A Criterion on Core Disking in the General State of In-Situ Stresses.

Based upon the results of finite element analysis for an axisymmetric rock mass with a long HQ core, which is subjected to nonaxisymmetric far-field stresses, tensile stress near the base of the core was analysed to obtain a criterion on core disking in the general state of stresses. The stress conditions of all 77 cases in this study were divided clearly into two groups by the direction of the maximum tensile stress in the central part of the core. In one of the groups the tensile stress is nearly in the direction of the core axis and in the other the tensile stress is nearly perpendicular to the core axis. Only 26 cases in the total 77 cases gave the stress conditions necessary for core disking. In these cases, the difference in the magnitude of the maximum tensile stress between in the central part and in the outer part of the core was small, which suggests that the stress conditions enable a crack initiated at the outer part of the core to penetrate through the entire core. These cases included the stress conditions where the far field minimum principal stress is not in the direction of the core axis. To produce a through crack in the core by tensile stress, the stress contour plane of the tensile stress must exist throughout the cross section of the core. Therefore, it is assumed that core disking occurs if the tensile stress whose upper and lower contour planes just touch each other reaches the tensile strength of the rock. This tensile stress, called the critical tensile stress in this study, is given by the minimum principal tensile stress among the maximum principal tensile stresses which are determined by searching the maximum value along the core axis for a set of the coordinates in the core cross section. Thus, we have proposed the following criterion on core disking: