Abstract
Most of the overcoring method for rock stress measurements use a large diameter drilling, what is called the overcoring, to relieve the stress around a pilot hole in which a transducer is installed. When the overcoring is advanced into hard rock subjected to high rock stress, a phenomenon of a core disking is sometimes observed. As the recovered cores which have a constant thickness have a center hole like a doughnut, this core disking is called a hollow core disking. In this paper, simulating the overcoring under three dimensional initial stress state in which the axis of a principal stress coincides with that of a borehole, the hollow core disking initiated during the overcoring is discussed, based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary conditions. Firstly, the surface stress intensity coefficients in the tangential and rotational directions of the borehole are analyzed under the conditions in which each principal stress of initial stress is subjected independently. It is clarified that the coefficients is not changed beyond the advance of the overcoring more than the radius of the borehole. Secondly, using the surface stress intensity coefficients, the surface stresses of the borehole are analyzed under three dimensional initial stress state, then the fracture criterion which represents the fracture initiation due to the tensile stress induced by the overcoring is formulated as a function of the advance of the overcoring. Finally, analyzing the surface stress of the borehole induced by the overcoring, it is made clear that the location of the fracture initiation, the type of the fracture and the thickness of disk are dependent upon initial stress state.
