Reinterpreting the saturated clayey soil behaviour undergoing desiccation

Desiccation cracks affect the stability of earth-works or natural slopes. The available simplified models for crack initiation and propagation are based on the hypothesis that desiccation cracks start from the surface of an evaporating soil layer. Experimental results available at Politecnico di Milano and at Delft University of Technology shows that in some conditions the cracks may start and propagate in the core of the layer. The presence of embedded cracks can compromise the reliability of a geotechnical structures. Based upon experimental results, a simple one-dimensional model was set up to analyse the behaviour of a homogeneous soil layer undergoing surface drying. In particular the role of geometrical constrains, the soil properties dependence on water content and the dependence on desiccation rate were discussed. In the model the idealised crack initiation condition is defined as the moment when the total tensile stress attains the same value as the tensile strength. The theoretical formulation point out that the governing equation of total horizontal stress is dominated by shear modulus. The results of the model show that dependence of material behaviour on the rate of water content change is a necessary requirement for cracks to initiate below the surface. The key factor is the dependence of the drying branch of water retention curve of the compressible soil on the rate of drying. This would be justified by a rate dependent fabric evolution. For this reason MIP analysis were performed on clay samples. The results show that different drying paths has different pore size distribution. In conclusion this framework was used in order to reinterpret tensile tests performed at the Delft University of Technology.