Percussion Drilling Rig

Percussion drilling is employed when auger or wash boring is not possible in very stiff soil or rock. It also can be used in most soil types. 

Here the advancement of a hole is achieved by alternatively lifting and dropping a heavy cutting or hammering bit that is attached to a rope or cable that is lowered into an open hole or inside a temporary casing (casings are hollow cylindrical pipes used for borehole stability and to prevent the loss of drilling fluid through the boreholes). 

Usually a tripod is used to support the cable. The stroke of bit varies according to the ground condition. The major disadvantage of this method is that it is not possible to get good-quality undisturbed samples. In very hard rock (and especially fractured hard rock), down-the-hole (DTH) drilling can be employed. In this case the hammer, applying repeated percussive pressure, is located just behind the drill bit inside the hole, unlike the open percussion drilling, where the hammer is on top of the drilling string. 

The drilling string provides the necessary force and rotation to the hammer and bit, as well as compressed air or fluids to the hammer and for the flushing of cuttings. 

This arrangement also allows for much deeper percussion drilling. However, the DTH drills are typically more expensive.