To provide a waterproof barrier, a cut off trench enriched with bentonite is installed in the dam.
With the water level high, an excavator is used to dig a trench through the core of the embankment, in the area of the leaks. The spoil is set aside and the clay content enriched by the addition of sodium bentonite powder. The spoil is then returned to the trench and as the bentonite is wetted it swells to provide a barrier to water movement.
In certain circumstances particularly around structures, in large embankments or where surface disruption must be avoided a slurry of sodium bentonite, or bentonite and cement are injected below the surface using pressure grouting equipment. The bentonite fills the voids created by water piping through the soil so sealing the leaks. This technique causes less disruption than trenching but it is difficult to ensure that every leak is filled.