Borehole breakouts and core disking in Westerly granite: Mechanisms of formation and relationship to in situ stress

Laboratory experiments were carried out in which vertical holes were core-drilled in cubical specimens of Westerly granite. Throughout drilling the blocks were subjected to a truly triaxial state of stress (three unequal principal stresses). Depending on the state of stress, borehole breakouts and/or core disking were observed.; A thin section study of borehole cross-sections revealed that transgranular extensile cracking subparallel to and behind the borehole wall, along the springline line of the least horizontal far-field stress, precedes the development of breakouts. Under higher stress conditions, microcracking intensifies, giving rise to thin rock flakes, which flex and buckle into the borehole. Progressive spalling of thin flakes creates the well-known 'V' shaped breakouts. Breakout dimensions are directly related to far-field stresses. Our study verified that a polyaxial strength criterion relating the octahedral shear stress to the mean normal stress on failure planes aligned with the direction of the intermediate principal stress, correlates well with the state of stress at breakout boundaries on the borehole wall, and can be utilized for the calculation of one of the in situ stresses from knowledge of the breakout span and estimation of the other two principal stresses.; Core disking was also detected when the specimens were subjected to high far-field stresses. Core disks are characterized by nearly uniform thickness for a given stress condition and by a 'trough' shape with the trough axis oriented in the direction of the far-field maximum horizontal principal stress. A thin section study revealed that the 'trough' shape is the result of subhorizontal extensile cracks developed at the root of the core stub. The variation in disk thickness with the far-field stress suggests a potential for correlation. In fact, the thickness decreased monotonically with the increase in the maximum horizontal principal stress for given magnitudes of the other two principal stresses. A quantitative relationship between disk thickness and far-field stress magnitudes was determined by a nonlinear regression analysis. The relationship enables the estimation of the magnitude of the maximum horizontal principal stress from disk thickness if the values of the other principal stresses are independently known.