Air Percussion Down-The-Hole Hammer A method close to the direct rotary method, called down-the-hole, is used for hard rock formation and employs a pneumatic drill at the end of the drill pipe that strikes the rock while the drill pipe is slowly rotated. The percussion effect is similar to the blows delivered by the cable-tool bit. The air used to drive the hammer as the hole is advanced removes cuttings continuously. Unlike the conventional cable-tool bit that is constantly striking previously broken rock fragments, the bit on the air hammer always strikes a clean surface, thus rending the hammer very efficient. The method permits soil sampling as cuttings are delivered to the surface through a discharge swivel. The method is well suited for boring in hard formations, where there is low risk of caving. Hammer bits 6 inches in diameter are most commonly used, although sizes range up to 17 inches. The down-the-hole hammer requires internal lubrication, which is provided by hydrocarbon lubricant added by means of in-line oilers. This need for lubrication eliminates consideration for monitoring work where hydrocarbons will be sampled. PROS Rapid removal of cuttings; No use of drilling mud; High penetration rate, especially in resistant rock formation (e.g. basalt); Easy soil and groundwater sampling during drilling; Possible to measure yield estimate at selected depth in the formation. CONS Restricted to semi-consolidated to consolidated formations.
Posted on: Mon, 27 Jan 2014 04:23:19 +0000
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