Discrete element simulation for seismically-induced soil liquefaction

Presented in this thesis an investigation of the phenomenon of seismically-induced soil liquefaction. In particular, liquefaction under small amplitude cyclic loading is numerically simulated using the Discrete Element Method (DEM) The main programming effort in the presented study involves minimizing the computational time for contact detection. This has been achieved by adopting the grid subdivision method. Two and three dimensional models are analyzed, and an extensive parametric study is carried out for each of them. Accordingly, sand particles are modeled as perfect discs for the two dimensional analysis and as perfect spheres for the three dimensional case. The diameters of the discs or spheres have a probabilistic distribution and the particles are located randomly in space. A method is developed to create arrays of random-sized particles that conform to a target grain-size distribution. All numerical simulations are strain-controlled. The load is applied as a simple shear for the two dimensional simulations. For the three dimensional simulations, two loading conditions are considered: simple shear strain and deviatoric strain loading.; The calibration and tuning of the numerical model is carried out by its application to a number of monotonic shear loading situations. The parametric study has been performed to decide on the number of particles to include in the assembly, the largest permissible time step used in the numerical solution, the maximum shear loading rate and the void ratio.; Three methods of packing the particle arrays are used in two-dimensional analysis to study the effect of the packing method on the fabric characteristics.; The variability in the behavior of specimens is examined by generating three dimensional, statistically identical specimens. The three specimens are subjected each to sinusoidal shear strains of 0.4% and 0.8%. The effects of the initial stress state, the loading direction with respect to the principal stress, the strain amplitude and the void ratio are examined.; The major findings and contributions of this study are summarized as follows. Firstly, this study presents the first extensive investigation of the behavior of granular soil under cyclic loading. Secondly, the study can thus be viewed as validating the use of DEM for liquefaction analysis of soils. Thirdly, different packing methods are found to result in different micromechanical and macro-mechanical parameters for the granular assemblies. Fourthly, the initial stress state is shown to have a measurable influence on the liquefaction potential. Specimens loaded under conditions similar to those of triaxial tests fail under fewer loading cycles. Fifthly, sliding between particles is found to contribute significantly to the build up of the pore water pressure. Finally, the soil liquefaction is found to be highly dependent on the effective mean stress values. (Abstract shortened by UMI.)...