Progressive failure of jointed rocks

Abstract

As jointed rocks consist of joints embedded within blocks of ‘intact’ rock, the behaviour of these rocks will depend on the behaviour of the joints and the intact rock blocks. In jointed rocks, there are two levels of heterogeneity: (1) the differences in properties between the rock blocks and the joints and (2) the heterogeneity within the rock blocks due to the differences in the degrees of weathering and the distributed flaws. Furthermore, as water normally exists in jointed rocks, the pressure and flow of the water will also have a significant effect on these parameters. In this paper, a finite element model, based on Biot’s theory, is developed to predict the behaviour of jointed rocks by taking into account the above factors. The joints are modelled by elements with low modulus and strengths; whereas the heterogeneity of the rock properties of the ‘intact’ rock is taken into account by assuming that they follow Weibull’s distribution. Four specimens with joints inclined at different angles, namely 0ͦ, 45ͦ, 90ͦ and 135ͦ are employed to study the effect of the orientation of the joints on the slip surface. The joints are discontinuous and they are modelled by adopting a weaker material without considering the effect of the pore fluid flow. The results show that a rock specimen with joints normal to the shear direction is more brittle. Furthermore, the shear strength is largest when the pre-existing joints are normal to the shear direction. It is minimum when the joints are horizontal. The minimum shear resistance is only about 60% of the uniaxial compressive strength, indicating the strength of the rock mass can be very much reduced by the joints (Fig. 1). An example of a vertical rock slope is used to demonstrate the effects of the water table on the failure of slopes.