Evaluation of core disking rock stress and tensile strength via the compact conical-ended borehole overcoring technique

To evaluate core disking rock stress and tensile strength via the compact conical-ended borehole overcoring (CCBO) technique, the mechanism of core disking by means of the CCBO technique is discussed. It is simulated that overcoring of the CCBO under three-dimensional initial stress state coincides with borehole axis and is one of principal directions. The characteristics of induced stress and initiated failure due to tensile stress on the borehole bottom surface during overcoring are discussed. They are based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary element conditions. From the results, an initiation criterion of core disking is suggested. It is shown that the failure is initiated from outside surface of core and that the failure initiation criterion on the longitudinal tensile stress outside of core can define the criterion of core disking. Secondly, since tensile strength of rock is closely related to core disking, an estimation procedure of it is suggested using strains during compact overcoring. It is shown that representative rock volume for tensile strength estimated by the filed measurement data is smaller than that by Brazilian test, and that the fracture initiation criterion may estimate rock stress using tensile strength obtained by Brazilian test with small volume. Finally, an estimation procedure of rock stress is suggested by means of core disking phenomenon on the CCBO technique.