Borehole Drilling

The Drilling Method

The drilling method employed is known as “down-the-hole” air flush rotary percussion. A pneumatic hammer and drill bit operated at the end of the drill pipe rapidly strikes the rock while the drill pipe is slowly rotated. The shattered splinters of rock are removed from the borehole continuously by the air used to drive the hammer. The drilling process is very noisy and can be very messy (either dusty or muddy). Drilling in urban areas is restricted to normal working hours and it is advisable to notify your immediate neighbours (to close their windows) the day drilling is to commence. The client needs to identify underground water pipes, sewage pipes, electrical, telecommunication or any other pipes or cables, as the drilling contractor cannot be held liable for any damage that might occur while drilling a domestic borehole can usually be completed within a few hours.

Drilling usually proceeds until either sufficient water is intersected (i.e., successful borehole scenario) or the client's budget is exhausted.

The Depth of the Borehole

Deciding on the final depth of the borehole is probably a decision that creates the greatest amount of misunderstanding between client and contractor:

Groundwater is found at varying depths in Southern Africa, in some areas of the Cape it is 300 metres deep, on the whole, however, most boreholes intersect usable water at depths of between 30 and 80 metres. The geologist or drilling contractor will advise on the depth at which water can be expected, just because “hard rock” is intersected, there is not necessarily any reason to stop.

Alternatively, the competent driller can advise the client on the borehole's prospects by continually examining the rock chipping samples and also from his own experience in the area.

The final stage of the sequence of events is to pump and pipe (reticulate) the water from the successful borehole into your hole / factory / garden. The end use of the water will to a large extent determine what type of pumping and reticulation system is to be installed. Of paramount importance to the correct sizing of the pump are the results of the pumping tests – how much water can be pumped out of the borehole for how long a period?

In general, there are two options for the pumping system. With a high yielding borehole one can either pump from the borehole via a pressure tank and then into the main supply line or into a storage tank (on surface) and then pressurized into the main supply line. Low yielding boreholes generally have to pump into a surface holding tank before being pressurized out to the main supply line.

A correctly sized and installed pump should last for many years with a minimum of maintenance and the extra cost of having a system professionally installed will pay for itself time and time again.