Changes

The Stone Hammer is a form of [[Percussion - general|percussion drilling]], used when boulders or hard clay are is encountered. A hollow drill bit is attached to a hammering tool. The hammering tool consists of a heavy weight which moves up and down in a pipe. Once the bit and hammering tool are lowered into the borehole by a cable, the same cable can be used to do the hammering. Lifting and dropping the hammer drives the drill bit down. When 60cm has been drilled, the unit is pulled up to the surface to empty the hollow drill bit. The stone hammer is only successful when boulders are encountered, not in homogeneous or solid stone layers, where displacement of the material is not possible.