The slurry wall or cut off trench is a very good way to slow the water that is seeping through a dam.
I have only read about the slurry. It is a mixture of water and bentonite that is pumped into a trench that has beed cut with a track-hoe. It needs to be about as thick as wet concrete.
This is the same idea as a cutoff trench that is backfilled with clay.
The trouble that I have had with cutoff trenches is the walls of the trench caving in before the material is in place.
