by Steve Schoonover

A precast sump pit of 5 ft. dia. by 6 ft. height, located at the base of an approximate 7 story Oxygen Pit (OP) water holding tower (the cleaned and oxygenated water would be discharged into the adjacent river), began leaking groundwater into the pit. This pit was located approximately 30 ft. below the groundwater table. As time went on the leak went from a nuisance to an approximate 125 GPM of gushing water. The leak was so severe it was estimated it took only a few hours for the water to rise to the same height as the exterior water table. The water, entering into the bottom of the sump pit, was also causing the sandy soils from under and around the OP to enter into and fill, the sump pit. This created grave concern of the possibility of the OP tilting, and possibly even falling into the adjacent river. The soils were continually being replaced by the plant. Further, at least two large industrial-sized sump pumps had been ruined due to the sandy soil running through and stripping the pumps. 

Initial discussions with the customer indicated they wanted to have Portland based grout injected in an effort to plug the hole. Explaining this product/method would likely not be successful due to the amount of flowing water, which was only approximately 50 gpm at that time — were asked about injecting a low slump mix. We expressed the product would most likely would wash out prior to it ever setting up enough to stop the water. At this time it was estimated the hole at the base of the concrete pit was only a couple of inches in diameter. On our second visit to the site it was estimated the the flow had increased to about 75 gpm (which, of course, meant the hole in the base of the concrete sump pit was also expanding).

The owners had decided to try stopping the water infiltration by inserting a steel “sleeve” into the pit. This sleeve was smaller than the original sump pit by approximately 1.5 ft. dia. and 1 ft. depth. The new plan was to have us inject a bagged, specialty cement grout into the annulus space between the pit and steel sleeve, through the 12 valves they had installed into the steel pit walls (three each (low, middle, and high) at 12, 3, 6, and 9 o’clock. Again, we explained this would likely not have a fully successful outcome for the project since they wanted all of the water infiltration stopped. Therefore, a cement-based grout would likely allow for water infiltration over a short period of time since the injected product would not bond with both the existing concrete and new steel. Instead, we suggested utilizing a urethane-based foam grout. 

Steve Schoonover — after consulting with Ed Paradis (BASF) and Tim Fitzgerald (Coastal Construction Products) — decided to use BASF’s MasterRoc® MP 355 1K for the project. Our reasoning was this product had a proven track record of stopping gushing water. In addition, the product is a one-component grout (plus accelerator), making it easier to maneuver around within the limited space at the base of the seven-story “silo” containing various obstacles (stairwell, large diameter pipe, etc.), not to mention having to crane all the materials and equipment down into the pit through a narrow skylight. 

On the day of the injection, the estimated water flow was around 125 GPM. A single valve had been placed in the center of the bottom of the steel insert. It was through this single 2 in. valve that we would insert a 1 in. tremie tube to pump the MP 355 1K, utilizing the valves located on the side of the pit as exit/relief valves. When we inserted the tube, it went approximately 4 ft. below the base of the pit without any resistance, and the water pressure was high enough to spew out our injection port! Now we knew there was at least a 4-foot deep void, but how wide was it? The base of the OP was 40 ft. in diameter. We didn’t think it could be very large as the structure was still standing. 

Initially, we had estimated it would take approximately 50 gal. of the BASF product to stop the leak. In the end it took about 4 hours of continuous pumping, and 90 gallons of product! Since no one knew the size of the hole at the bottom of the sump pit to begin with, but we knew our void was at least 4 feet in depth, this meant the diameter of the void was quite a bit larger than the client had initially thought, as was the opening at the base of the pit, allowing the water to infiltrate. 

Upon completion, not a single drop of water was exiting any of the valves — not one drop!  For more information or assistance with your project, you can contact Steve Schoonover, President of Superior Structural Solutions, Inc., at 205.876.6999 or or click here.