An empirical criterion of intact rock failure under the general state of polyaxial compressive stress is developed. The criterion is shown to predict an elliptic failure envelope for the case of biaxial compression, and a parabola of the Hoek and Brown type for the case of triaxial stress. The model prediction for failure of intact rock under multiaxial compression is shown to conform to experimental observations. A new rock mass index, ë, which takes into account the geometric (joint orientation, size, frequency, and the number of joint sets) and mechanical (joint friction coefficient) properties of the discontinuities in a rock mass, is introduced. This new rock mass index eliminates the elements of subjectivity generally associated with the determination of similar indices. The new rock mass index is demonstrated to capture the anisotropy of jointed rock strength for the case of uniaxial compression, and is related to dimensionless forms of the discontinuity geometric and mechanical parameters by an exponential decay function. This index is then used to generalize the three-dimensional intact rock failure criterion developed herein to that of jointed rock failure. The obtained jointed rock failure criterion is demonstrated to conform reasonably well to experimental observations for the case of biaxial compression. The decay constant in the exponential decay function is shown to be related to the normalized confining stress for the case of biaxial compression.


The typical state of in situ stress in a rock mass, as observed in field measurements [1], is polyaxial, which is the state of stress where the three principal stresses are generally unequal (ó1 = ó2 = ó3 = 0) Failure of intact rock under a state of polyaxial stress is assumed to occur when some functional relation between the three principal stresses ) , , (ó1, ó2, ó3) is satisfied. This relation, termed the criterion of failure, has the functional form given


[Equation available in full paper] (1)

where ç1=( ç I,1,..., ç I,N)is an N-dimensional vector

representing all intact rock model parameters. The failure criterion given by equation (1) represents a geometric surface in a three-dimensional space defined by the principal stresses. Much research has been devoted to mapping out this surface for different materials with the intent of developing the exact form of the function that could be applied to all rock materials [2]. The general information about the failure surface available at present has been obtained through uniaxial, biaxial and triaxial compression experiments. Uniaxial tension and compression tests yield, respectively, uniaxial compressive, ó u , and tensile, T o , strengths.

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