Detournay and Tan (2002) performed experiments with a rock cutting device to measure the load required to fail the rock under confining stress. They proposed models describing the correlation between the specific energy and confining stress for shear dilatant rocks as a function of unconfined specific energy at failure, cutter rake angle (?), internal friction angle (f) of the rock, and an assumed interface friction angle (?) between the rock and the cutter. The aim of this paper is to evaluate the model proposed by Detournay and Tan (2002). Their model assumes that the interfacial friction angle between the rock and the cutter is equal to the internal friction angle (?). Their model also assumes that the rate of change coefficient (m) is constant for different bore hole pressures (pm). To evaluate the model, data on three (3) rock formations (Mancos Shale, Pierre Shale, and Catoosa Shale) was used in the study. The data on Mancos shale and Pierre shale was obtained from Zijsling (1987) and data on Catoosa Shale was obtained from Smith (1998). Quantitative assessment of rate of change coefficient (m) of specific energy was also accomplished to evaluate the assumption that m is constant for different bore hole pressures. The results show that the interface friction angle varies with the bore hole pressure and is not equal to the internal friction angle. The results also show that rate of change coefficient (m) of specific energy estimated using the equation proposed by Detournay and Tan (2002) and an assumed internal friction is not representative of m obtained from known values of internal friction angle, interface friction angle, and back rake angle.
Behavior of rock under a range of confining pressures has a vital role in its applications to many engineering and scientific disciplines. The strength and stress - strain behavior of rock under confining stress are particularly important to geophysicists analyzing plate tectonics, earthquakes, and other fault movements. They are also useful to engineers designing deep foundations for buildings, off shore structures, and dams on the rocks. It is significantly important for drilling engineers to understand the behavior of the rock under various confining stresses to understand the behavior of drill bits on different kinds of rocks and to develop techniques for enhanced rate of penetration (RoP). Confining pressures affect the unconfined strength of the rock and the energy required to cut a rock during drilling.