Borehole

The Process of Drilling a Borehole

The appointed operator will set up his rig and equipment on your property, a process that takes up to half a day. Drilling usually commences with a 8 or 10 inch drill bit. In the Cape Peninsula the geological formation requires that a process called Rotary Mud Drilling (‘RMD’) is used, a specialist technique that allows the successful drilling through sand, clay and unstable gravel beds in the weathered overburden (top layer). During RMD water is flushed down the hole, carrying the drill cuttings to the surface where they are , deposited in flow pits (that are dug next to the rig for this purpose). The water is mixed with a chemical that helps to carry debris and prevents the formation from collapsing during the casing (a steel or PVC sleeve) installation.It is possible that water is found during this process, at which point the hole will be sleeved with a PVC lining (‘cased’) of which the last 15m or so is slotted (screened). Note that the drilling stops well past the point where water has been struck, as there may be variance in the exact water level over time and during periods of extended drought. The thickness/strength (‘class’) of the PVC is determined by the depth of the hole, but typically the sleeved part consists of Class 12 PVC (very strong wall thickness, the thinner classifications being class 9). The annular cavity surrounding the PVC is packed using a specific grade of filter pack depending on the formation. This “filter-pack” acts to keep the screen from clogging up with sand or clay particles. The all-important next step is then to ‘DEVELOP’ the hole.

Should harder formation be encountered before water is found (which, depending on the area, is the case more often than not), the RMD process is halted and the hole cased with a steel sleeve, 6 or 8 inch in diameter depending on the depth. The 3.5mm thick steel casing comes in 6m lengths that are welded together and lowered down the hole to the point where it rests hard against the solid formation (rock). This steel pipe can be screened (slotted) to get benefit from any water in the unstable formation.

The drilling process is now changed from RMD to what is known as a “down the hole” Air Flush Rotary Percussion method or simply Air Percussion Drilling (APD). A smaller diameter percussion drill bit is used, and drilling then continues inside the steel sleeve. During this process air is forced down the hole inside the drill. This compressed air drives a hammer that in turns actions the percussion drill bit, and the air then carries the pulverized cuttings to the top of the hole where it is blown out. As with Rotary Mud Drilling, the APD process is continued until water is struck (usually in weathered or fissured rock – very clean, beautiful water). In this situation it may not be necessary to line, the hole with PVC (e.g. in hard sandstone formation), but that is unlikely. Mostly the formation is unstable and a PVC casing is inserted all the way down

Developing the hole

Pressurized air is blown into the hole via a pipe fitted with a special nozzle. This process blows out the chemically treated water, clears all the muddy sediment from the screen and gravel pack and settles the surrounding formation. This process is continued until the water flowing from the hole is clear. (NB: Not developing the hole, or simply pumping out the mud, will have a severely negative impact on the yield of water from your borehole.)

The last steps before the submersible pump is fitted are for the operator to do a yield estimate on the hole and to take a water sample for analysis. The flow rate and the depth of the hole (the ‘head’) will determine the size of the pump to be installed. The submersible stainless steel pump is lowered down the hole and suspended 2 to 3 meters from the bottom. The electrical cord and return supply pipe runs to the surface, where it is connected to an electrical control box and a ball valve (on a base plate) respectively. The top section of the annular area is grouted and cement sealed in order to prevent ground water contamination

As the borehole is often sited on a lawn, a protective, green glass-fiber covering can be fitted over it so that it is flush with the ground.

Borehole constructionGreg

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