1 March 1997
REHABILITATION OF EXISTING SYSTEMS
11-1. REHABILITATION VERSUS REPLACEMENT.
a. Infiltration. Techniques for rehabilitating sewer systems to eliminate infiltration are
presented in detail in USA-CERL Technical Report N-88/25. Their evaluation and applicability to
problems are briefly discussed in the following paragraphs along with other techniques of
trenchless. All of these techniques work best in small areas and are generally not effective in
correcting problems throughout an old, deteriorated system. It is generally best to replace
shallow lines where excessive or difficult trenching is not required. These correction techniques
also work best when there are leaks at joints but the sewer is structurally sound. Systems with
extensive deterioration most often must be replaced rather than repaired. It is important to note
that in most systems the major source of infiltration is building laterals which are generally
shallow and can be excavated and replaced.
b. Inflow. Inflow problems are generally much easier to correct than infiltration. Manhole
covers in roadways or depressed areas are sources of inflow and can be prevented by covering
holes with bolts and sealing around the cover with tar, caulk, insulating foam, or a flexible gasket.
All cross connections must be eliminated.
11-2. GROUTING. Once locations requiring grouting has been identified using remote television
inspection and flows stopped, grout is applied manually in large lines and at manholes and by
remote control in smaller lines or where hazards exist. The type of grout to use depends on the
type and size of the crack, experience of the installation personnel and external pressure. This
method should be used only for spot repairs and where further cracking or settling is not
anticipated. It is best at joints where the grout has deteriorated or was not properly installed and
not where the joints have separated or been damaged by external stresses.
11-3. CEMENT LINING. Use of cement lining is restricted to sealing large cracks and adding
protective liners in wetwells, manholes, and sewers over 36 inches in diameter in which workers
can enter the pipe. It is commonly used in brick manholes where hydrogen sulfide accumulations
have decimated the cement mortar. Portland cement or shotcrete (at least 100 mm (4 inches)
thick) is the most common type of cement liner but is not impervious to hydrogen sulfide attacks.
There are, however, other types of cement, such as that with a calcium aluminate base, which
are advertised to better resist deterioration caused by hydrogen sulfides.
11-4. SLIPLINING. This technique, which involves pulling or pushing a new pipe through an
existing pipe, will have the greatest application for systems where joint compounds have
deteriorated but the pipes are structurally sound and have not experiencing differential settling.
Sliplining with flexible plastic pipe, especially polyethylene, after debris and roots have been
removed, has been effective in temperate climates but should not be used if large temperature
fluctuations are anticipated unless methods are used to control temperature-related contraction
and expansion. Grouting or casing spacers are needed to hold the new pipe firmly in place. If
the structural damage to the existing pipe is severe, continuous grouting is needed. If there is
the possibility for significant external pressure to be exerted, such as from a high water table, the
pipe specified must be rigid enough to withstand the pressure. Even though there are methods
for making connections to laterals, it is best to use sliplining on sections that do not have laterals.