Abstract: A dense “boss” in the hole bottom by the conventional drilling in the hard rock requires a more efficient drilling technique with slow bit wear, a new kind of hard rock combined drilling method with the abrasive water jet and mechanical bit is developed in this paper. In this method, it is proposed to introduce the abrasive water jet to erode the “boss” for a “pilot hole”, and then the drilling mechanism of conventional rotating bit is changed to improve the hard rock drilling efficiency. Meanwhile, the mechanism of the new drilling method to improve drilling efficiency in hard rock was revealed by using fracture mechanics theory and energy conservation principle. Moreover, by designing a hard rock breaking bit and a set of hard rock combined drilling equipment system, a series of comparative experiments between the new and conventional techniques were conducted. The results show that the drilling depth has increased by about 63%, the axial force and torque have reduced by about 15% and 20% respectively, and the bit wear is reduced significantly.

Key words: hard rock, drilling, abrasive water jet, mechanical bit, combined breaking rock, drilling efficiency