A novel 3D-FDEM method using finite-thickness cohesive elements to simulate the nonlinear mechanical behaviors of rocks

The crack closure stage and a high ratio of unconfined compressive strength (UCS) to tensile strength (TS) are frequently observed in laboratory tests of highly fractured rocks. However, few numerical methods can capture the crack closure behavior and high UCS/TS ratio in three dimensions. Hence, in this study, a novel three-dimensional finite-discrete element method (3D-FDEM) using finite-thickness cohesive elements was proposed, and random initial microcracks were introduced to the method. The crack closure stage and high UCS/TS ratio of highly fractured rock (Lac du Bonnet granite) are well matched through the present method. The simulated results show that the crack intensity determines the occurrence of the crack closure stage and the crack width significantly influences the evolution process of tangent modulus and crack closure strain. Due to the combination of stiffness degradation and failure of cohesive elements and initial microcrack closure, this method can capture the evolution trends of tangent modulus of various rock materials. The decreases in the simulated UCS, TS and elastic modulus with an increase in crack intensity are consistent with the experimental results of rocks after thermal treatment. A calibration procedure, which can ease the calibration of the parameters of this method, was proposed.