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PUBLIC HEALTH ENGINEERING DEPARTMENT
Foremost IN THE FIELD OF WATER SEWAGE AND WASTE TREATMENT
Overburden Drilling Helps To Avert
Atlas Copco have introduced a new technique for sub-soil and underwater drilling— Overburden Drilling Method. The Overburden Drilling Method help to avert sinkhole disasters. It has been successfully used at West Driefontein Gold Mine to establish whether any further sinkholes, such as that into which the crushing plant disappeared in Decemberlast year, are likely to occur. One of these holes, drilled next to the massive bulk of the winding engine house was taken to 426 ft., overburden equipment to 96 ft. and deepened with standard 14 in. steels and 2 in. tungsten carbide tipped bits to the final depth. On the conclusion of drilling operations, a 1 in. pipe was lowered through the overburden casings to the bottom of the hole after removing the standard steels. The overburden casings were then removed and replaced with 96 ft. of 2 in. pipe to collar the hole and so prevent the overburden from filling it. The bottom 15 ft. of 1 in. pipe was grouted in. This pipe projects about 15 in. above the surface and 9 in. above the 2 in. pipe. Should any sinkholes develop, the 1 in. pipe will be drawn into the hole through the 2 in. pipe by the subsidence, thus giving warning that the area should be evacuated. The drillers at West Driefontein struck loose ground consisting of soil and boulders up to a depth of 95 ft. From 96 ft. to 125 ft. the drill penetrated damp wad, after which it passed into solid dolomite with numerous chert bands. At 400 ft. the drill was penetrating the rock at the rate of 12 in. a minute.
In the overburden drilling method the hole is drilled simultaneously with a 2% in. drill pipe and standard 14 in. round steels. The drill pipes are jointed together with external coupling sheaves, the first pipe down the hole having a 3; in. detachable ring bit. The 2 in. tungsten carbide bit of the leading steel projects one in. beyond the ring bit and acts as pilot. When bedrock is reached, both the ring bit and the gross bit on the leading steel are collared a few inches into the rock for the secure support of the