PUBLIC WORKS/Process makes manhole rehab safer and easier
Flawed manholes can cause a number of problems for cities and counties. “We estimate that up to 25 percent of our inflow problems are directly related to leaking manholes,” says Dennis Blakley, superintendent of the McCandless Township (Pa.) Sanitary Authority. McCandless Township estimates that, before its manhole re-habilitation program, 750,000 gallons per day of additional rainy day flow could be attributed to leaking manholes.
Besides the costs of treating such unnecessary inflow, city and county employees often spend hours dealing with corrosion and with cleaning and relining leaking manholes. The potential exposure of workers to hydrogen sulfide gas in the manholes can make the rehabilitation process dangerous.
McCandless Township’s manholes had suffered the deterioration of two or more inches of concrete in some areas, primarily the result of corrosive chemicals in the manholes. Calcium and other mineral deposits also were evident in a number of the township’s manholes.
Before the manholes could be rehabilitated, they had to be cleaned, and all loose or contaminated concrete had to be removed. Traditionally, a worker with a hand-held hydroblast wand would have handled the job. However, the problem of exposure to hydrogen sulfide gas was exacerbated by the danger of having a worker inside a confined space with a high-pressure water system.
So, Blakley looked for help. Lawrence, Pa.-based WaterBlasters developed a hydrojetting system that involved a specially made nozzle, which allowed workers to clean entire manholes from ground level. Capable of blasting 3,000 to 7,000 psi of water at a rate of 50 gallons per minute, the rotating nozzle presented two benefits: It cleaned the surface of the manhole from one central point, and it left a roughened surface that allowed for better application of bonding substrate for subsequent linings. By using the system, Mc-Candless Township went from cleaning one or two manholes a day to cleaning eight or more.
Once the manholes were cleaned, substrate had to be repaired and a water infiltration material applied. Using conventional methods would have required workers to enter the manhole and apply the substrate material by hand. But Pittsburgh-based Sauereisen and Farmingdale, N.Y.-based RFI Construction developed a process involving a specialized pump, materials and a “spinning nozzle” that allowed for application of substrate repair from the street.
A quick-curing substrate resurfacer was formulated for use with the system. Additionally, all manholes directly downstream from the pump stations were coated with a corrosion-resistant epoxy, also developed specially for the system to prevent microbiologically induced corrosion.
Using the same equipment to apply all products made the work easier on installation crews. Additionally, because of the speed with which the rehab can be completed, the companies have dubbed the process the “60-Minute Solution.” “It’s a cost-effective method for quickly removing inflow and infiltration from a sanitary sewer system, while, at the same time, providing a safe method of application,” Blakley says.