Dam Outfall Pipe Abandonment
Dam outfall pipe abandonment, Front Royal, Virginia
After completing an outfall pipe abandonment with cellular concrete, an engineering firm wanted us to check out another project they had, which was somewhat similar. A pond owned by an HOA had lost a few acre feet in just a few days.
Fifteen years ago, the outfall structure on the pond had been rebuilt. The only part of the project which the engineer at the time wouldn’t sign off on was the abandonment method for the original outfall. The contractor plugged the 12″ CMP culvert by filling it with prepolymer chemical grout, but only on the last five feet. The reason the pipe had to be abandoned was that the invert was starting to rust out.
Over the 15 years from when the last 5 feet of the pipe was plugged, the invert continued to rust. All the while, it was under 20+ feet of head pressure. In May, 2015, what little pipe & soil was holding back the water pressure gave way, and the pond quickly dropped.
Upon inspection by CJGeo, it was apparent that the original foam plug had also shrunken from exposure to UV light, letting water leak around it. The foam used was just semi open-cell prepolymer, which is not ideal for bulk void filling in high head environments.
Access to the area was incredibly difficult; it was more than 400 feet from where a polyurethane grouting truck could park. From there, hoses had to be run through the woods and across the slope and toe of the dam to the end of the pipe.
Our first approach was to grout the pipe under normal leak flow. This was successful in cutting off the seeps which had developed around the outfall. However, this concentrated the water flow immediately around/through the pipe.
A siphon was used to draw down the pond elevation to reduce the head, and flow through the pipe. This slowed the velocity of the leak to the point where the polyurethane grout wouldn’t wash out before reacting.
We excavated the pipe back to uncover the top of the pipe behind the end of the original plug that had failed. We injected plural component, structural geotechnical grout into the pipe through the top of the exposed pipe. The inlet riser of the pipe had been ripped off during the original rebuild. During polyurethane grouting, a significant surface disturbance with bubbling and foam pieces extruding to the surface showed in the pond.
Injection back pressure slowly built, as the surface roiling slowed. Grouting was stopped at refusal; there was no evidence of water flow at the outlet afterwards.
Subsequent monthly visits showed the pond level gradually rebuilding, and no water flow from the pipe. The seeps around the pipe have also continued to stay dry. The cost of grouting the pipe with geotechnical polyurethane for the outfall pipe abandonment was significantly less than open cutting the dam to remove the pipe.