Manholes are the access points to underground utility lines. Round, although there are some square manholes, the access lines are often made of concrete with metal or fiberglass steps. However, the use of fiberglass composite material is standard for manholes with hydrogen sulfide; sewer gas is present.
Since they are designed to last fifty years, these staples of utility infrastructure are slowly deteriorating around the world. Rehabilitation is an ongoing effort to correct the damage and keep the current access points in place.
Why Rehabilitate Manholes
If a manhole is damaged, it makes it difficult for workers to access the underground utility lines that may need repair safely. Digging up the structure through conventional trenching is costly and time-consuming. It can take weeks or even months to dig the necessary trenches and remove the current line before reinstallation and connection take place.
Due to the number of manholes to renovate in each city, there is no time to remove the existing access points. With rehabilitation, cities can repair potentially hundreds per each year. (Learn more in "Dig We Must: Small City Manhole Rehabilitation.")
Rehabilitation is a necessary part of manhole maintenance. It helps to remove infiltration within the access point and restores structural integrity. Additionally, sewer repair addresses public safety issues. A damaged or deteriorating hole can contribute to sewer overflow, which is a serious public hazard.
How to Conduct Manhole Rehabilitation
To start a manhole rehabilitation, an engineer must inspect the manhole before determining what type of repair is appropriate, defining each defect and determining which restoration method works best. The use of the present manhole condition as a baseline is imperative.
Other questions to ask before rehabilitation work begins including:
- What are the conditions of the substrate?
- Are there any application requirements concerning noise, dust or vibration?
- Are there any material conditions? If so, what are they?
Manhole rehabilitation engineers need to note any application constraints they may encounter during the project. They also must be aware of any uncontrollable environmental issues, such as high levels of hydrogen sulfide, low oxygen, live water flow or constant moisture.
Only after assessing the manhole’s conditions can workers choose the appropriate lining. Each lining has conditions in which they work best. Once determined, the area is prepared by pressure washing the manhole. Filling any large voids takes place before lining installation. For CIP lining, installation of a unique ring attaches to the upper portion of the liner. This ring holds the liner in place during the installation process. Other liners may have special holders as well.
For most liner installations, the weather is not an issue. Slight adjustments are made to allow the compounds to cure appropriately.
Due to the nature of manholes, and the fact that they are often composed of multiple types of materials, various techniques are used to complete the rehabilitation. A full rehab takes approximately three to seven days with minimal traffic disruptions as there is no digging required. (Read more in "Trenchless Sewer Repair and Cleaning 101.")
Types of Pipe Lining Used in Manholes
Rehabilitation experts have several lining choices. Each has a different benefit and work better under certain conditions. During the inspection, the engineer determines which lining is best in the given situation.
Cured-in-Place Liners (CIPP)
Cured-in-place, or CIPP, lining is a fast, cost-effective solution the conventional cylindrical manhole. This lining is a felt tube impregnated with resin. The tube is inflated to be tight against the existing sewer. The ambient temperature of the structure cures the resin tube to create a solid liner. CIPP works well with access tubes of any size and shape and is water resistant.
The epoxy lining is a solid coating suggested for manholes in treatment plants or wastewater infrastructures. This lining is hydrogen sulfide resistant and withstands immersion in water. Also, the epoxy coating is guaranteed not to release volatile organic compounds into the surrounding soil.
If only the top part of the manhole needs rehabilitation, a chimney liner may be the best choice. This method seals the upper portion of the manhole. The chimney endures the bulk of the wear and tear on a utility hole through exposure to traffic, freezing and thawing conditions. In some instances, the chimney damage is due to poor maintenance or faulty installations. Using a chimney liner is a cost-effective solution to repair the most affected portion of a manhole.
A final lining type is the cementitious lining. This coating is applied by using either a centrifugal spinning process or low-pressure spraying. The cementitious lining is a fiber reinforced cementitious material. Use of this filling material adds groundwater resistance and integrity to the existing manhole.
In addition to coatings, there are other options to rehabilitate manholes. In instances of minor damage, workers can chip away the damaged area and seal the sections with cement. If there are cracks, workers restore them with chemical grouting, which is a two-part gel squeezed into the cracks. The grout seals the cracks making them waterproof and root infiltration resistant.