Bentonite is a very fine powder of clay particles which swells in contact with water to become a waterproof clay. Two basic types are available, either sodium bentonite or calcium bentonite. Sodium bentonite is imported from the USAand is the higher quality but more expensive material, absorbing five times more water, weight for weight, than does calcium bentonite. In the past, bentonite was supplied in powder form for mixing into the surface of excavations to create a waterproof layer, or for sprinkling into existing ponds to repair leaks. Neither use has proved very reliable, and many failed bentonite ponds have been subsequently lined with butyl or other flexible linings.
To overcome the problems of getting even coverage, bentonite is now supplied sandwiched between two geotextile layers, called a geosynthetic clay liner (GCL). To avoid problems of losing the dry powder during transport, and difficulties with on-site wetting, a further development is to pre-hydrate the bentonite layer during manufacture. This product is called Rawmat High Density Bentonite.
The great advantage of bentonite linings is their ability to self-heal when punctured, as the clay component swells to fill the gap. Posts for revetments, boardwalks and other features can be driven into the lining without causing leaks. Sealing around inlet or outlet pipes is a simple procedure, and joining two sheets of Rawmat is done by means of a self-sealing overlap.
As Rawmat is supplied pre-hydrated, it is heavy, at about 8kg per square metre. It is supplied in rolls, 1m by 5m length, and 2m by 25 or 50m length, with the 2m width needing machinery for handling. For further details contact the manufacturer, Rawell Water Control Systems Ltd.
