Generalized crack initiation and crack damage stress thresholds of brittle rock masses near underground excavations

The rock mass failure process is characterized by several distinct deformation stages which include crack initiation, crack propagation and coalescence. It is important to know the stress levels associated with these deformation stages for engineering design and practice. Extensive theoretical, experimental and numerical studies on the failure process of intact rocks exist. It is generally understood that crack initiation starts at 0.3 to 0.5 times the peak uniaxial compressive stress. In confined conditions, the constant-deviatoric stress criterion was found to describe the crack initiation stress level. Here, generalized crack initiation and crack damage thresholds of rock masses are proposed. The crack initiation threshold is defined by sigma(1) - sigma(3) = A sigma(cm) and the crack damage threshold is defined by sigma(1) - sigma(3) = B sigma(cm) for jointed rock masses, where A and B are material constants and sigma(cm) is the uniaxial compressive strength of the rock masses. For a massive rock mass without joints, sigma(cm) is equal to sigma(cd), the long-term uniaxial strength of intact rock. After examining data from intact rocks and jointed rock masses, it was found that for massive to moderately jointed rock masses, the material constants A and B are in the range of 0.4 to 0.5, 0.8 to 0.9, respectively, and for moderately to highly jointed rock masses, A and B are in the range of 0.5 to 0.6, 0.9 to 1.0, respectively. The generalized crack initiation and crack damage thresholds, when combined with simple linear elastic stress analysis, assist in assessing the rock mass integrity in low confinement conditions, greatly reducing the effort needed to obtain the required material constants for engineering design of underground excavations. (C) 2004 Elsevier Ltd. All rights reserved.