Modeling static liquefaction in granular deposits

Static liquefaction failure takes place in loosely packed, water saturated granular deposits, such as soil or tailings. Undrained triaxial compression tests on loosely prepared and fully saturated sand specimens show a peak in shear strength followed by a decrease. For specimens with higher preparation density, the strength beyond the peak decreases and then increases. Monotonically increasing shear strength is obtained for very dense specimens. The existence of peak in shear strength leads to instability during monotonic load-controlled process.; This Thesis focuses on describing this behavior by a particular elastoplastic model, the Superior sand model, and aims at analyzing rigorously the conditions and regimes of instability.; First, to obtain a better agreement between experimental data and the model predictions, the original two invariant formulation of the Superior sand model is revised. In particular, the form of the yield condition is generalized and a new plastic potential is proposed.; To describe adequately the response of the material in three-dimensional loading conditions other than triaxial, a generalized formulation is proposed, where the influence of the third stress invariant on the yield condition and plastic potential is accounted for. The three-invariant model predicts adequately the higher tendency for instability in undrained triaxial extension than in compression as observed experimentally.; The instability states for a given density of the material are represented in the stress space by a surface. This surface intersects the triaxial compression and extension planes along instability lines, which are linear. To measure the severity of liquefaction, the energy of liquefaction is introduced by considering the difference between the applied load and the decreasing/increasing material strength.; Implementation of the two-invariant Superior sand model in the explicit mite difference program FLAC is presented. Simulations of undrained biaxial testing show that the model predicts shear banding if the undrained condition is enforced locally. When allowing for internal flow shear bands may not form.; Finally, the influence of the rate of loading on the material undrained response and instability occurring during undrained creep are described with the help of an elastic viscoplastic model.