Raise boring is widely used for the excavation of shafts in the deep level mines of South Africa. As depths of mining have increased, problems have been encountered, and have resulted in failure of the equipment on numerous occasions. The consequence has been the plummetting of the cutter head to the bottom of the bore. Cases of such occurrences are included. One of the reasons for such failures is the spontaneous fracture of the rock which occurs ahead of the face. This results in the formation of blocks of rock which cause high cutter wear and jamming and oscillating of the head. Analyses of this spontaneous fracturing are described and from the results it is possible to predict the depth to which such fracturing will occur ahead of the face.
Raise boring has become a routine activity in the mines of South Africa. At present there are 55 raise borers and 46 blind hole borers available in the country, and, for the year ending December 1988, approximately 35km of raise boring and 17km of blind hole boring had been completed. Unfortunately the rock mechanics aspects there of are rarely considered in any detail. This is to be regretted, since mining depths are becoming greater, and the trend in raise boring is towards larger diameters, with single-stage holes of the order of 4m now being common. As mines become deeper, the stresses become greater, and, as a result of the more arduous conditions, technical problems have been encountered in raise boring contracts. These have been compounded by the refusal of insurance underwriters to cover the costs of damage to, or loss of, down the hole equipment.
The main sources of problems in raise boring are threefold:
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mechanical failure of the equipment
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unfavourable geological structure
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induced fracturing of the rock.
Mechanical failure of the equipment can arise from weaknesses in the materials, inadequate maintenance with resulting non-replacement of fatigued parts, and poor operation of the equipment leading to excessive stressing of components. Unfavourable geological structure may lead to instability of the face and walls of the bore (McCracken and Stacey 1989). This might lead to jamming and irregular movement of the cutter head which will aggravate any mechanical deficiencies, and may cause failure of well maintained equipment. Under high stress conditions fracturing of the rock may be induced, resulting in the formation of rock slabs at the cutting face, which cause high cutter wear. "Dog ear" fracturing (Hoek and Brown 1980), which occurs on the sidewalls of the bore as illustrated in Figure 1, can cause excessive perimeter cutter wear. Owing to the jamming and oscillating of the cutter head which results from uneven cutter loading, failure of the equipment may occur, usually at the threaded connection between the last drill pipe and the cutter head. The consequence is the free fall of the head to the bottom of the hole. In the present paper, this concept is extended to the raiseboring situation.
