Instability phenomena in hard soils-weak rocks: Importance of the anisotropy

Hard soils and weak rocks constitute a borderline material group where, in many instances, it is difficult to apply properly behaviour models either from pure soil or rock mechanics. This situation becomes more evident in slope stability analyses where the usual geotechnical models for soil (for example, slices method) or rocks (for example, wedge analysis) tend to underpredict the factor of safety, leading to a design of the slope on the unsafe side. This panel ilustrates this fact by means of two examples, taken each one from the hard soil and the other from the weak rock case. The cases shown indicate the important role of the strength anisotropy of the material in the triggering of the failure.