip-lining involves putting a pipe within a
pipe and grouting the resulting annulus between the new lining and the old
pipe. This causes a reduction in capacity and the process has now been
modified using polyethylene to reduce the thickness of the liner and to minimise the size of the annulus. Spray linings using
cement or resin are widely used on water pipelines. Spray lining materials have to be used
carefully and approved by regulatory authorities due to the potential for
releasing solvents and residues. Spray linings are suitable for dealing with
leaks but not where there are structural defects. Localised repair techniques make
use of robots in conjunction with CCTV cameras to clean, prepare and fill
cracks and voids with epoxy mortar. This is often a cost effective way of
dealing quickly with an isolated problem in an otherwise sound pipeline. The
ease of transport and mobilisation of the equipment
is an advantage. All these methods discussed in here is a
sincere attempt to provide a reference tool to all the stakeholders facing
the requirement to rehabilitate or manage a Subsurface Utility Network where
open trenching is not possible. Users are advised to exercise utmost care in
using these methods as they are meant to provide the introductory information
only and these should not replace the project designs and sound engineering
selection processes. Dr. Niranjan
Swarup
Director General
Indian Society for
Trenchless Technology
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