Comparison of Some Rock Hardness Scales Applied in Drillability Studies

In this paper, the influence of hardness of rock material on drilling rate has been studied. During the research, eight various rock types were subjected to drilling and hardness tests such as; Mohs hardness, Indentation Hardness Index (IHI) and L-type Schmidt hammer. Mean Mohs hardness of each rock was calculated based on the hardness of contained minerals and other two scales are carried out based on ISRM standards. For drilling studies, rock samples have been drilled using actual pneumatics top hammer drilling machine with three inches diameter cross type bit. Regression analyses between mean Mohs hardness and the drilling rate reveal that in soft rocks, with increase in hardness, drilling rate decreases logarithmically but in hard rocks, with increase in hardness, drilling rate decreases linearly. In total, with increase in Mohs hardness, drilling rate decreases exponentially. Also, with increase in Indentation Hardness Index and Schmidt hammer value, drilling rate decreases logarithmically. The regression analyses showed that Indentation Hardness Index has the best and stronger relationship with the rate of percussive drilling.