A damage model for unsaturated natural bess submitted to cyclic loading

High speed railway from Northern France has encountered several stability problems in zones where loessic soils are present. Important sinkholes have been observed and were mainly due to the collapse susceptibility of the encountered bess when submitted to the cyclic loadings imposed by the passage of the high speed trains. This collapse susceptibility seems to be related to the degradation of the cemented bonds and to either the collapse under wetting at constant applied load or liquefaction depending on the natural water content of the soil. In this paper, a constitutive model is developed to gain insight into cyclic behaviour of theses soils. This model is an extension of a model previously proposed by the authors for modelling degradation of bonds and liquefaction potential of natural cemented soils under saturated states. The platform model, from which the extension is carried out, is based from one hand on the bounding surface plasticity theory for the description of the cyclic response of the soil and is inspired on the other hand from the work of Vaunat & Gens (2003) concerning bond degradation modelling. Influence of non-saturation effects is introduced following an approach similar to that of the Barcelona Basic Model (Alonso et al. 1990). The developed model is thus capable to describe the mechanical behaviour of unsaturated bonded soils under cyclic loading.