Influence of the spatial variability on the slope stability of mainly cohesive deposits

Artificial and natural deposits are formed by successive layers of materials with varying properties. The variability prevailing in such a structure influences the stability of slopes. In a first part of this paper, a simple method is proposed to model the main features of the spatial variability of the properties in a layered medium. These features include the overall trend and its dispersion. The latter is described in terms of an auto-correlation function, able to reflect the more or less regular pattern related to the periodicity of the dumping phenomena. The proposed model can often be reduced to a small number of independent random variables, smaller than the number of separate layers forming the deposit. The second part introduces a probabilistic computational procedure allowing the use of large numbers of random variables with standard engineering methods. Finally, the application to a slope in a cohesive layered deposit is presented, and the effects of the variability of the properties are analyzed. For large dispersions of the cohesion, the behavior becomes complex. The slope surface can be "attracted" by a randomly located weak layer, modifying the geometry, and accordingly the expected value of the safety factor and its related uncertainty. As a conclusion, deterministic methods, based on mean properties for the deposit materials, can overestimate greatly the safety of slopes in layered natural or artificial deposits.