It can be difficult to find resources that deal with manhole and catch basin issues that arise in Minnesota. In a recent project, researchers interviewed 83 municipalities and interviewed other engineers and product representatives to collect data on manhole and sump installation and maintenance practices. The resulting report provides up-to-date information to municipal engineers on these critical facilities.
Municipalities in Minnesota with underground storm and sanitary sewers also have manholes and sumps (also called storm drains). Typically spaced every 400 feet along streets, manholes allow workers to access sanitary and storm systems for inspection and maintenance. Catchment basins are also located along streets and in parking lots to collect and transport rainwater.
For example, Minneapolis has about 18,000 storm manholes and 32,000 sanitary manholes. In addition, 55,000 catchment basins are part of the city’s stormwater management effort. Although almost invisible despite their large number, these facilities are an essential part of the infrastructure of every municipality.
“The final report is a valuable tool, bringing together in one document all of the best standards and products to prevent and resolve common manhole and catch basin problems, especially those faced by municipalities in Minnesota facing frequent freeze-thaw cycles, âsaid Steve Bot, City Administrator / Director of Public Works, Town of St. Michael.
Some state municipal engineers were concerned about cases of settling and heaving around manholes and sumps. After reviewing data from an initial study, a Local Road Research Board (LRRB) research committee learned that there were no information resources for the construction or maintenance of catchment areas, and that the existing information on manholes was insufficient and obsolete. Many new products targeting the installation needs of these facilities could improve construction techniques and maintenance practices. Municipal engineers needed an accessible and comprehensive resource on the installation and maintenance of manholes and sumps.
What was our goal?
The objective of the project was to provide an information resource on manholes and sumps that covered installation techniques, products and their application, as well as common maintenance issues. This resource would also include information on practices that mitigate settlement and heave around facilities.
What have we done?
To gather information for this resource, the researchers interviewed representatives from 83 municipalities and the City Engineers Association of Minnesota. The survey gathered information on product selection, installation techniques, specifications and maintenance issues.
Members of the research team interviewed some municipal employees to further document the processes and concerns. They contacted industry professionals to learn more about suitable materials and proper application techniques.
Researchers gathered information on construction methods and products, developing an in-depth review of construction and repair products, and practices to prevent settling and heaving. They also collected engineers’ experiences with the products, considerations while working in the field, and advice for new installation and repair of existing units.
What was the result ?
Manholes and sumps are generally constructed from three materials: precast reinforced concrete, cast-in-place concrete, and manhole bricks or blocks. All the municipalities studied used precast concrete, five municipalities used cast-in-place concrete, and six cities used bricks and blocks only at the discretion of the engineer. In general, a 5 inch wall thickness has been specified for prefabricated and cast-in-place manholes and sumps.
The manhole chimney section, which connects the manhole cone to the pavement surface, showed the greatest variation in construction among municipalities. All used manhole adjustment rings (also called extension rings), which cover the distance from the top of the chimney to the surface of the roadway and aid in leveling.
âCatchment and manhole problems can be difficult for a municipal engineer to study. Resources are scattered, with limited detail. This project provides a focused resource with a particular focus on Minnesota experiences and practices, âsaid Derek Tompkins, senior civil engineer, American Engineering Testing, Inc.
Municipalities used both precast concrete rings and engineered polymer rings; 40% of respondents were in favor of polymer rings and 21% not. The number of respondents who preferred concrete rings was almost equal to those who found concrete rings less effective. This variation revealed that some preferences were a matter of background and experience.
The methods of sealing manholes varied from municipality to municipality, but certain products were commonly used, such as rubber or butyl o-rings between the joints; packaging materials, such as Infi-Shield Gator Wrap; sealants such as Cretex internal seals for high groundwater areas; and other barrier wraps. These construction characteristics all stem from the fact that the facilities are built underground. They are subjected not only to hydraulic pressure, but also to the tremendous destructive forces of the freeze-thaw cycles that occur in cold states like Minnesota.
To avoid settlement and uplift problems, the information in the report includes methods to effectively backfill and compact the soil around the unit during construction.
The project report will provide municipalities statewide and beyond with up-to-date information on installation, products and maintenance practices for manholes and sumps.