Gypsum is used to stabilise dispersive soils on dam embankments so that both surface erosion and potential tunnelling and piping failures are reduced. A ﬁne-grained gypsum is preferable because it is more soluble.
Piping or tunnelling occur through a dam wall or embankment when water seeps along a line of weakness (ie crack in dry clay material) creating a pip or tunnel. This can lead to rapid loss of water stored above the pipe.
The gypsum is generally mixed into the ﬁrst 150–200 mm of surface soil at up to 2% (about 2-8 kg/m2) , and then the treated area is compacted with a roller. Remember, dry soil will not compact well. Good compaction can only be achieved with a soil near its optimum moisture content (seek civil engineering advice).
Bentonite is a naturally occurring clay which is commercially mined. In dam building it is useful because, when it is wet, it swells to many times its dry volume.
Bentonite may be used in several ways depending on the soil type on site and whether it is practical to empty the dam. On light or loam soils a mixed blanket is worked into the soil. On heavy soils, a pure blanket would be required (similar to a clay lining). In both cases the dam would need to be emptied and allowed to dry.
A third option, which is hit and miss, would be to broadcast the bentonite on the water surface.