Description: Sewer systems act as giant french drains. Groundwater follows the cut and grade of the sewer system relentlessly looking for yet an easier path to its own level. Water and gravity work together to make sewer systems vulnerable to infiltration and exfiltration. This understanding combined with other very basic concepts and the limited success of coating systems alone make it clear that infiltration is best approached from the positive or outside of the structure.

After the obvious leaks in lines, laterals, have been stopped we look at where the hydraulic pressure has been transferred to locate the source of additional infiltration. The weakest point will leak first. When hydraulic pressures are diverted new weaknesses are divulged. Sometimes it is not obvious where the leaks or weak points are. In order to evaluate the leak potential of a given manhole it is necessary to simulate high water table. It is not feasible or perhaps possible to evaluate each manhole during times of high water table. This will be obvious to anyone that has grouted pipe joints, lined pipe lines, and coated manholes only to have unacceptable infiltration during high water events.

We know that water will take the path of least resistance. This law when applied to manholes takes us first to the lowest part of the manhole, the invert, bench, and penetrations. As the water table rises leakage occurs at other joints, mortar joints, pipe sections, structural cracks, and around lid rings.

The procedure for locating these additional points of infiltration is very simple. Small holes are drilled into the structure, above penetrations, at joints, and in the bench will tell the story when pumped with water at low pressures (20 PSI ). This procedure will reliably divulge where the leaks are. All that happens is that water fills the voids behind the manhole structure following its "natural" route around the structure and most frequently into the structure. Dyes or chemical grout injected with the water will make the revelation more dramatic. Many times a "Good looking" structure will have infiltration of from 3 to 50 gallons per minute during high water table events.

Many times epoxy and cement based products, are expected to stop these flows. Limited long term success is typical of systems that do not address active leaks from the positive side prior to application of the structural repairs. It is important to understand that structural rehabilitation is necessary in many situations. While structural systems do not answer infiltration problems positive side waterproofing DOES NOT replace structural repair.

We find that polyurethane chemical grout can be effectively used with water to locate and seal leaks in manhole structures.

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