Minnesota municipalities and counties use varying practices to construct and repair catch basins and manholes. They also choose from a variety of products to build a catch basin or manhole, seal structure joints, and help reduce infiltration and intrusion.
There is little documentation, however, on the local experience with these structures in Minnesota, as most resources are promotional literature or articles from industry publications. A new guidebook provides foundational information for municipal engineers about these essential installations.
The Minnesota Department of Transportation (MnDOT) and the Minnesota Local Road Research Board (LRRB) initiated this project to document best practices for managing the effects of settlement and heave at catch basins and manholes. After feedback from municipalities indicated that settlement and heave were not dominant issues, the study was broadened to include more general municipal experiences and best practices.
Key findings
According to a survey of Minnesota municipalities, manholes and catch basins are typically constructed from three materials: precast reinforced concrete, cast-in-place concrete, and manhole bricks or blocks. All surveyed municipalities used precast concrete, five municipalities used cast-in-place concrete, and six cities used bricks and blocks only at the engineer’s discretion. Generally, wall thickness of 5 inches was specified for precast and cast-in-place manholes and catch basins.
The chimney section of the manhole, which connects the cone of the manhole with the pavement surface, showed the most construction variation among municipalities. All used manhole adjustment rings (also called extension rings), which span the distance between the top of the chimney and the pavement surface and assist with leveling.
Municipalities used both precast concrete rings and engineered polymer rings; 40 percent of respondents favored polymer rings and 21 percent did not. The number of respondents who favored concrete rings was nearly equal to those who found concrete rings less effective. This variation revealed some preferences were a matter of context and experience.
Methods to make manholes watertight varied among the municipalities, but some products were commonly used, such as rubber or butyl O-ring gaskets between joints; wrap materials, such as InfiShield® Gator Wrap; sealing products such as Cretex internal joint seals for high groundwater areas; and other barrier wraps.
Valuable tool with best standards and products
“The final report is a valuable tool, collecting in one document all the best standards and products for preventing and addressing common manhole and catch basin issues, especially those experienced by Minnesota municipalities faced with frequent freeze-thaw cycles,” says Steve Bot, city administrator/public works director with the City of St. Michael and the project’s technical liaison. The project was sponsored by the LRRB.