About the author: 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.
Tracy Favre, P.E.
undefinedThe 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.
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.