Pipes: Choosing the Right Rehabilitation Method

Aging infrastructures and sanitary sewer overflows (SSO),
combined with limited funding and increasingly stringent compliance requirements,
make keeping their systems in good working order a daunting task for collection
system managers. To accommodate the lack of funds needed to replace large
portions of a municipal sanitary sewer system, many managers are investigating
techniques for extending the life of their collection system assets through
pipe and manhole rehabilitation.

The first thing to determine when considering rehabilitation
of pipes or manholes is what is the purpose of the rehabilitation. Does the
system need structural repair? Does Infiltration/Inflow (I/I) need to be
eliminated, does system capacity need improvement? Or a combination? The
answers can direct a manager to the most cost effective, efficient use of
limited resources.

Chemical Grouting

One option for non-structural repair is chemical grouting.
Ideally suited to stopping leakage and reducing soil loss, grouting consists of
injecting a self-setting grout into structurally sound leaking joints or small
wall cracks. The grout travels outside of the joint or manhole wall into
surrounding soils and bonds with those soils to create a seal collar of
material around the leaking joint or wall defect.

If protruding taps or significantly dropped joints do not
impede the passage of the grout packer, grouting can effectively be performed
on pipes 6" and larger in diameter. Pipes with significant soil voids
behind leaking joints or those with pipe under-drains may be better suited to
other rehabilitation techniques. Acrylamide and Urethane are the most common grouts
available, and various additives can increase/decrease setting times and
increase strength, flexibility and durability.

Sliplining

When existing capacity is sufficient, sliplining is a
relatively cost-effective rehabilitation method in which a pipe is inserted
into an existing line by either pulling or pushing continuous or short-length
pipes, frequently HDPE pipe. With traditional sliplining, a lead-in trench is
excavated for installation and pipes are butt-welded on the surface of the
ground before being winched or jacked into the existing pipe.

To stop infiltration from migrating down the annual space
between the pipes and to provide additional strength, grouting the annular
space between the lining and the original pipe is recommended. Significantly dropped
joints, roots and other impediments to the insertion of the pipe must be
repaired or removed prior to installation. Laterals must be reconnected by
excavation. Bypass pumping is required.

Particularly deep installation may make the method less desirable
due to increased costs. While relatively cost efficient in most instances, the
installation can be disruptive due to the excavation required for the
installation trench and lateral re-connection. Lack of access may make
traditional sliplining impractical.

Cured-in-Place Pipe (CIPP)

Using a polyester flexible sock or sleeve impregnated with
resin that is inverted or winched into the pipe, CIPP liners are considered a
trenchless repair. The sock is inverted using winch inversion, water inversion
or air (steam) inversion. Inversion allows the liner to conform to the existing
pipe contours providing for minor irregularities or slight changes of pipe
direction. Increasing the thickness of the polyester sock increases the
thickness of the pipe.

The CIPP method usually does not require grouting of the
annual space because the lining is designed to fit closely to the existing
pipe.  Excavation is not usually
necessary and lateral connections can be reconnected internally, making the
process ideally suited for locations with restricted access. If I/I reduction
is one of the rehabilitation objectives, the sealing laterals using internal
robotics is often required to curtail leak migration.

Fold-and-Formed Pipe

The fold-and-formed method, another trenchless repair, uses
deformation to reduce the pipe diameter of conventionally formed pipe. The new
pipe is then pulled into the existing pipe in the manner of traditional
sliplining. Once winched into place, the pipe is expanded back into its
original shape, forming a close fit with the existing pipe.  Grouting of the annular space is
usually not required because of the close fit of the new pipe to the old.
Laterals may be reconnected internally or by excavation. As with CIPP, laterals
may require sealing or lining in order to eliminate migration of I/I. By-pass
pumping is usually required.

Pipe Bursting

Pipe bursting uses a pneumatic or hydraulic bursting mandrel
to crush an existing pipeline while pulling a new pipe into place. The crushed
pipe is forced into surrounding soils, acting as bedding material for the new
pipe. A sleeve pipe is pulled immediately behind the bursting mandrel during
the process. When complete, the sleeve pipe is lined with a new pipe, typically
HDPE. Pipe bursting is well suited to brittle pipes and allows for the
size-for-size or increased size replacement of the existing pipe.

Depending on surrounding soils and ground conditions, pipe
bursting can typically be performed on distances as great as 300 feet or more. Offset
joints and short or minor line swags can be eliminated during the process.
Excavation of an insertion trench is required, usually at a manhole, and
laterals must be reconnected by excavation. Some ground upheaval may occur
where pipes are close to the ground service or where surrounding soils are not
easily compressed. Bypass pumping is required during the process.

 

The five rehabilitation methods discussed here are not a
comprehensive listing. As a collection manager strives to meet the challenges of
maintaining today’s aging infrastructure, traditional open-cut, point
repairs and a multitude of exotic methods all have their place. Setting
priorities, accurately evaluating existing conditions and proper selection of
the appropriate technique will be the keys to a successful rehabilitation
program.

Tracy Favre, P.E. is the Marketing Director of Pipeline Services at Severn Trent Services, Inc. For further information, phone STS at 800-375-0119.

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