Curtain Walls are underground constructions designed to isolate one section of the ground from another below the surface.  Frequently the purpose of a curtain wall is to prevent the migration of groundwater into or out of a particular section of ground.  Varous grades of Kentish Minerals Bentonite, usually KM SW, are used in conjunction with cement to provide the impermeable element of the wall, and to impart a modicum of flexibility to the wall, thus preventing any future breaches or cracking of the impermeable barrier.