Chemical Grouting Combats Water Intrusion

TASA ID: 11934

This article was originally published in Transmission & Distribution World, www.tdworld.com, September 2016.

Typically, utilities have leaks that occur inside conduits, along cracks and at pipe penetrations. The conduit problem, for example, occurs where the conduit penetrates inside a structure such as a building foundation, a manhole or a vault. Water infiltration occurs inside the structures at the annular space between the cables and the conduit. This leakage can fill an underground vault with water, depending on the water table elevation. Structures located near the ocean also are subject to accelerated corrosion due to salt water infiltration, which can damage other equipment inside the structure.

Chemical grouts; however, have been used successfully to seal off the annular spaces between the conduits and the cables.

Sealing Conduit

As part of this process, one option is to use clean sections of oakum that are inserted into a pail of expanding urethane based chemical grout that needs water to react. Then the oakum is placed inside a leaking conduit about six inches deep, beyond the conduit opening.

The expanding urethane and the oakum create the first barrier. The barrier will still have pinholes and small amounts of water may penetrate through the barrier, but this typically seals off 80% of the water. Next, the process is repeated and a second section of oakum and expanding urethane is placed about three inches deep, beyond the conduit opening.

After the barriers have cured for about 15 minutes, a needle pipe is inserted between the barriers. The needle pipe has a sharp tip to cut through the barrier material. Next, water and urethane chemical grout are pumped between the barriers.

The void between the barriers is pumped because specific urethane chemical grouts used for this purpose must be in a dense plastic-like condition in order to seal off water for long-term results. Simply allowing urethanes used for conduit sealing to expand freely into foam will not result in a cured matrix that can stop water seepage/infiltration.

Urethane based chemical grouts must be flexible because thermal changes can cause conduits and cables to move. Cables must be separated to allow the chemical grout injected to seal their entire circumference around the cable, as water can leak later between the cables. This is especially important with wrapped steel for lightning protection or small conduits.

People often ask if the cables can be pulled out of the conduit later or if new cables can be inserted into the conduit. The answer is yes, the cured chemical grout and oakum can be removed with a knife if necessary; however, the process would need to be repeated to reseal the conduit.

Remediating Leaking Cracks

For cracks in concrete, drill holes 1/8 inch in diameter are drilled into the shear plane of a crack, with spacing between the drill holes every one-foot to three feet apart. Water is injected into the drill holes to clean out any dust. Then packers are placed into the drill holes and a bolt is turned to force a rubber sleeve to expand against the drill hole walls. Chemical grout is injected into the packers to react with the water and cure to a rubber like matrix while penetrating into the crack.

Pumping pressures can range up to 3000 psi with chemical grouts that are in the viscosity range of 300 centipoises, similar to maple syrup. Basically, the cured chemical grout matrix is flexible and unlike conventional brittle repair options, such as hydraulic cement or epoxy, the chemical grout matrix will not crack. This type of repair is used most often for cracks in manholes, vaults, foundations, tunnels, walls and roof slabs.

Results are guaranteed as long as the correct application technique was used and the appropriate chemical grout was selected that will allow for the expected movement/elongation limits. Expecting a chemical grout to withstand the movement at a bridge expansion joint would not result in a successful long-term repair, for example. After the injection is completed, the excess chemical grout must be sanded off and the drill holes are patched with non-shrink hydraulic cement.

Preventing Leaking Pipe Penetrations

Finally, a third common problem is pipe penetrations, when the space between the pipe and the wall is leaking. Chemical grouts are used in these situations by drilling 3/8 inch to 5/8 inch diameter holes through the wall and intersecting the space between the pipe and the wall.

The drill bits used for drilling holes are rounded masonry bits that will bounce off any metal intersected to avoid damaging a metal pipe. Drilling depths are measured to ensure the drill bit does not penetrate concrete or PVC pipe.

The exposed annular space is often wide enough to allow visual observation of the drill bit, as drilling depths are usually about six inches deep. After the drilling is completed in two or three holes, depending on the diameter of the pipe, the annular space is sealed with an expanding urethane and oakum.

With wide annular spaces, two sections of oakum and urethane can be used to form barriers six inches deep and three inches deep. Chemical grout and water is pumped after 15 minutes to seal the annular space between the barriers, similar to sealing leaks inside conduits.

Smaller annular spaces that do not allow placement of oakum and expanding urethane are sealed by “stage grouting.” This process consists of injecting a chemical grout with water to activate it and waiting until the crack is filled and the grout begins to cure. Next, a second injection through the same drill hole is used to pump chemical grout and water that is trapped inside the crack/annular space, thus allowing the second stage of grout to obtain the proper density for long-term results.

This technology can offer considerable savings over excavation with waterproofing membranes, for example. Also, guaranteed results have been completed for long-term results successfully so the technology has a time tested record.

For best results, it is imperative that electric utilities select the proper material, use an experienced applicator and partner with firms that specialize in chemical grouting. That way, they can prevent water leakage into their vaults, pipes, manholes and other structures; therefore limiting water infiltration, corrosion and extending their life.

The expert (TASA ID#: 11934) is a consulting engineer with more than 34 years of experience, specializing in chemical grouting. He is a professional engineer.

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This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

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