News | October 4, 1999

Sanitary sewer system rehabilitation techniques vary

By Francis T. Kung'u, Ph.D., P.E.
Senior Associate
USInfrastructure, Inc., Consulting Engineers
Birmingham, Alabama

Editor's note: The following is the first in a two-part series on different techniques available today for the rehabilitation of sanitary sewer systems. The first discusses the techniques and the second involves a case study in Jefferson County, AL.

Contents
Cured-in-place liner pipe technique
Deform/reform liner pipe technique
Fold and formed liner pipe technique
Slip liner pipe technique
Pipe bursting technique

Although the evidence of sanitary sewers has been uncovered in the ruins of ancient cities, the establishment of engineered sanitary sewer collection systems has been accomplished only during the 19th and 20th Centuries. Sanitary sewer collection systems consist of sewer pipes and sewer appurtenances such as manholes, inlet and outlet structures, and pumping stations. Sewer pipes are made of various types of materials such as clay, concrete, reinforced concrete, cast iron, ductile iron, polyvinyl chloride (PVC), and polyethylene. Most manholes are predominantly made of clay bricks or reinforced concrete.

Some collection systems may be more than 100 years old
The sanitary sewer collection systems of major cities in the United States were constructed during the 1800's or early 1900's. As a result of physical, chemical, and biological phenomena, some of the components of sanitary sewer collection systems for major cities have incurred defects such as collapsed pipes, cracked pipes, offset joints, and cracked manholes. During periods of wet weather, defective components of sanitary sewer collection systems allow the entry of surface runoff and/or groundwater (infiltration/inflow, I/I).

In addition to I/I, rainwater enters the sanitary sewer collection system through illicit connections from buildings. The combination of wastewater, I/I, and rainwater from illicit connections sometimes exceeds the capacities of some segments of the sanitary sewer collection system. This results in surcharges or overflows from manholes located upstream of the pipe segments whose capacities are exceeded. The total flow also often exceeds the capacities of wastewater treatment plants resulting in bypasses of untreated wastewater into receiving waters. This bypass has the potential to impair human health and the environment.

Excavation disruption led to trenchless technology
The traditional technique for rehabilitating defective sanitary sewers and associated appurtenances is excavation and replacement. Since sewer pipes in urbanized areas are, for the most part, located in the middle of streets, excavation and replacement creates traffic disruptions which results in loss of productive time for workers within such areas. In addition to the cost associated with lost productive time for workers, additional costs are incurred during repair of excavated roads and other structures.

In an attempt to reduce the cost and disruptions associated with excavation and replacement, the sanitary sewer collection system rehabilitation industry has developed "no dig" (trenchless) technologies for sanitary sewer collection system rehabilitation. Trenchless technologies include: cured-in-place liner pipe, deform/reform liner pipe, fold and formed liner pipe, slip liner pipe, and pipe bursting.

Cured-in-place liner pipe technique:
Cured-in-place liner pipe is formed by inserting a resin-impregnated felt tube into a defective sewer pipe and curing it with hot water. After the liner pipe is installed and cured, a remote-controlled cutting device is used with a closed circuit TV camera to reopen service connections. The cured-in-place liner pipe method is best suited for sewer pipes with high I/I and defects such as missing pipe segments, offset joints, and cracked pipes. It is also suited for sewers with multiple bends. The cured-in-place liner pipe method is probably one of the oldest and most effective method for sanitary sewer collection system rehabilitation. It has been used in Houston, Texas; Birmingham, Alabama; Atlanta, Georgia; Boston, Massachusetts; Chicago, Illinois; Dallas, Texas; Denver, Colorado; Miami, Florida; Washington, D.C.; and Los Angeles, California. Some of the advantages of the cured-in-place liner pipe method include the following:

  • No excavation is needed to accomplish the installation of a cured-in-place liner pipe. The resin-impregnated felt tube is inserted through existing manholes and service connections are reinstated remotely.
  • The cured-in-place liner pipe method can be used to correct almost all sewer line defects. The flexibility of the resin-impregnated felt tube allows it to navigated through defective sewers realigning offset joints, filling missing sewer sections, and re-sealing cracked sewer pipes.
  • The cured-in-place liner pipe realizes high reductions of I/I due to its ability to hold tightly to the host pipe and to form a good seal at pipe joints.
  • Unlike deform/reform and fold and formed liner pipes, the cured-in-place liner pipe bonds well with manhole rehabilitation products and hence reduces significant amounts of I/I at manhole connections.

Some of the disadvantages of cured-in-place liner pipe method include the following:

  • For the same diameter pipe, the cured-in-place liner pipe method is more expensive than other sanitary sewer collection system rehabilitation methods, (except pipe bursting). The cost to rehabilitate an 8-inch sewer pipe using the cured-in-place liner pipe method may range from $40 to $65 per linear foot depending on location and depth of the sewer pipe.
  • Curing the resin-impregnated felt tube can be difficult for long (greater than 1,500 feet) sewer segments and when there exists an underground spring within the sewer pipe segment being rehabilitated.
  • Correcting defective installations of a cured-in-place liner pipe can be very time consuming and cost prohibitive. Sometimes the sewer segment has to be excavated and replaced.

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Deform/reform liner pipe technique:
Deform/reform liner pipe is made of high-density polyethylene. The liner pipe is extruded in a round shape during the manufacturing process. The round pipe is then deformed using a combination of heat and pressure and wound onto spools ready for installation. During installation, the deformed pipe is pulled off the spool and inserted into an existing pipe through a manhole using an electric winch. Once in place, hot water or steam under pressure is fed through the inside of the deformed pipe to soften and reform the pipe. A remote-controlled cutting device with a closed-circuit TV camera is used to reopen service connections. The deform/reform liner pipe method is best suited for the rehabilitation of straight sewer pipe segments with few or no service laterals. The deform/reform liner pipe method has been used in North Miami, Florida; Dania, Florida; Fort Lauderdale, Florida; Orlando, Florida; and West Palm Beach, Florida. Some of the advantages of the deform/reform liner pipe method include the following:

  • No excavation is needed to accomplish the installation of the deform/reform liner pipe. The liner pipe is inserted through existing manholes and service connections are reinstated remotely.
  • Unlike the cured-in-place liner pipe, no inversion is needed to install a deform/reform liner pipe.
  • For the same diameter pipe, the deform/reform liner pipe method is less expensive than the cured-in-place liner pipe method. The cost to rehabilitate an 8 inch sewer pipe using the deform/reform liner pipe method may range from $35.00 to $50.00 per linear foot depending on location and depth of the sewer pipe.

Some of the disadvantages of the deform/reform liner pipe method include:

  • The deform/reform liner pipe has the potential to shrink considerably after installation into a defective sewer pipe. In some cases, shrinkage of the deform/reform liner pipe may cause serious dislocations of the openings at service connections. Once an opening to a service connection has dislocated, the only way to reinstate the service connection is to cut another opening leaving the dislocated opening as a point of entry of I/I. Unless sufficient lengths of the liner pipe are left protruding into the manholes associated with lined sewer pipe segments, the liner pipe may shrink into the host pipe. This results in a potential point of entry of I/I at the manhole connection. Available manhole rehabilitation products do not bond well with the deform/reform liner pipe at the manhole connection unless special bonding material are applied to the liner pipe before the application of manhole rehabilitation materials.

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Fold and formed liner pipe technique:
Fold and formed liner pipe is made of polyvinyl chloride (PVC). The liner pipe is extruded in a folded shape during the manufacturing process. The folded pipe is then wound onto spools while it is still pliable. During installation, the folded pipe is inserted into an existing pipe through a manhole using an electric winch. The folded pipe is then unfolded using hot water or steam under pressure. A remote-controlled cutting device with a closed-circuit TV camera is used to reopen service connections. The fold and formed liner pipe method is best suited for the rehabilitation of straight sewer pipes. The fold and formed liner pipe method has been used in Huntsville, Alabama; Albuquerque, New Mexico; Baton Rouge, Louisiana; Denver, Colorado; Miami, Florida; Los Angeles, California; and Seattle, Washington. Some of the advantages of the fold and formed liner pipe method include the following:

  • No excavation is needed to accomplish the installation of the fold and formed liner pipe. The liner pipe is inserted through existing manholes and service connections are reinstated remotely.
  • In some cases, the fold and formed liner pipe method may be less expensive than the cured-in-place liner pipe method. The cost to rehabilitate an 8 inch sewer pipe using the fold and formed liner pipe method may range from $ 30.00 to $ 45.00 per linear foot depending on the location and depth of the sewer pipe.

One disadvantage of the fold and formed liner pipe is that it has the potential to fold back to its original folded shape after installation resulting in re-introduction of I/I and reduction of the hydraulic efficiency of the sewer pipe.

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Slip liner pipe technique:
The slip liner pipe is a round pipe, which is inserted into an existing pipe through a process called sliplining. Slip liner pipes are manufactured of materials such as PVC, polyethylene, and fiberglass reinforced polyester. During installation, the liner pipe is pushed or pulled into an existing pipe through an excavated section of the existing pipe. The annular space between the existing pipe and the liner pipe is then grouted with a cement or chemical-based grout. Service connections are reinstated by excavation and reconnection using "Tees" or similar accessories. The slip liner pipe method is best suited for the rehabilitation of large diameter sewer pipes with few or no service connections and pipes where the wastewater flow is difficult to divert. This method has been used in San Diego, California; Albuquerque, New Mexico; Crane, Indiana; Lakeland, Florida; Rockford, Illinois; Alexandria, Virginia; Baton Rouge, Louisiana; Oklahoma City, Oklahoma; and Madison, Wisconsin. Some of the advantages of the slip liner pipe method include the following:

  • For the larger diameter pipes (greater than 21 inches in diameter), the slip liner pipe method is less expensive than cured-in-place, linear pipe method. The cost to slip line an 21 inch sewer pipe may range from $88.00 to $162.00 per linear foot depending on the pipe material, location, and depth of the sewer pipe.
  • No curing is needed during installation.
  • Flow diversion is not necessary for pipes with no service connections.

The disadvantages of the slip liner pipe method include the following:

  • Installation requires excavation of entry and exit pits.
  • Service connections are reinstated through excavation and reconnection.
  • The slip liner pipe method may result in a significant loss in pipe diameter for pipes smaller than 12 inches.

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Pipe bursting technique:
The pipe bursting method involves breaking a pipe and inserting another pipe of equal or greater diameter. During installation, a cone-shaped tool is pushed or pulled through the inside of the pipe to be replaced. In the process, the cone-shaped tool breaks the existing pipe and forces the broken fragments into the surrounding ground. The cone-shaped tool tows the new pipe behind it, simultaneously installing it in place as it bursts the old pipe. The bursting tool has a slightly larger outside diameter than the new pipe and it has a diameter greater than the inside diameter of the old pipe. Service connections are reinstated through excavation and reconnection using a variety of methods including mechanical saddles and "Tees". The pipe bursting method is best suited for areas where the size of an existing sewer pipe is too small to handle current or anticipated wastewater flows. The pipe bursting method has been used in Huntsville, Alabama; Seattle, Washington; Houston, Texas; New York, New York; Dallas, Texas; Orlando, Florida; Atlanta, Georgia; and Memphis, Tennessee.

Some of the advantages of the pipe bursting method include the following:

  • The pipe bursting method can be used to install a pipe with a larger diameter than an existing pipe resulting in an increase in capacity to handle increases in wastewater flow.
  • The pipe bursting method can be used to install sewer pipes of different materials of construction including PVC, polyethylene, and ductile iron.

The disadvantages of the pipe bursting method include the following:

  • It requires the excavation of entry and exit pits.
  • For the same diameter pipe, the pipe bursting method is more expensive than cured-in-place liner pipe, deform/reform liner pipe, fold and formed liner pipe, and slip liner pipe. The cost to rehabilitate an 8 inch sewer pipe ranges from $40 to $70 depending on type of pipe material, location, and depth of the sewer pipe.
  • Service connections are reinstated by excavation and reconnection.

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Rehabilitation of manholes done in several ways
Several technologies are used to rehabilitate defective manholes. Some of the most common technologies include spraying the inside surface of a defective manhole using urethane resin, epoxy, or cementitious materials; installing a cured-in-place epoxy resin liner; and installing a fiber grass insert. The sprayed-on urethane resin is probably the most effective material for rehabilitating defective manholes. The cost to rehabilitate a manhole with urethane resin may range from $275 to $350 per vertical foot.

First step is to perform a system evaluation survey
Trenchless technologies have been implemented in the United States with varying degrees of success. In order to determine the most appropriate products for the rehabilitation of any sanitary sewer collection system, each community should perform a sanitary sewer evaluation survey (SSES). The purpose of the SSES is to determine the condition of the sanitary sewer collection system, to identify the types and locations of defects allowing I/I, to quantify the extent of I/I, and to set priorities for rehabilitation of various defects based on the desired reduction of I/I. The SSES can be accomplished through the following methods: continuous flow monitoring, manhole inspection, smoke testing, flow isolation, dye-water testing, cleaning and television inspection, building inspection, and hydraulic modeling. Following the completion of the SSES, each defect should be analyzed and the most appropriate rehabilitation method identified.

It should be noted that trenchless technologies have not completely replaced excavation and replacement of defective sewers and manholes. There still remain circumstances where excavation and replacement is the best method to rehabilitate a segment of a sanitary sewer collection system.

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