Abstract: Reasonable and accurate evaluation of rock brittleness is the precondition of engineering practices, such as the assessment of rock burst risk and fracturing of oil and gas reservoir. In this paper, the applicability and limitation of energy drop coefficient for characterizing the failure process of brittle rock are analyzed in detail. In order to evaluate the brittleness of rocks more effectively, based on the stress-strain curve of the whole process of rock failure and the energy drop coefficient, a revised energy drop coefficient is developed by considering the ratio of released elastic energy in the total energy of the pre-peak stage. It is considered that brittleness is a comprehensive manifestation of releasable elastic energy stored in large quantities in the pre-peak stage and released rapidly in the post-peak stage. The experimental data of marble and granite under different confining pressures are analyzed and compared. It is found that the revised energy drop coefficient can not only reflect the brittle-ductile behavior of same rock under different confining pressures, but also effectively evaluate the degree of brittleness of different rocks under same confining pressure. In addition, the influences of Poisson’s ratio and damage variable on rock brittleness evaluation are discussed, the brittleness of granite is found greater than that of marble under each confining pressure with the increase of Poisson’s ratio; and with the increase of damage variable, the brittleness of granite is firstly greater than that of marble and then less than. The test results verify the reliability of revised energy drop coefficient, which may be expected to provide some references for rock brittleness evaluation.

Key words: rock mechanics, brittleness evaluation, energy drop coefficient, stress-strain curve, energy characteristics, damage variable