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It is shown that the mechanism of rock fracture is similar both when a blunt drag bit is used to cut strong rock and when a flat-bottomed punch is pressed into the rock surface. In order to study this fracture mechanism an experimental technique was developed which involved indenting the rock specimen in a quasi-static manner, using a drag bit as the punch. The effect of directing waterjets adjacent to the bit was investigated and it was found that these jets caused a rock chip to form with lower than normal forces applied to the bit. This finding agrees with results of previous experiments where lower forces were applied to the bit during the cutting operation when waterjets were used. A hypothesis is proposed to explain the action of the waterjets on the rock to produce this reduction in the indentation force.
drag bits, waterjets, hard rock, cutting, indentation, rock fracture, erosion
Assistant professor, University of California, Berkeley, Calif.