ABSTRACT
The mechanical properties of the rocks forming the upper part of the earth crust are affected by weathering, i.e. by the physical and chemical action of environmental agents. The amount of bonding between rock grains and their strength decrease with time so that, at the macroscopic level, a progressive reduction of rock cohesion takes place. The height of a rock cliff is directly proportional to the value of the cohesion. Its progressive reduction causes therefore slope failures and the retreat of the slope crest. Examples of such a phenomenon are reported by e.g. Hutchinson (1973), Kirkby (1984), Yokota & Iwamatsu (1999). Various types of materials are affected: clay shales, sandstone, chalk, pyroclastic deposits. The speed at which slope retreat occurs is usually rather low, but, in volcanic or carbonatic rocks, it is fast enough to jeopardize the safety of buildings or infrastructures located nearby.
