As shown clearly in the figure 1, the percussion drilling process involves a heavy drilling bit in the form of a chisel or a clay cutter, attached to the end of a cable operated by a Winch of the drilling machine.
- Percussion boring rig
- Boring tools and chisel
- Clay cutter
That drilling bit is called a chopping bit. During the drilling process, the bit or cutter is lowered or raised in the borehole and in order to break the soil into fragments, water is added. The amount of water to be added must be such that it covers an area only at the boom of the borehole and the rest of the hole is dry. As the cutter is taken out or driven into the ground, the broken soil fragments and other materials are produced. These materials are removed from the borehole by using the various drilling tools/machines. Some of them are:
- Wash boring
- Clay cutter
- Sand pump or bailer
The shell used in removal is either open or is provided with a flap valve. In case of clayey soil, an open shell is replaced by clay cutter to remove the soil. A shell with a flap valve is mostly used to remove sandy soil or very soft clays. The casing is driven by a weight of 180 kg. In the structure, generally the drilling rig is designed to be speedily erected and is mounted on two wheels. The rig is so heavy that it can be operated only by two men. The boring capacity of this machine in general is about 100 m with a hole diameter of 150 mm or more. The addition of hydraulic power take off in the recent years has allowed many accessories to be added to the basic rig. Power auger attachments, casing jacking units and diamond drilling attachments are some of the lately used accessories. Percussion drilling is mostly suited for the area where soil constitutes strata of gravels and boulders. Since sands and clay consolidate instead of breaking, this method is not suitable in that case. The samples taken are completely disturbed and not representative. For rock boring below the soft strata, diamond drilling is the appropriate method.