Le forage manuel fait référence à plusieurs méthodes de forage qui nécessitent de l'énergie humaine pour réaliser un trou de forage et assurer un approvisionnement en eau. Différentes techniques peuvent être utilisées dans des zones où les formations sont assez molles et où les eaux souterraines sont relativement peu profondes.
Le forage manuel peut fournir de l'eau potable. L'équipement peut facilement être transporté vers des populations éloignées ou difficiles à desservir, qui seraient autrement laissées pour compte. Les particuliers, les entreprises et les gouvernements apprécient les coûts moins élevés que ceux du forage mécanique. Le forage manuel fournit également des emplois locaux.
Des méthodes de forage manuel sont utilisées pour fournir de l'eau potable pour les besoins domestiques dans au moins 36 pays à travers le monde. Dans certains endroits, les méthodes de forage manuel sont bien établies.
Le forage manuel fait partie du sujet sur la professionnalisation du secteur des forages d'eau.
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Well jetting is also known as washboring, and in Nigeria as hand turning.
Description: Jetting is considered to be a manual drilling technology, even though it utilises a small pump. The technique involves pumping water (with a hand, or motorised pump), down the drill pipe, which is held vertically in the hole. The water passes through the bottom, open end of the pipe and carries the drilling spoil up the annulus. The drill pipe is held vertically and slightly rotated and/or reciprocated. It is the washing action of the water, which forms the hole. The drill pipe is usually up to 50mm in diameter, while the hole is 100 to 150mm. The equipment comprises a centrifugal pump, suction hose, flexible delivery hose, elbow and swivel and jetting pipes. Temporary casing can be used, but a technique which enables the self jetting of wellscreens has also been developed. In cases where the ground is very compact, a special jetting point is used.
Description: This technique is a variation of hand percussion drilling. A 60cm long cutting tool is fitted to the base of a drill pipe. A 70kg steel weight (or hammer) is lowered into the drill pipe. This hammer is raised and dropped onto the cutting tool, forcing it to penetrate the formation, before being lifted out carrying the cuttings.
Description: Hand sludging (also known as Asian, or Indian sludging) is a traditional technique used in parts of Nepal, India and Bangladesh. It involves reciprocating a steel pipe (of 25 to 40mm diameter) vertically in a shallow pit, which is kept full of water. The reciprocating action is achieved by a lever, which is attached to a bamboo frame. One operator operates the lever while the other uses his hand over the top like a flap valve. On the up-stroke the hand covers the pipe, while on the down stroke it lifts off. This action enables the cuttings to be carried up through the drill pipe and exit at the top.
Description: The rota sludge technique is similar to hand sludging and the Pounder rig. It involves raising and lowering a steel pipe, which is weighted at the bottom and fixed with a drill bit on the base to drill the hole. A simple wooden frame and lever are used to enable the reciprocating action. Water mixed with cow dung is used as drilling mud – ie to carry the cuttings to the surface and prevent collapse of the drilled hole. A hand, placed at the top of the pipe acts as a valve, lifting up and releasing the cuttings on the down stroke and covering the pipe on the up stroke. A pit (Figure - above centre) enables the drill cuttings to settle out and thus the mud to be recycled. A handle is clamped to the drill pipe. This allows rotation of the drill pipe, which assists to scrape and break the formation.
The stone-hammer technique is used in conjunction with the rota sludge technique to penetrate hard formation.
Description: Percussion (also known as cable tool) drilling refers to the alternate breaking of the formation and cleaning the hole. Percussion drilling is often undertaken with different tools – eg a chisel to break followed by a bailer to remove the spoil. There are also clay-cutting tools available that can both cut and remove the spoil. The drilling tools and weights (referred to as the tool string) is suspended from a rope or steel cable and reciprocated through a stroke of 1 to 3m. Small amounts of water are usually added to the hole to help loosen the formation. It is often necessary to line the hole with temporary steel casing to prevent collapse.
Description: Hand augering can be undertaken with a heavy tripod and winch (such as provided by the Vonder Rig). Alternatively, very light equipment can also be utilised such as that common in Niger. Common to both of these rigs is the auger bit. A bailer can also be used to remove the spoil form the hole in the form of a slurry. Drilling is undertaken by rotating the auger into the ground, and adding additional drill pipe as the hole deepens.
Description: EMAS Drilling, developed by Wolfgang Buchner in Bolivia combines jetting with a percussion action. Drilling mud (water mixed with clay or bentonite to a suitable density) is pumped down through the drill stem using a hand operated metallic version of the EMAS pump. The mud flows back up around the drill stem, carrying up the drill cuttings. Sand and small stones are decanted, and the drilling mud is recycled through the pump. When progress is difficult, due to particularly course sand or small pebbles, the technique is changed to a suction system.
Description: A driven well refers to driving a well point and well screen directly into the ground using a hammering tool (figs right). The material is forced aside rather than excavated by this technique. This technique is sometimes used in conjunction with hand augering.
Description: Terry Waller of Water for All in Bolivia developed Baptist drilling in 1993. It is a hybrid between sludging and percussion drilling. The main difference is that while hand sludging relies on a person’s hand at the top of the drill pipe as a valve, the Baptist method uses a valve, incorporated into the bit at the bottom of the drill stem.