The effects of strength anisotropy of rock masses in slope stability problems are the focus of this paper. The source of the strength anisotropy can be attributed to many factors and the motivation of this study is to concentrate on the effects of planes of weakness, such as joints and bedding planes on the slope stability analysis. The limit equilibrium approach and finite element with shear strength reduction method are used for numerical simulations of slope stability problems. The finite element simulations take advantage of a constitutive model with embedded weak planes. The results obtained from the two methods are compared with each other.
Natural soils and sedimentary rocks, such as shale, limestone and mudstone, are typically formed by deposition and progressive consolidation during formation. Such formations usually have a distinct internal structure, which is characterized by the appearance of multiple sedimentary layers. Besides the bedding planes, the geometric layout of networks of joints and other types of discontinuities in a rock mass are significant contributors to the complex behavior of such geomaterials (e.g. Hoek & Brown 1980, Hoek 1983, Zienkiewicz & Pande 1977).The presence of these fissures and planes of weakness significantly influence the response of geotechnical structures such as slopes, tunnels and excavations (Goodman et al, 1968, Bandis et al, 1983).
The jointed rock mass here is considered to be composed of an intact material that is intercepted by up to three sets of weak planes. The spacing of the weak planes is such that the overall effects of the sets can be smeared and averaged over the control volume of the material. Such a configuration with two sets of weak planes is illustrated in Figure 1 where the weak planes are oriented at an arbitrary angle θ1 and θ2 in the rock mass.
In this paper, it is assumed that the failure of the matrix can be described by the Generalized Hoek-Brown criterion presented in Equation 1. The strength criterion of the weak planes is formulated by a simple Coulomb criterion as in Equation 2.