: To verify the feasibility and efficiency of rotary-percussive drilling assisted by waterjets in very hard rocks, experiments on the performance of Polycrystalline Diamond Compact (PDC) cutters resisting different combined loads of static thrust, impact, cutting and waterjets on Missouri red granite and Halston limestone were investigated. The experiments were conducted by means of a drop hammer and linear cutting-impact table. The effects of static thrust (WOB), impact energy, impact spacing and waterjets on the rate of penetration in hard rocks are described and the results are analyzed by measuring the depth of the craters or cuts penetrated by PDC cutters. Results from the study confirmed that the combined mode of cutting-impact is very effective in very hard rocks.
Since the introduction of PDC bits in the mid-1970s, tremendous improvements in drilling rate, efficiency, economy and usage have been made. Nowadays, PDC bits can be used successfully in drilling formations ranging from soft to mediumhard rocks. Currently, PDC bits are applicable on hard formations with an unconfined compressive strength of between 120~150 MPa (Rubin 1985, Fear et al. 1995). Hitherto, no successful cases have been reported in drilling extremely strong and abrasive formations such as granite, volcanic material or old dolomite.