Specifications Part 1 - Water Supply, Sewerage and Tube Well Works

particularly those having clay or sticky shale strata, is much lower as comparedto direct or reverse rotary method.3.1.3 Percussion drilling in hard rock is a slow process and is being graduallyreplaced by pneumatic rotary drilling because of economy and speed ofcompletion regardless of the higher initial cost.3.2Pneumatic drilling with top-hammer and eccentric bit and pneumatic drillingwith down the-hole hammer are the two principal method available for drilling inconsolidated (hard rock) formation:3.2.1 Down-the-Hole Hammer3.2.1.1 This drilling method, called DTH for short; permits rapid and effective drilling inrock and through over-burden which is not susceptible to collapse. In thismethod the impact mechanism blows directly on the drill bit and accompanies itdown into the hole compressed air for the impact mechanism is suppliedthrough drill tubes which are jointed as required as the drilling advances.3.2.1.1.1 The same air is, after it has passed the hammer, made use of for flushing. Thenecessary rotation is supplied from a rotation unit connected to the upper drilltube.3.2.1.2 As the drill tubes are not required to transmit the violent impact energy of thehummer, they can be manufactured with large diameter and still be relativelythin walled.3.2.1.2.1 This gives the method better flushing characteristics than conventional tophammer drilling. Theoretically the rate of penetration is independent of the holedepth with the DTH method no water is required during drilling. The equipmentis also cheaper and lighter as a much smaller compression is required than fortop hammer drilling.4. Casing of wells4.1 Well in soft soils must be cased throughout. When bored in rock, it is necessaryto case the well at least through the soft upper strata to prevent caving. casing.Casing is also desirable for the purpose of excluding surface water and itshould extend well into the solid stratum below. Where artesian conditions existand the water will eventually stand higher in the well than the adjacentgroundwater, the casing must extend into and make a tight joint with theimpervious stratum; otherwise water will escape into the ground above.4.2 If two or more water bearing strata are encountered, the water pressures indifferent strata are likely to be different, that from the lower usually being thegreater.4.2.1 Where different pressures thus exist, it is only possible to determine theiramount by separately testing each stratum as reached, the others being casedUrban Administration & Development Department Page 210