Another primary consideration in project planning is the availability of existing water sources and water points. There may be completed dug wells and boreholes already in the area. Are they in use? If not, can they be rehabilitated to augment water availability or to reduce the cost of the programme? Drilling equipment, such as compressors, can be used to bring disused boreholes back into use; the question of rehabilitation will be addressed in Section 10 of this review. This section outlines the factors that must be considered when choosing a drilling method.

                                                                                 

Common drilling methods 

Essentially, a drilling machine consists of a mast from which the drilling string components (tools plus drill pipes or cable) are suspended and, in most cases, driven. Modern systems are powered rotary-driven, but it is probably worth a short digression to describe some methods of manual drilling for water. Simple, low-cost methods include:

 

A) Hand-auger drilling

Auger drills, which are rotated by hand, cut into the soil with blades and pass the cut material up a continuous screw or into a ‘bucket’ (bucket auger). Excavated material must be removed and the augering continued until the required depth has been reached. Auger drilling by hand is slow and limited to a depth of about 10 metres (maximum 20 metres) in unconsolidated deposits (not coarser than sand, but it is a cheap and simple process.

                              

B) Jetting

A method whereby water is pumped down a string of rods from which it emerges as a jet that cuts into the formation. Drilling may be aided by rotating the jet or by moving it up and down in the hole. Cuttings are washed out of the borehole by the circulating water. Again, jetting is useful only in unconsolidated formations and only down to relatively shallow depths, and would have to be halted if a boulder is encountered.

 

C) Sludging

This method, which may be described as reverse jetting, involves a pipe (bamboo has been successfully employed) being lowered into the hole and moved up and down, perhaps by a lever arm. A one-way valve (such as someone’s hand at the top of the pipe) 

provides pumping action as water is fed into the hole and returns (with debris) up the drill pipe. There may be simple metal teeth at the cutting end of the pipe, and a small reservoir is required at the top of the hole for recirculation. The limitations of sludging are similar to those of the previous two methods, but it has been used effectively in Bangladesh

 

D) Percussion drillingDrilling by percussion is done by simply dropping a heavy cutting tool, of 50 kilograms or more, repeatedly in the hole. This may well be the original method of drilling for water, pioneered by the Chinese (probably using bamboo) 3000 years ago or more.

 

The drilling tools are normally suspended by a rope or cable; and – depending on the weight of the drill string, which, for manual operation, is obviously limited – it is possible to drill to considerable depths in both soft and hard formations. Basic percussion drilling systems are still widely used in Pakistan to drill shallow boreholes for hand-pumps. They consist of a strong steel tripod, cable and power winch, percussion tools, and a baler. These systems are seriously hindered when the ground is hard, and can accidentally change direction along weaker zones, causing boreholes to 

become crooked or tools to jam. Unconsolidated materials, although easy to drill with cable tool, become very obstructive when boulders are present. Sticky shales and clays are also difficult to penetrate with cable tool rigs, and loose sand tends to collapse into the hole almost as fast as it can be bailed.

                               

These manual shallow drilling techniques might be used as low-cost alternatives in groundwater investigations for dug well sites, particularly if geophysical surveys prove to be ineffective, unavailable or impracticable because of ground conditions. In such instances, when the drilling is done solely for the purpose of prospecting, only small holes are drilled rapidly.

 

E) Rotary drillingMost borehole applications in the field will require rotary drilling.

 

True rotary drilling techniques allow much deeper boreholes to be constructed, and use circulating fluids to cool and lubricate the cutting tools and to remove debris from the hole. Circulating fluids usually take the form of compressed air or of pumped water with additives, such as commercial drilling muds or foams.