An analysis of geotechnical problems involving strain softening effects

The phenomenon of strain localization and the associated softening effects are here approached from a macroscopic viewpoint. A finite element model is proposed which includes two possible aspects of the phenomenon. The first one, referred to as structural softening, leads to a criterion for detecting the onset of localization which depends on the current values of the stress components. A simple analytical example is discussed which shows how this approach can lead to strain softening even for elastic perfectly plastic materials, in the presence of a non-associated flow rule. The second approach to the phenomenon, referred to as material softening, assumes that the initiation of softening depends on the accumulated plastic strains. Some applications of these approaches are discussed, concerning the situation of small-scale model tests on shallow tunnels. On the basis of the comparison between experimental and numerical results some comments are presented on the effectiveness of the procedure in capturing the collapse mechanism of the opening.

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