Slope stability for planar, polygonal and wedge—shaped sliding surfaces: A formula for the design of anchors in rock slopes is presented. Since several parameters can be lumped together in two factors and the cohesion appears in an explicit form the formula can be used to carry out parametric studies with little expenditure of time. Due to an analogy between plane failure and the sliding of a wedge on two planes the same formula can be used to describe the three dimensional problem. An extension of the work to problems involving polygonal sliding surfaces is presented, which is based upon the hypothesis that due to kinematic considerations a discrete number of internal slip surfaces must exist in the rock mass. These internal slips may take place on preferred surfaces, which arise from the actual geological situation. In the majority of cases, however, new ruptures are created, that depend only partially on existing planes of weakness. In the numerical procedure the rock mass is divided up into discrete elements governed by the assumed internal slip surfaces. The basic formula mentioned above is then applied to each element separately. The internal forces acting on the interfaces between elements are defined by an additional failure condition. In an example taken from rock engineering practice it is shown how great the influence of the rock mass in resisting the development of such internal slip surfaces is with respect to the stability of the slope.

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