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What Is Necessary For Manhole Rehabilitation Success? Correct Application, Use Of Manhole Rehab Technologies
Editor’s Note: This is the second of three articles focusing on the current state of manholes, manhole assessment and manhole rehabilitation. Part three will be published in the November edition of Underground Construction.
Manholes consist of multiple components and are made of different materials, each serving a different purpose but contributing to the complete manhole structure. Each component typically has its own set of defects and deterioration mechanisms. Each component therefore has specific repair or renewal requirements due to the various depths at which manhole components are installed and the materials used for construction.
The most common materials used to construct manholes fall into two major categories; bricks and mortar or precast concrete. Newer materials are being innovatively used in manhole construction today, such as high density polyethylene, polymer concrete and fiberglass, but these are not the traditional materials that will be renovated in older collection systems. Other types of structural materials have been used in different regions, such as segmental block, but these generally are more limited in their use or confined to specific collection systems where that material or approach was incorporated in the system. Manholes are also access points for the wastewater collection system, so each is typically finished with a cast iron frame and cover.
There may be dimensional differences between manholes constructed of brick laid in the field versus factory precast concrete manholes. Brick manholes tend to be more conical in shape from the manhole rim to invert and more slender than precast concrete manholes. The brick manhole wall is typically thicker at the base than a comparable precast manhole in order to support the traffic and loads necessary for deeper manholes. Despite the differences between various manhole materials, the terminology for describing each manhole component is consistent.
Shallow manholes located at the surface or within four feet of the ground surface can be refurbished or replaced at a relatively low expense since they are readily accessible. Manholes deeper than four feet below grade are more often repaired or renewed as opposed to being replaced.
There are many technologies available for rehabilitating the different components of existing manhole structures without the need for excavation and replacement. These can include cementitious, polymer, cured-in-place, panel, grouting, mechanical and more, for both internal and external coating, lining and sealing techniques.