RIVISTA ITALIANA DI GEOTECNICA 4/2005 Modelling Damage with Generalized Plasticity J.A. Fernández-Merodo,* ,1 R. Tamagnini 1 , M. Pastor* ,1 , P. Mira* ,1 Abstract This paper presents some improvements of the generalized plasticity model proposed by [PASTOR et al., 1990] that can be used to reproduce “damage” phenomena in geomaterials. First, a simple improvement is introduced to reproduce the mechanical behaviour of bonded soils or weak rocks. In this case, the bond degradation depends only on the plastic strains and affects the plastic modulus. In the limit case, when destructuration is complete, the proposed model coincides with the original one. Then, a hierarchical enhancement of the basic stress-strain relation is also presented for unsaturated soils. It is obtained by introducing a second mechanism of plasticity. Hydraulic hysteresis is reproduced by taking into account the water storage mechanism. In the limit case, when the saturation degree is equal to one, the basic saturated model is reco- vered, and the transition between saturated and unsaturated conditions takes place without discontinuity. Other degrada- tion phenomena as those caused by thermal or chemical effects can be modelled with the same constitutive assumptions. The main advantage of using generalised plasticity theory is that it gives a simple yet efficient framework within which it is possible to reproduce not only monotonic stress paths but also cyclic behaviour. Finally, some numerical validations of the proposed improvements are described. 1. Introduction Flowslides characterised by a sudden failure and a rapid and extensive runout, such as those caused by the earthquake in El Salvador in February 2001, Figure 1, are responsible for major damage to lives and property. It is therefore important to study this kind of landslides to asses the risks and to propose remediation measures. Prediction of the conditions under which failure will take place, and its conse- quences is of paramount importance. Failure mechanisms can be of different types. In some cases, the failure mechanism consists on a clearly defined surface where shear strain concen- trates, while in other cases the collapse is due to an important increase of pore pressures and a corre- sponding decrease of the effective stress. This is the case, for instance, of the liquefaction of a layer of very loose saturated sand induced by an earthquake. This mechanism of failure that affects a much larger mass of soil, can be referred to as “diffuse” [DARVE & LAOUAFA, 2001] and it is characteristic of soils pre- senting very loose or metastable structures with a strong tendency to compact under shearing. It has to be mentioned here that this mode of failure can be exhibited also by non-saturated soils such as those of volcanic origin. Indeed, collapse of the material under the loading induced by an earth- quake can cause a major pore air pressure increase and eventually lead to a “dry” liquefaction [BISHOP, 1973]. In the absence of obvious evidence for the ex- istence of positive pore-water pressure and the ob- servation of the extensive run out in Figure 1, Las Colinas flowslide could be explained by this mecha- nism [PASTOR et al., 2002], [EVANS & BENT, 2004]. Be- fore arriving to the needed loose state, the loss of in- itial strength in Las Colinas can be explained by a destruction of suction forces in the partially satu- rated soil, i.e. by a destruction of menisci during the earthquake, and/or a destruction of cementation in the pyroclastic material, i.e. destruction of bonds during the earthquake. Simple constitutive models cannot be applied to reproduce the collapse and the behaviour described * Centro de Estudios y Experimentación de Obras Públicas (CEDEX), Madrid, Spain 1 M 2 i (Math. Model. Eng. Group), Dept. of Applied Mathema- tics ETS Ingenieros de Caminos, UPM Madrid, Spain Fig. 1 – Las Colinas landslide (El Salvador), February 2001. Fig. 1 – Frana di Las Colinas (El Salvador), febbraio 2001.