Percussion - Stone hammer
The Stone Hammer is a form of percussion drilling, used when boulders or hard clay 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.
In Western countries the same principle is used with the percussion gouge. As opposed to the stone hammer, the hammering action of the percussion gouge takes place on ground level. The gouge is electrically driven and used to break hard surface layers (rubble) and to take soil samples.
The stone hammer and percussion gouge are not individual drilling methods, but used to supplement other methods when boulders, rubble or hard clay is encountered during drilling. Although it does not penetrate very hard stone such as basalt, it is a tougher option than other options such as the Rota-sludge method or the EMAS method.
The Stone-hammer method won a competition for innovative irrigation technologies organized by the World Bank, Winrock and International Development Enterprises (IDE).
Percussion drilling is suitable for unconsolidated and consolidated formations: Sand, silt, stiff clays, sandstone, laterite and gravel layers.
Manual percussion drilling is generally used for tube wells of 2" to 5" diameter up to depths of 25 meters.
| - The Stone Hammer can be used in combination with other drilling tools when gravel or a hard layer is encountered.
- Stone hammer wells are cheaper to drill than machine drilled wells: due to input of local labour ($600 versus $1500).
| - It is difficult to remove the drill bit after it has been hammered into the formation. |
- The stone hammer is only successful when larger boulders are not encountered. Smaller ones are okay.
Compared to hand-dug wells, the Stone Hammer has the following advantages:
- Some 40 % cheaper ( $600 versus $1000 for a 20 meter well).
- Better water quality because of deeper penetration in the aquifer.
- Less accidents during construction.
- Less risk of contamination of the water.
Construction, operations and maintenance
Equipment is commercially produced in western countries. Local production is possible. Percussion drilling is hard work. Use of a small engine may be appropriate.
The costs are 20-60% less then those of hand dug wells. The cost of introduction are between US$ 15,000 - 30,000 per project, including a production drill set, drawings, hands-on training, and the first wells.
The stone hammer is mainly used in India and Nicaragua, and sometimes in Madagascar. The percussion gouge is mainly used in western countries.
"The Stone-hammer is slow but does not give us the headache of the cost and maintenance of motorized drilling rigs". A. Fajardo, Director of the Nicaraguan NGO Cesade .
- Rota-sludge and Stone Hammer drilling, part one: Drilling Manual by Practica Foundation.
- Rota-sludge and Stone Hammer drilling, part two: Production Manual by Practica Foundation.
- UNICEF - Percussion manual drilling: Technical Training Handbook on Affordable Manual Well Drilling
- Manual Hand Drilling Techniques. Rural Water Supply Network. An analysis and comparison of all the drilling techniques, including 3 in-depth manuals of comparisons.