Sewer System Repairs

Background

In the past, the primary reason for rehabilitating a sewer was to restore the structural integrity of a line that had failed and, as a consequence, discharged raw wastewater into the environment. Today, however, the situation is much more complex. A large number of factors contribute to the rapidly declining integrity of major portions of the wastewater collection systems throughout the country.

To begin with, many of the collection systems in use today are very old and the materials used in their construction have lost their structural integrity due to corrosion and natural deterioration from use. This gradual breakdown allows greater infiltration, particularly during periods of heavy rainfall and under high groundwater conditions. The additional flows, in turn, generally produce two negative effects: (1) the surcharged sewer flows accelerate deterioration of the system and allow leakage into the environment and (2) the increased inflow/infiltration overloads the treatment facilities to the point where they sometimes fail and discharge partially treated wastewater into the environment.

Other factors that contribute significantly to the deterioration of collection systems include rapid system expansion to keep pace with population growth in some areas. Not only can expansion the entire system, but it may result in a diversion of effort and attention away from the long-term preservation of existing facilities. Another effect of population growth is the stress that is created by vibrations from construction and the laying of roadways and other utilities over sewer systems. Sewer systems also are failing due to unstable foundation soils or improper pipe bedding material.

A smoothly functioning sewer system is a service the public has come to take for granted. Because the system is largely out of sight, there is little awareness of its importance or of the serious consequences that result form continued neglect. As more and more large municipal systems reach the end of their useful lives, major programs of systematic rehabilitation will have to be undertaken. An ongoing program of long-range analysis and rehabilitation could greatly extend the service-ability of existing systems, could reduce operating costs, could reduce the need to expand the system, and could protect the environment from the consequences of a major collection system failure.

Sewer Pipe Repairs

Pipe Linings. Pipe linings may be installed inside the sewer in either continuous or short lengths. A large variety of sewer sizes and cross sections can be lined, using sliplining (continuous pipe and short pipes), cured-in-place pipe (CIPP), deformed pipe, or spiral-wound pipe.
Sliplining. Sliplining is a method by which pipes are inserted to an existing line by pulling or pushing continuous or short-length pipes into the sewer. The annulus between the existing pipe and liner pipe should be grouted. There are many pipe types available.
Continuous Pipe. Sewers may be sliplined with continuous lengths of solid-wall PE or polybutylene pipes, commonly supplied in (40-ft) lengths and butt fusion-welded together before insertion to the sewer pipe. Pipes are drawn or jacked into the sewer, subsequent lengths being added as required. Where the annulus between the lining and the existing sewer is (1 in.) or more, it should be grouted to provide support for the flexible lining. Where the annulus is not grouted, the liner should not be considered a structural lining. Small-diameter, continuous, extruded pipe can be used. A nose cone is attached to the front of the pipe to reduce the likelihood of snagging and as a point for attaching the winch cable.
Cured-In-Place Pipe. The CIPP lining process involves inserting a flexible lining to the sewer using either an inversion process or a winching process. The lining is inserted through existing manholes and, depending on the system selected, installed using water inversion, winched inversion, or air inversion. With water inversion, the lining is inverted under water pressure and cured by circulating hot water. With winched inversion, the lining is winched into place and inflated against the sewer wall by a calibration hose inverted to the lining under water pressure. The lining is cured by circulating hot water. With air inversion, the lining is inverted under air pressure and cured by introducing steam.
Nu-Pipe and U-Liner Deformed Pipe. These deformed pipe systems involve inserting HDPE or PVC thermoplastic U-shaped pipe that is expanded in the sewer to form a tight fit with the existing sewer.
Rolldown and Swagedown Deformed Pipe. Lining systems using these deformed pipes reduce the diameter of conventionally formed circular PE pipe to allow installation inside an existing pipe. When the diameter is returned to its original size, it forms a tight fit with the existing sewer, increasing the hydraulic capacity of the pipe.
Spiral-Wound Pipe. Using spiral-wound pipe involves inserting a pipe fabricated at the bottom of a manhole or access shaft. A PVC strip is pulled through a winding machine that incorporates a series of rollers that form the pipe to the correct size. The spiral joint is made using either an interlocking PVC clip, twin rubber gaskets, or a water activated polyurethane adhesive. The pipe is literally wound into the existing pipe, and the annular space is grouted with a cement-based grout. Three basic forms are available: remote spiral winding for non-person-accessible sewers, person-entry spiral winding for larger sewers, and manually installed panels.
Trenchless Replacement. In recent years, trenchless replacement and construction techniques have been used more frequently, including pipe bursting, microtunneling, directional drilling, fluid jet cutting, impact moling, impact ramming, and auger boring. Minimum excavation techniques, such as narrow trenching, are also being used more in less-congested environments.
Pipe Bursting. Pipe bursting is a method for inserting a new pipe of equal or larger diameter to an existing pipeline by fragmenting the existing pipe work and forcing the material into the surrounding soil. The new pipe then is inserted into the enlarged hole.
Conventional Replacement. The ultimate solution to I/I problems is complete replacement of the pipeline. To some degree, this is achieved with some of the lining and trenchless replacement options in that a new pipe is provided.
Pointing. Conventional repointing techniques may be used in brick or masonry sewers to reduce infiltration or replace deteriorated mortar. The technique frequently is associated with internal or external grouting designed to restore side support to the structure. The existing sewer still must be physically intact, with no deformation, disturbed bricks, or closed joints. Traditional sand and cement mortars and special pressure pointing mixes typically are used, although the mortar must be of a similar strength to the original as spalling of the brickwork may occur if stronger mortars are used.
Internal Grouting. Internal, or chemical, grouting is the most common method for sealing leaking joints in structurally sound sewer pipes. Using special techniques and tools, chemical grouts can be applied to pipeline joints, manhole walls, wet wells in pumping stations, and other leaking structures. Small holes and radial cracks also may be sealed by chemical grouting.
Small and Medium Sized Pipes. A hollow, cylindrical packer with inflatable collars at either end is inserted to the sewer and positioned across a pipe joint. The rubber collars are inflated and the joint tested under pressure. If the joint fails the test, a chemical grout is pumped through the joint to the surrounding ground until the grout solidifies and an acceptable back pressure is achieved. This technique reduce the potential for migration of groundwater infiltration through other unsealed joints in a sewer segment.

Manhole Repairs

Coating Systems. Coating systems have been used to restore manhole structures. In each application, a cementitious material containing Portland cement, finely graded mineral fillers, and chemical additives is applied in one or more layers to the interior of a manhole that has been adequately cleaned and prepared. Most coating systems provide for both mechanical and chemical bonding. The system can be used to coat the entire manhole, including reconstruction of the bench and invert. Coatings are ideally suited for brick structures that show little or no evidence of movement or subsidence, as the coatings have little intrinsic structural qualities in shear and tension.
Chemical Grouting. Chemical grouting has been successful in reducing I/I in manhole structures. It is important to note that grouts do not add to the structural integrity of the manhole. The success of grouts in reducing manhole I/I largely depends on soil and groundwater conditions, injection patterns, gel time and grout mixture, containment of excessive grout migration, and selection of the proper type of grout There is a wide range of grouts for pressure application, including acrylamide, acrylate, urethane foam, and urethane gel. The common applications for pressure grouts are for brick manholes with somewhat tight joints, active I/I, structurally sound manholes, cohesive soils with optimal moisture content, and improving and filling voids to stabilize the surrounding soil.
Corrosion Protection. Manholes subjected to corrosive atmospheres must be protected with a non-cement type coating. The market place offers a variety of plastic, polymer, and epoxy coatings effective in protecting manhole walls from the corrosion of sulfuric acid. Bituminous coatings have not proven to be effective in corrosion control of manholes. The effectiveness of corrosion protection depends on adequate preparation and cleaning of the substrate wall of the manhole and proper application of the coating.
Frame, Cover and Chimney Rehabilitation. Leakage problems common with manhole frames and covers include surface water entering through the holes in the cover, through the space between the cover and the frame, and subsurface water entering under the manhole frame. Tests consistently have shown that these sources account for a significant portion of manhole leakage. Manhole covers can be sealed by replacing them with new watertight covers, by sealing existing covers through the use of rubber cover gaskets and rubber vent and pick holes plugs, or by installing watertight inserts under the existing manhole covers. Selection of watertight inserts (plastic, fiber glass, stainless steel) must consider installation condition's (nontraffic, heavy traffic) effect on the required strength and durability of the insert. The manhole frame-chimney joint area can be sealed internally without excavation when frame alignment and chimney condition permit, or internally or externally when realignment or replacement of the frame or reconstruction of the chimney or cone requires excavation. This sealing can be achieved by installing a flexible manufactured seal or by applying a flexible material to either the surface of the chimney and frame or between the adjusting rings and under the frame.

(Source: Existing Sewer Evaluation & Rehabilitation, Water Environment Federation and American Society of Civil Engineers, 1994)

(Source: Operation and Maintenance of Wastewater Collection systems, Vol. II, California State University, Sacramento,1998.)