Assessing shear strength of rock discontinuities through laboratory testing on replicas

Abstract

Natural slopes and rock masses exposed by excavation are susceptible to sliding failure due to the presence of discontinuities. Discontinuities are considered as planes of weakness in the intact rock, along which failure may occur. Kinematic analysis of rock masses is generally followed by a stability analysis, with shear strength properties being the critical input parameters when considering sliding planes. Direct Shear Box Testing is a standard test method that is used to obtain the shear strength characteristics of rock discontinuities. This dissertation presents a method for the creation of replica synthetic discontinuities utilizing liquid silicone rubber to generate moulds from which Plaster of Paris specimens have been cast. The method is proven to enable the repeated production of identical specimens, and thus eliminating the inherent natural heterogeneity of discontinuities. Four different surface configurations were tested using synthetic replicas, to investigate how the shear strength is affected as roughness, interlock and contact area vary. Testing showed that Plaster of Paris is a suitable synthetic substitute to Lower Globigerina Limestone (LGL), but at the lower extreme of strength and density. The highest shear strength envelope was achieved by the rough and fully interlocking joint. It was seen that the linear Mohr Coulomb failure criterion may not be applicable in this case since it tends to overestimate shear strength at lower normal stresses. Non-linear envelopes, such as the Barton criterion which utilizes the JRC and JCS parameters, provide a better fit for rough interlocking joints even at lower normal stresses. The lowest shear strength envelope resulted from the smoothrough joint due to minimal contact, which was further exacerbated by the smooth surface. The Barton criterion should be used cautiously or perhaps even avoided for highly mismatched joints, that is those having a low JMC. Dilation and contraction were also recorded with the lowest normal displacements recorded for the smoothsmooth joint as expected. The Mohr Coulomb failure criterion is seen to be applicable in this case.