As an example, for a 50 MPa rock (E=12.5 GPa, E/G=100, K=1.8), the predicted failure height above a 5 m by 3 m roadway is 6.25 m. Building on this work, this paper proposes that, for highly overstressed sedimentary rocks, a spalling limit of 7.5 can be used to predict the maximum height of failure. Fortunately, one of the case studies used by Martin, Kaiser, and McCreath (1999) to develop their brittle rock model involved a tunnel in sedimentary rocks.
Stress redistribution depends on the ratio of Young’s Modulus to the Independent Shear Modulus (E/G)įor layered rocks, the Generalised Hoek-Brown criterion is not considered valid as at no scale can the rock mass be considered isotropic. As an example using Rocscience RS2 is a simple metric for anisotropy that is the ratio of Young’s Modulus to the Independent Shear Modulus (E/G): thinly layered rock has an E/G ratio of 100, moderately layered rock a ratio of about 25, and massive rock (=isotropic) a ratio of 2.5.The models presented in Figure 1 taken from RS2 show contours of the ratio of the major and minor principal stress for a horizontal stress ratio of 1.8. One result of adopting transverse anisotropy is the recognition of significantly different patterns of stress redistribution around rectangular roadways such as those used in coal mining. This paper presents an approach to examining the deformation and support of highly stressed sedimentary rock that assumes an anisotropic continuum with an equivalent Independent Shear Modulus (E/G) readily calculated from the spacing of the layer discontinuities and joint stiffness in the order of 1000 MPa/m. Although this type of rock can never be considered as an isotropic continuum, it is also not practical to explicitly model all the possible discontinuities as joints. In addition to there being a discernible texture within the matrix, layers can be in the form of discontinuities with spacing sometimes measuring less than 10 cm. Seedsman Geotechnics Pty Ltd, Mittagong, Australiaįine-grained sedimentary rocks such as mudstones and sandstones are layered and of relatively low strength.
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