dam outfall pipe seep grouting

Dam Outfall Pipe Seep Grouting

Fairfax, VA

Extensive seeps at the joint between the pipe and end wall.


This dam outfall pipe seep grouting is located in Fairfax, Virginia at Northern Virginia Community College. A 36″ RCP outfall pipe for a dam terminated at an end wall. As part of a dam rehabilitation, small seeping leaks at the end wall/pipe joint needed to be stopped. The leaks had to be stopped so the wall could be resurfaced.

The engineer specified chemical grouting.


Access to the area was quite difficult. It was more than 150 feet from the closest vehicle access, which was only accessible by 4×4 vehicles down a steep slope.

While unknown during the planning of the chemical grouting repair, what was thought to be 36″ RCP was actually a larger diameter RCP that had been lined with steel casing pipe. An interior poured-in-place concrete liner had been installed afterward.  


Super low viscosity prepolymer chemical grout was selected. This was for two reasons: 1) the ability to easily pump more than 150LF from the lay-down area. 2) the ability to seek out and stop leaks through very tight cracks in the structures.

Prepolymer chemical grouts are water-reactive, so can be injected into active water flows. The grout expands when it comes into contact with the water, which seals the leak.  

After chemical grouting of seeps.

The initial grouting plan was to install the chemical grout through the pipe wall starting beyond the first joint. However, because the pipe had been cased, placement had to be done through the end-wall structure only. Multiple injection holes were drilled through the end wall structure, and the chemical grout was pumped through the end-wall structure.  

Extensive catalyzation was used to first seal the leaks at the end wall and then chase the water flow pathways up along the outside of the pipe. The job was messy due to the forced proximity of the injection points and the leaks. But all the leaks were successfully stopped.

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