City Innovates in Manhole Rehabilitation
by George Downs
Lexington, Missouri is a historical river town (population, 5,000) with many 19th century homes and historical sites. The City of Lexington recently completed a successful manhole rehabilitation program, reducing I/I while minimizing cost.
A total of 75,000 lf of sewer of vitrified clay pipe and brick manholes is located in the historical downtown district. The city has retained the services of Alliance Water Resources Inc. to operate and maintain the sanitary sewer system.
In response to sewer backups and overflows during wet weather, the city retained George Butler Associates Inc. of Lenexa, Kansas to complete a sewer system evaluation study (SSES) in the fall of 1997. The study area included the historical downtown and surrounding areas.
A comprehensive field inspection of the manholes was conducted. Condition ratings were assigned to each component of the manhole. The data results were input into the GBA Sewer Master software program and run through the Manhole Rehabilitation module. This program prioritizes manholes needing repair based on user-defined criteria.
Prior to the SSES project, no rehabilitation had been completed on the manholes. Out of 160 manholes inspected during the initial study, 150 needed some type of rehabilitation. A rehabilitation schedule was produced, identifying 14 distinct types of manhole rehabilitation methods.
After the contractor was selected, the City of Lexington enlarged the study area and chose to inspect and rehabilitate an additional 100 manholes. A design/build approach was used to select the actual method of rehabilitation for the additional manholes. These manholes were inspected by the contractor, Spray Com Inc. of Sedalia, Missouri, a certified Permaform/Permacast applicator, and George Butler Associates to determine the most cost-effective rehab method.
Manholes scheduled for total replacement and manholes located at the discharge of force mains were also inspected to review recommended rehabilitation methods. The rehabilitation schedule was modified based on the joint inspections.
A wide variety of rehabilitation methods was used to ensure that the most cost-effective solution was achieved.
- Replace/Adjust Frame & Cover; Replace Grade Adjustments
The Permaform Encapsulation & Protection Plate were used. Repairs were made by excavating to a depth of 6 inches below sound brick and mortar. At the top of the manhole, setting frame and cover to grade and placing prefabricated forms inside the manhole top, and pouring a monolithic structure from the existing brick up and around the frame.
- Rehab Wall, Pipe Seals, Bench/Trough
Remove unsound mortar and patch with a non-shrink, high-strength repair mortar. All manholes were pre-cast with isolated defects at the joints. Many manholes had a flat bench and invert that would not direct the flow out of the manhole. Repairs were made by removing all unsound material and installing a new bench and invert using the non-shrink, high strength repair mortar.
- Install Cementitious Liner
Generally included manholes located in collector streets with light traffic. The Permacast method of centrifugally casting a cementitious liner at a minimum thickness of 1 inch provided sealing and structural reinforcement to deteriorated brick manholes. The cone sections were patched and coated by hand with repair mortar prior to spraying.
- Install Epoxy Liner
Installation of a cementitious liner prior to the epoxy liner. The Permacast method of centrifugally casting COR+GARD, a two-component, 100 percent solids epoxy, was used on manholes that were subject to severe attack of microbiologically induced corrosion.
- Replace Total Manhole
Various methods, including replacement with pre-cast concrete manhole sections, cast-in-place, and Permaform and T-Lock existing manholes, were used. Shallow manholes presented a problem because pre-cast concrete base and cone sections are generally too large. A cast-in-place, monolithic, poured Permaform process was used for rehab.
The city is still implementing recommendations as detailed in the study. Not all identified sources of I/I have been removed to date. Kenny Maib, P.E., city engineer, and John Harris, local manager of Alliance Water Resources, indicated that peak flows seen at the treatment plant have been reduced slightly, but that infiltration has been reduced significantly. Flows at the treatment plant return to normal much sooner after a wet weather event than prior to the manhole rehabilitation program.
George Downs, CET, is a lead project designer for George Butler Associates