A regional airport in the Midwest has miles of pipe and storm sewers that stretch under the runways. These pipes carry surface runoff from rain events into the storm sewers. The airport sits near the Missouri River, in the river bottom, so the water table is extremely high. Cracks and leaking joints caused erosion around the pipes during major rain events.


Eventually the erosion would compromise the runways, potentially leading to sinkholes. The exfiltration had to be stopped. An airplane had already fell into a void and necessitated costly repairs. The need for soil repair was urgent.


When contractor Bob Mixan, owner of Mixan Mudjacking, proposed chemical grouting, engineers were unsure why mudjacking wouldn’t be the preferred method. That is, after all, a service he offers. In fact, it took weeks to convince them that the water pressure from the leaks made this method a bad fit.


“A few years ago, I did a similar project using my mudjacking equipment, but the hydrostatic pressure prevented our ability to pump the material. Pressure against pressure meant you couldn’t pump anything. Also, given the length of the stormwater pipes, the weight of the pumping pipes filled with concrete slurry make it impractical. The Prime Flex 920 is water-activated, so it was a much better permanent fix than what they originally hired us to do.”


He did a demonstration for the engineers, showing how the 920 from Prime Resins worked. Once repairs were performed on one of the manholes, the choice became clear. The ease and speed with which the gushing leaks were sealed off earned the product a quick approval for the entire scope of the job. The chosen method was to curtain grout the pipes with Prime Flex 920 from Prime Resins, a highly expansive structural polyurethane foam.


The result?


“Now this is all they spec,” says the contractor.


Curtain wall grouting is a method that involves injecting a polyurethane grout completely around the structure to create a waterproof barrier. From inside of the manholes, injecting through the wall completely seals off the structure from infiltration. Think of it as being similar to a koozie surrounding a drink can. The Prime Flex 920 polyurethane foam creates an impermeable mass.


Prime Flex 920 was chosen for a few reasons. This hydrophobic, super low viscosity polyurethane reacts with water and expands to form a closed cell, watertight, rigid foam. This would seal the leaks in the concrete pipes. Due to its low viscosity, 920 is also used for permeation grouting of loose soils to consolidate soil particles and increase the load-bearing capacity. When you’re working around airport runways, load bearing is a crucial consideration.


This was a large project, with 28 different areas on and around runways needing work. Manholes and culverts surrounding the runways had leak issues, which in turn washed away soils and created voids.


Outcome


On Phase 1, the crew pumped 700 to 800 gallons of Prime Flex 920 to encapsulate several leaks. “We sealed everything 100 percent and they were happy with it,” Mixan said. All areas were inspected by camera at completion, and all passed on the first try.


Challenges


The project was fairly standard, so the crew didn’t have to tackle any major obstacles. Some voids were larger than expected, which had a budget impact but were not a problem from an execution standpoint. One of the leaks was a particularly large gushing leak, and a faster reaction time was needed on the Prime Flex 920. Kick Fast catalyst from Prime Resins was used to speed up this process. This catalyst allowed Mixan to speed up the initial reaction time to as quick as five seconds, depending on the mix ratio used. This was an easy adjustment.


Alternatives


The alternative would have been to tear out and replace all five runways. This would have been cost prohibitive due not only to the large construction costs (in the millions) but also due to the opportunity lost via runway shutdowns. This work would have taken approximately a year to complete.


chemical grouting


Summary from the contractor


“I wish more people understood how well this product works. This is right next to the Missouri River. Mud would not have been able to flow far enough to seal it. Water would have kept the mud from setting, and it would have been washed away, so mudjacking wasn’t an option. The 920 won’t get washed away. The beneficial cost of using chemical grout compared to replacement costs is probably what many engineers and property owners don’t realize,” Mixan said.


The owners of the airport were so pleased with the work that they are now using Prime Flex 920 on their citywide projects.



Kevin Anthony is marketing manager of USL America. 



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