1. Introduction

THE PROBLEMS entailed by the stability of rock slopes are among the most difficult with which the profession is faced. Yet they are not exclusively academic, as amply evidenced by disasters such as the Malpasset Dam abutment failure and the Vajont rock slide.

Owing to the scale effect, strength properties vary generally with specimen size, and little or nothing is known about the laws governing this variation. Fortunately, however, there is practically no scale effect when the residual shear strength of a continuous layer of soft material is considered. This is the case of fault gouge or sedimentary clayey joints which are responsible for most failures of natural rock slopes. In view of this, and in the present state of knowledge, only rock slopes with surfaces of separation of large extension can be satisfactorily analyzed for stability.

A three dimensional method of analysis of such slopes has been worked out. The principle of the method will be described first and then a few examples will be given of its application to dam abutments. It is obvious that the process is perfectly valid for rock slopes other than dam abutments provided there exist large surfaces of separation.

2. The principle of three-dimensional analysis

Assumptions. The stability of the slope is analyzed in terms of the stability of a given rock volume (Fig. 1). The internal faces of the volume are plane: the surface (O, B, C) or plane P1; the surface (O, C, A) or plane P2; the surface

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