Testing and modelling of silty and sulphide-rich soils

Abstract: Silt and sulphide soil are two types of soil which are dominating in northern Sweden, especially in the coast area of the Baltic sea. The stress-strain and strength characteristics of these two soils have so far not been as much explored as those for e.g. Scandinavian clays. With increasing use of numerical methods in geotechnical engineering, better knowledge of the constitutive behaviour of soils is needed as well as more accurate models. The work reported in this thesis is an attempt to achieve such a knowledge for silt and sulphide soil. The drained and undrained stress-strain characteristics of silt have been investigated by means of triaxial compression and extension tests. The compressibility, the dual characteristic of contractancy and dilatancy during shearing, the effect of the clay content and the dependence of the failure surface on the Lode angle have been specially discussed. The drained and undrained stress-strain characteristics of sulphide soil were investigated by means of triaxial and CRS oedometer tests. The compressibility, the effect of the excess pore pressure and the anisotropy have been specially discussed. A plasticity model was proposed for modelling the constitutive behaviour of silt. Among others, the model takes into account the dual characteristic and the dependence of the yield criterion on the Lode angle. An analytical solution was carried out for the undrained triaxial state which links the dilatancy parameter to other parameters. Conditions of material stability were also derived for the triaxial stress state. A plasticity model was proposed for modelling the constitutive behaviour of sulphide soil. Among others, the model takes into account the cohesion and the initial and stress-induced anisotropy. An explicit relation was derived to relate failure or critical state parameters in the isotropic case to those in the anisotropic (K0) case. A calibration method based on optimization was designed to improve the overall quality of model parameters. By using this method, the two proposed models were then calibrated to experimental data from several sets of tests on silt and sulphide soil. Two boundary-value problems, with available data from field tests, were analyzed using a finite element code with the proposed models implemented to demonstrate the capability of the proposed models and of the numerical method.