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.
This report culminates one of the first projects undertaken for study in the Local Road Research Program. The primary purpose of this project when it was initiated was to develop correlation between the plate bearing test and the Benkelman beam test. Such a correlation, or other acceptable procedure for using the Benkelman beam, would provide a practical and relatively economical means for our local highway engineers to obtain a measure of the strength (load carrying capacity) of flexible pavements.
This study was originated in 1960 at the request of the Minnesota Local Road Research Board. Its basic objective is to determine a means of backfilling slit-type trenches so that little or no settlement results. In the first four years personnel from the Research Section, Office of Materials, Minnesota Highway Department, collected data by observing trench backfilling during regular construction operations and then measuring the subsequent settlements. In 1964, 1965, and 1966 dummy trenches were excavated and backfilled under more controlled conditions. This work was done by the City of St. Paul and Minnesota Highway Department maintenance forces. All testing was done by Research Section personnel.
In 1963, a progress report titled BACKFILLING TRENCH EXCAVATIONS was published by the Materials and Research Section, Minnesota Department of Highways. Included were general recommendations for street preparation, backfilling, and patching. Also, the effectiveness of various types of compactors was discussed and specific compaction procedures for given soils and traffic conditions were recommended. The results of the controlled backfill studies reported herein have confirmed certain of those recommendations for compaction procedures but have contradicted others. The investigation to date has not satisfied its objective and is being continued.
