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.
One repair option that the city experimented with was oakum packing and cement patches. That combination works on small leaks, but the cracks and failures in the manholes were just too large and water pressures too high.
Chemical grouting seemed a perfect solution for the situation. A trenchless method that has proven itself over 50 years, grouting works by filling the soil voids behind leaks and cracks with an acrylamide solution that quickly sets up into a durable rubbery gel, creating a gasket-like seal that plugs leaks and stabilizes the surrounding bedding, thus ending the process that caused the problem in the first place. Fixes can last decades and are far less expensive than other trenchless methods. If long sections of pipe don’t need replacing or relining, grouting is often the best repair technique available.
But there is one catch: grouting requires skillful application to work well – it is not as straightforward a process as replacement or relining. The village contacted Visu-Sewer, a Wisconsin-based company that they had worked with previously. “They were the only ones we knew who did this work,” Williams explains, “They’d done good work in the village on other projects. We knew they were experienced and that they were a good choice for this project.”
A tricky job done right
Injection grouting can seem like a fairly simple process. Holes are drilled near leaks and a solution of acrylamide and catalyst is injected into the bedding outside the leak. The solution is mixed right at the applicator tip as the chemicals are injected. If all goes well, the solution flows to the infiltration areas and sets up there, plugging the leak and filling the external voids caused by soil washing into the pipe.
“But every manhole is different,” says Visu-Sewer Foreman Keith Mauzer. “In this case, some inverts were cracked, and there were also cracks around the ‘bench’ where the precast manholes were set into the trench. You have to develop a sense of where to drill ‘away’ from leaks, and gauge the curing time correctly so that the solution flows to the leak and sets up as it gets there. It’s a bit of an art.”
The two-man Visu-Sewer crew worked on one manhole per day and dewatered with an upstream inflatable plug as they went. But the plug didn’t make things dry. “There was a lot of groundwater in the surrounding bedding and it got higher and higher as the job progressed” Mauzer explains. “Even on the first manhole, water was coming into the pipe through leaks at about five to 10 gallons per minute at each manhole — and it got worse as we went along.”
The specific product used was AV-100 from Avanti International, which has some unique features. It’s shipped as liquid or granules and when mixed, its viscosity is quite low, the same as water. That means it will flow anywhere water flows and find its way to the leaks being repaired. Also, its cure time can be manipulated within a wide window, anywhere from five seconds to 10 hours.
In this case, Mauzer mixed the AV-100 thicker than normal, to account for the especially large voids, and timed the solution to set in 12 seconds. Given the high rate of flow through the leaks, this turned out to be the right approach.
Solution grout sets up well in wet conditions; in fact, some flow is necessary to convey the solution to the leak. After oakum packing, and before grout injection, a tracking dye is pumped into the bedding, to verify that injection holes have been drilled in the right area — if dye isn’t seen coming through the leaks, the drilled hole has to be relocated.
The Visu-Sewer crew worked upstream and actually made two passes; the plugging of initial leaks and some wet weather raised the water level in the trench outside the manholes, making it possible to identify and plug remaining leaks. Mauzer says that the results on this project were particularly gratifying. “You could actually see the leaks slow and stop,” he says. “The village told us that after just three days we’d already cut daily flows by 200,000 gallons — it was dramatic.”
On average, about 140 gallons of grout were pumped per manhole, which is high compared to similar projects. Quality assurance was straightforward. Bradley inspectors did a visual inspection and the evidence of stopped leaks was quite clear. To complete the job and for “insurance,” loose mortar and visible grout were chipped away from cracks, and the area was patched with a high-strength hydraulic cement mixed on site and made of Portland cement, silica sand and Anti-Hydro, a commercial waterproofing agent.
“The Visu-Sewer crew was very informative,” says Williams. “I was on the site fairly often, and they would always explain what was going on. I appreciated learning more about grouting; it’s an interesting process.”
Good to go
This interceptor feeds directly into a lift station, so the village could precisely monitor the daily flows. Before the grouting project, daily flows during wet weather were more than a million gallons. After grouting? “During a recent a storm,” says Williams, “we recorded 515,939 gallons — just about half.” Williams says this cut in flow began immediately after grouting and has held up well. He considers the project a complete success.
Total cost of grouting was about $85,000, much of which was covered by a state grant. “I’m grateful that there was money out there for this,” Williams says, “People tend to think of police and fire programs when they think about municipal grants, but there is sometimes money available for infrastructure — villages should be sure to look into it.”
It was certainly a good use of the funds. The project will pay for itself quickly just in lowered electrical and treatment costs. Currently, Bradley’s sewage is treated by a separate agency at the Kankakee Wastewater Treatment Plant, operated by the Kankakee River Metropolitan Agency (KRMA). “KRMA called us up after the project, and asked what was going on,” says Williams, “They’d noticed the reduction — I have to admit, it was pretty sweet getting that call.”
Like any municipality, Bradley is struggling to keep up with needed infrastructure maintenance in a time of limited budgets. Having to cope, suddenly, with a badly failing piece of nearly new sewer could have been a disheartening disaster. But by calmly considering their options, identifying funding and choosing a practical, low-cost repair method that was appropriate for the situation, the village avoided a budget-wrecking expense. In fact, when all costs are considered, they may even have come out ahead.
FOR MORE INFO:
Visu-Sewer, (800) 876-8478, www.visu-sewer.com
Avanti International, (281) 486 5600 or (800) 877 2570, www.avantigrout.com