MUNICIPALITIES USE CHEMICAL GROUPT TO Stretch Sewer Rehabilitation Budgets.
Infiltration and inflow (I/I) may be responsible for more avoidable expenses in a sanitary sewer system than any other item. I/I often surcharges systems and causes raw sewage to back up into homes, or it overflows the system and creates unhealthy situations, sometimes killing thousands of fish and other life forms and destroying their habitat for years. Infiltration almost always erodes soil into the pipeline; over time, the erosion can wash away sidefill support, creating large voids that can lead to serious subsidence or even street collapse.
I/I also creates extra costs at treatment plants. While transportation and treatment costs vary widely across the country, two dollars per 1,000 gallons seems to be a conservative, but reasonable number to use for an average.
Most infiltration enters sanitary sewer systems at five critical points: manholes, pipe joints, pipe cracks and missing sections, service line connections, and the first few feet of service lines. Many different technologies offer long-term solutions to these problems. The specific method chosen will depend on the condition of the pipe and the budget for the project.
Chemical grout is specifically designed to stop infiltration at pipe joints and laterals, and its use to seal laterals after lining is a cost-effective alternative. For example, grouting laterals after lining costs between $300 and $700 per lateral.
Many municipal engineers and consultants have found that by aggressively using chemical grout to stop leaks in structurally sound pipelines and other structures, they have more money left to reduce infiltration throughout their system. Three examples of this strategy follow.
Jim McWade, wastewater conveyance-project manager, CH2M-Hill (Corvallis, Oregon), reports using chemical grout to stretch the Dallas, Oregon, rehab budget and solve a serious infiltration problem.
Dallas, a Willamette Valley community of about 10,000 people, has an annual 120-day rainy season that produces between 50 and 60 in. of rainfall. In that season, the sanitary sewer system would frequently surcharge and even overflow through manholes during heavy rain events.
With the aid of a source-detection program, CH2M-Hillidentified a basin with severe infiltration. The basin included about 19,000 ft of old concrete and clay pipe as well as 80 manholes, mostly precast. The pipes and manholes were structurally sound, but joints were leaking badly in some areas, as were the connections where pipes entered some manholes. The most serious infiltration was limited to about 10,000 ft of pipe and 52 manholes. CH2M-Hill investigated several different rehab technologies and selected chemical grout to stop the leaks.
CH2M-Hill contracted with Gelco Services, Inc. (Salem, Oregon), to clean, test, and seal the pipejoints and manholes. The contract cost was $36,000. After thoroughly cleaning the lines, Gelco pulled a remote-controlled TV camera and chemical grout injection packer through the pipeline. The packer was stopped at each pipe joint and a pressure test was made. If the joint could hold a specified pressure for a set length of time, the packer and camera were moved to the next joint. However, if the joint could not pass the test, chemical grout was immediately pumped into the packer void and through the leak into the surrounding soil.
David Magill, president, Avanti International, a chemical grout manufacturer headquartered in Webster, Texas, explained that when most chemical grouts are applied, they have about the same viscosity as water, but usually thicken, or gel, quickly. The gel-time, which can be adjusted to any point on a scale from almost instantaneous to several hours, is normally about 30 seconds.
Magill explained that grout permeates the soil outside of a leaking joint or crack before it gels, thus forming a waterproof, monolithic mass that cannot be forced back through the opening. If there is a soil void outside the opening, the grout gels into a solid, rubber-like, waterproof mass that groundwater cannot penetrate. If groundwater pressure increases, it simply presses the mass tighter against the structure.
To seal the manholes, Gelco drilled small holes through the leaking joints and applied the grout with hand-held injection guns. Once the chemicals were outside the structures, they formed tight-fitting, waterproof masses.
McWade estimated that the city could eliminate 13 MG of infiltration annually from the manholes and another 145 MG per year from the pipelines. Since it costs Dallas about $1.50 per thousand gallons to transport and treat wastewater, savings at the treatment plant alone could top $240,000 per year from the $85,000 investment, which included source detection and engineering.
"Since we completed the grouting, the basin has not had a single overflow, and surcharges have been minimal," McWade said. "Regulatory people like that. Fines for sewer overflows can easily exceed the total cost of our grout contract, so all the other benefits we received are bonuses. One of those bonuses is that the money we saved in this basin can be used for more costly repairs."
NORTH OLMSTED, OHIO
Larry Griffith, assistant city engineer, reports that North Olmsted has been using chemical grout to reduce infiltration into the city's sanitary-sewer system for over l0 years. During that time, the city has grouted 191,688 ft of main line and 90,398 ft of lateral service lines. It has also lined 6,006 ft of main, made 158 point repairs, and completely replaced 27,663 ft of old pipe.
Joseph Tartabini, Jr., vice president, United Survey, Inc., Cleveland, Ohio, the company which did most of the chemical grouting in North Olmsted, pointed out that the average daily flow at North Olmsted's wastewater treatment plant used to be 5.6 mgd. Seven years later the flow was only 4.2 mgd, even though the city had grown significantly. In fact, North Olmsted is one of the few cities in the United States to be completely released from its Finding and Orders by the EPA.
"We started our grout program after the EPA filed a remedial decree against the city," Griffith explained. "We had to do something that was both very effective and quick because we were not allowed to issue any building permits until we came into compliance. The only time we did anything other than chemical grout was when the pipe was not structurally sound. There is no way we could have improved our system as much as we have without chemical grout," he explained.
ROLLING MEADOWS ILLINOIS
Robert Nixon, underground utilities superintendent, City of Rolling Meadows Public Works Department, had never used chemical grout when a consulting engineering firm, RJN Group (Wheaton, Illinois), suggested it as a cost-effective way to reduce infiltration into its sanitary sewer system.
"It's just too costly to line everything that leaks," Nixon said. "I had heard and read a lot about chemical grout over the years, but you never know for sure how something works until you try it yourself, so I was eager to get started. We contracted with National Power Rodding (Chicago, Illinois) to kill and remove the roots in 28,000 ft of pipeline, then clean, test, and seal the pipe joints."
The contractor first applied an herbicide to the roots, then waited eight weeks before cutting the dead roots out. Next, the pipes were cleaned and the joints were tested and sealed, when necessary. Some herbicide was mixed with the chemical grout to help keep new roots from attacking the sealed joints.
"It's been three years now since we sealed our first pipes, and they are still clean and dry," Nixon said. "Chemical grout has been very effective for us, and the least expensive way we've found to stop infiltration in a structurally sound pipeline. I don't know how long it will last, but our program has already paid for itself and there is no sign that a single joint has failed yet. We'll go back in a few years and look at it again. But, if it doesn't last another day longer, I feel it has been a very good investment, and I will definitely continue to use it. Without chemical grout, our rehabilitation program would have to be scaled back dramatically," Nixon said.
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|Date:||Oct 1, 2000|
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