Experimental study of fracture slip behavior in granite under different intermediate and minimum principal stresses

In deep rock engineering and deep resource exploitation, faults or fractures slip and reactivation usually leads to serious dynamic geological hazards (such as seismic activities, rock bursts and landslides). However, the effect of minimum principal stress (sigma(3)) and intermediate principal stress (sigma(2)) on rough fracture slip behavior remains enigmatic. Here we present an exploratory research on slip behavior of rough granite fractures under true triaxial (sigma(1) > sigma(2) > sigma(3)) stress conditions. The results suggest that the peak strength for the fractured granite specimen increases with increasing sigma(3), but first increases and then decreases with increasing sigma(2). Increasing sigma(3) restricts the slip deformation, while increasing sigma(2) tends to first increase and then decrease the slip deformation. A three dimensional (3D) asperity which considers the contact, friction and matching relationship of fracture surfaces is used to account for the influence of sigma(3) and sigma(2) on strength and deformation behavior. The AE hits with larger amplitude tend to increase with increasing sigma(3) or sigma(2), indicating severer fault damage under higher stresses, which is consistent with the micro damage observed from SEM. The changes of wear regions and gouges with increasing sigma(3) or sigma(2) are in good agreement with that in peak strength and deformation along sigma(3). The decreasing b value and increasing strain along sigma(3) can be used as a precursor for fracture slip. Moreover, four strength criteria developed for intact rock were used to fit the results of the fractured rock, and the comparison shows that the Mogi-Coulomb criterion provides the most satisfactory results and can be used to predict the peak strength of fracture or fault failure in the applied stress regime. This study will provide new insight into a thorough understanding of shear behavior under different sigma(3) and sigma(2), and can also contribute to the stability assessment of fractured rock in deep underground engineering.