Failures of the adjusting rings of manholes cost Minnesota cities about $7 million a year in damages in manholes, sewers, and paved surfaces. This study looks at the experience of Minnesota cities and other past research in evaluating the many available methods to repair rings and in recommending construction and repair methods that minimize the risk of damage. Poor construction of manholes and damage from frost heaving cause most failures. When repairs are necessary, survey results suggest three methods with the best cost/value ratios: The Cretex Internal Chimney Seal (Trademark), Flex-Seal Utility Sealant (Trademark), and Infi-Shield (Trademark) seal. One method still under evaluation, rings made of High-Density Polyethylene (HDPE), shows some initial indication of success. While lower in initial cost, other methods are less lasting, resulting in a poor cost/value ratio. Methods also must accommodate the tensile stresses and elongation strains associated with frost heaving through ice lenses. Materials must be elastic enough at sub-freezing temperatures to stretch without breaking when frost heaving opens a joint.
