Small Strain Response and Failure in Granular Soils

Geotechnical practices and research range over problems from small to large strains. The small strain modulus Gmax is widely recognized as a critical input for multiple constitutive models used to describe the static and dynamic behavior of granular materials (Santamarina et al. 2001), so is important for design and analysis of various geotechnical problems. In practice, Gmax is normally indirectly obtained by measuring shear (S-) wave velocity Vs , using laboratory or field techniques such as bender element and seismic cone. There are also previous experimental studies aim to correlate small-strain properties to large-strain response in soil. For cemented sands in particular, S-wave velocity are used to assess the cement degradation, which has significant effects on the stress-strain-volume response of soil under various loading conditions. Compared to the conventional drained triaixal compression, constant deviator shear (CDS) is a special loading condition that is less studied and not well understood. It is reported both in the laboratory and the filed to cause instability in soil in a diffuse manner under drained condition and before the conventional failure criterion is met.;Discrete element method (DEM) simulation is a useful tool for investigating the complex behavior of particulate materials in conjunction with laboratory tests. It allows for both macro-scale (specimen level) and micro-scale (particle level) observations, providing access to the parameters of interest which are difficult or very expensive to be directly observed in standard laboratory practices. The remainder of this dissertation is focused on utilizing DEM simulations to study the aforementioned topics for granular soils, regarding: fundamentals of S-wave propagation and measurement, cementation effects on small and large strain response and drained diffuse instability.;A DEM study of S-wave propagation in particulate assemblies in an environment similar to the laboratory bender element test was presented. Simulation outputs reveal the complex internal responses of particulate assemblies subjected to S-wave excitation, in aid of the interpretation of receiving signals. S-wave generation and measurement were presented first, followed by an examination of system response in both particle and specimen level. Receiving signals from multiple receivers were interpreted with different methods. Finally the effects of excitation signals, particle properties and stress state on S-wave velocity were analyzed.;Parallel bonding model (ITASCA, 2006) was used to simulate cement agent in granular soils. Special emphasis was placed on the analysis of decementation under isotropic consolidation and drained triaxial compression. The basic model parameters were calibrated base on the experimental results by Sharma et al. (2011). Then the effects of cement content, stress state and two distinct load-cement histories: load-before-cement (LBC) and cement-before-load (CBL) on S-wave velocity were shown and discussed. Finally the stress-strain and volumetric response, and the evolution of S-wave velocity during drained triaxial test of cemented specimens were presented and compared to experimental results (Sharma et al. 2011). Micro-scale analysis is highlighted in regard of bond network, normal contact force, local porosity and local coordination number.;The drained diffuse instability, as an intrinsic nature of granular material was reproduced by DEM modeling when the constant deviatoric stress path is applied. The loss of controllability reported in laboratory tests was observed. The onset of instability was predicted with satisfactory accuracy compared to laboratory and elastoplastic model simulation results. For the dense specimen, the onset of instability occurs with stress states above the CSL but below the FL in 'p-q' space. For the loose specimen, the onset of instability occurs below the CSL. Analysis on the evolution of contact orientation during CDS test using spherical histogram and fabric tensor shows increasing degree of anisotropy and a diffuse failure mode.