The purpose of this paper is to suggest a numerical method for the evaluation of the deformability modulus of discontinuous rock masses. The study is conducted by assuming that the fracture density and the length of cracks, contained in a rock mass, depend on a negative exponential statistical distribution. Compressive tests are simulated on rock volumes containing open and closed flat cracks through a numerical computation code based on the Displacement Discontinuity Method. It proved impossible to determine the deformability modulus of the Representative Volume Element (RVE) in numerical terms. However, for a set of specimens containing even as few as 9 open cracks, the mean value of the deformability modulus is seen to approach the value determine d for the RVE analytically. Based on this observation, the calculation method is applied to determine the deformability modulus of specimens containing closed cracks. The numerical results show that the deformability modulus of a rock mass decreases as function of the fracture density and the maximum crack length if it is compared to that of rock material.


The evaluation of the value of the deformability modulus to be assumed for the rock mass is a difficult task when a continuous equivalent approach is used. As a matter of fact, the values obtained through laboratory tests on small specimens, containing only micro- fractures, are different from those obtained through large scale tests on the rock masses containing natural discontinuities of different orders of magnitude. For this reason laboratory values cannot be directly assumed in the analysis of real rock structures. For engineering purposes, empirical correlations are used linking quality indices of the rock mass with the value of the deformability modulus determined in situ (Bieniawski, 1976; Barton et al., 1974).

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