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Rehabilitating A New Sewer: Village of Bradley Eliminates I&I With Chemical Grouting
Major rehabilitation of old sanitary sewer systems is more or less expected, if unpleasant. Public works departments know that time takes a toll on all infrastructure and when sewers get to be more than 50 years old they are likely to show their age and need significant care.
But when new sewers fail, that’s not only unusual, but disappointing as well. Nevertheless, it happens. And it recently occurred in Bradley, IL, when the city began to record extraordinary flow surges during storm events.
“When I took over this job, the mayor knew that something was wrong and asked me to look into it,” says Scott Williams, the village’s superintendent of building standards and utilities. “We started pulling lids during storms and checking flows.”
It didn’t take long to determine that a main sewer interceptor, about a quarter mile of 36-inch PVC pipe with 16 manholes, had major infiltration issues. But though it was obvious, it was still a little hard to believe. “We have three main interceptors and this was the newest,” Williams explains, “In fact, it’s just seven years old. But the flowmeter evidence was undeniable.”
The new interceptor had been laid in bedrock, which required blasting a trench. The first blasting pass left too small a trench so a second pass was completed which left a significantly oversized pass. However, everything looked good at the time of completion: the interceptor passed air tests and visual inspections showed nothing unusual. But apparently, the excess amount of bedding that the oversized trench required settled badly which caused major cracking around the precast manholes, especially at the inverts and at the pipe-to-manhole connections. Since the bedrock trench basically trapped water like a sleeve around the pipe, leaks and infiltration were severe.
How severe? Well, during storm events, daily flows would almost double to about a million gallons, and they would remain high until groundwater finally subsided. The added costs of treatment and lift station electricity were a substantial burden. Something had to be done -- and soon.
The village quickly settled on a solution. Grout, injected into the bedding around the manholes, was determined as the most viable rehabilitation method. According to Williams, trenching wasn’t seriously considered. “It was too expensive, for one thing,” he says. “And besides, the pipe itself was fine -- the leaks, which we could see, were confined to areas in and near the manholes.” Additionally, dewatering the bedrock trench could have taken months.