Core Disking Observations and In-Situ Stress Magnitudes

Tensile stress plays a critical role in core disking. Three dimensional numerical analyses were carried out to determine the distribution of theses tensile stresses in the vicinity of the advancing drill bit. A methodology was developed to examine the spatial distribution of the maximum, minimum, and average, maximum tensile stress. A criterion based on the Averaged Maximum Tensile Stress (AMTS) was found to give good agreement with the thickness of core disks measured on core from 75-mm-diameter boreholes. This approach was then used to establish a general core disking nomogram using site-specific geometry, the Brazilian tensile strength, and the AMTS. The approach was applied to AECL’s Underground Research Laboratory and found to be in agreement with field observations. It is also found that a large diameter hole with a rounded shape bit can significantly reduce the potential for core disking in and around core stub.