Stability assessments of large-scale rock slopes in mountainous regions and in deep open pit mines necessitate the implementation of approaches in which strain sensitivity of various slope forming materials is considered. As the rockmass is strained, a gradual degradation of rock slopes and thus its stability occurs with time. The progressive failure process of different rock types evolves differently depending on the rocks deformability or brittleness characteristics. This paper presents a strain-dependent mobilization law for the development of the shear strength along both the basal and the internal shear planes and uses this law to explain the gradual process of stability degradation of rock slopes. It is demonstrated that the stable geometry and the shape and location of the shear planes in slopes depend on the manner by which the strength in various slope-forming geo-materials is mobilized and lost, i.e. rock's brittleness.
The stability of rock slopes concerns both civil and mining engineering activities and may involve building, safe maintaining and economical operating of both natural and man-made slopes. Unlike other engineering materials such as steel or concrete the properties of the involved geo-materials in slopes are not pre-selected and one normally deals with a ground that is heterogeneous at all scales. The heterogeneities of rock masses affecting the stability of rock slopes include: micro-scale heterogeneities related to grains size distribution (fabric), mineralogy, the nature of bonding between the grains, porosity, etc., minor discontinuities (relative to the scale of slope) such as joints, fractures, foliation and bedding planes, major heterogeneity due to rock type changes, and finally major structures such as faults, folds, and other regional geological structures. These structures inside the rock mass interact in different ways, as a slope is deformed and thus affect the global behavior of a slope. Furthermore, the observed displacements and possible failure mode of a rock slope are related to changes in geometry and boundary conditions (stress and pore pressure), and material behavior with time. Various types of rock slope failures are first described. The strain-sensitivity of the failing geo-materials is then discussed and the implications of the strain-sensitivity of geo-materials are demonstrated in stability assessment of vertical and inclined rock slopes.
From a rock mechanics point of view the failure of rock slopes can be divided into two distinct categories. The first category deals with failure mechanisms that are primarily controlled by (nearly planar) geological structures. Examples belonging to this category include: plane failure, whereby slip is controlled by a single discontinuity with or without tension cracks (Fig. 1a), wedge failure where slip occurs on two or more discontinuities (Fig. l b), and irregular failure geometries with defined basal planes, that include major discontinuity (relative to the slope height) and/or several minor discontinuities (step-path failure surfaces) (Fig. 1e and Fig. 1d). Planar and wedge failures are common in small-scale (20m to 50m) slopes. In large-scale rock slopes these failure types may occur if there is a single pre-dominant discontinuity such as fault.