High lateral strain ratio in jointed rock masses

Rock masses encountered in the field are jointed and highly anisotropic. An important manifestation of this characteristic of rock masses is that the blocks, when slipping, separate and create voids in the mass resulting in high lateral strain even under low axial stress. It has been shown in this study that the ratio of lateral to axial strain may be very high, especially, if the joints are critically oriented. The assumption of isotropic linearly elastic material is not applicable in such situations. This observation is based on the outcome of an extensive laboratory testing programme, in which a large number of specimens of a jointed rock mass with various joint configurations were tested under uniaxial loading conditions. The trends of experimental results for both lateral strain ratio and rock mass strength have also been verified through distinct element modelling. The reason for high lateral strains has been attributed to the creation of voids and also to the fact that permanent deformations due to slip commence along rock joints right from the start of loading process. A simple mechanistic model has also been suggested to explain the high values of lateral strain for rough and dilatant rock joints. The lateral strains in such situations are important in the design of rock bolts. An example from a recently completed hydroelectric project has also been discussed where high lateral strain was found to be responsible for excessive wall closure and failure of rock bolts.