Mechanisms leading to degradation of shales are first described from a qualitative perspective. Stress changes and suction cycles are stressed as important contributors. The effect of these two effects on the degradation of clay rocks around tunnels are then described, following recent results of a few large scale experiments performed in underground rock mechanic laboratories. Also, examples of claystone degradation in excavations exposed to atmospheric action are reviewed. A number of laboratory experiments, which involve, in general, the application of loading and suction cycles are presented. They highlight the relevance of extreme suction cycles and the importance of rock mineral constituents. Constitutive models for bonded soils are aoutlined. The most recent developments describe the clay rock as a composite of an elastoplastic clay matrix and a brittle damage model for the bond constituents. The paper ends with the description of a model in which the clay matrix model expands and shrinks under suction cycles. Then, realistic predictions of weathering effect can be made.
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