ABSTRACT:

Mining at greater depths can lead to stress-induced failure, especially in areas of high horizontal in-situ stress. The induced stresses on the boundary of an opening are known to be in a biaxial state. Biaxial testing of intact rock mostly requires sophisticated loading systems making it expensive and time-consuming. This study investigated the impact of biaxial stress state on the failure response of intact rock using a simple and cost-effective design. The apparatus consists of two platens that apply biaxial compressive stress on a cubical specimen when placed inside the uniaxial testing frame. The platens apply equal loading in both directions (s1 = s2) until specimen failure. In addition, a confining device was used to perform separate biaxial tests under constant intermediate principal stress (s2 = constant). The failure modes and peak strength of sandstone specimens were compared with other biaxial devices to validate this design. Uniaxial tests were performed on both cubical and standard cylindrical samples. The biaxial strength of 50.8 mm cubical specimens was found higher than its uniaxial strength at any level of confinement. A quadratic polynomial, based on regression analysis, provided a good fit to the data points at failure. Spalling characterized the failure mechanism at free faces followed by shear failure of mid-section. In biaxial tests, shear planes at mid-section rotated with the increase in the intermediate principal stress.

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