Part I of this thesis investigates the behavior of cemented sands (i.e., bond breakage and stress strain response) with three bond strengths by numerical simulations using the discrete element method (DEM). Bond breakage at different axial strains follows a trend similar to the stress-strain curve trend. Concentrated patterns of bond breakage produce a pronounced local weakness and therefore a persistent shear band in the samples with strong and intermediate bond strengths; the shear-band thickness decreases with increasing bond strength or strain softening rate. In the weakly cemented sample, bond breakage and associated local weaknesses are always randomly formed as shearing proceeds. After the bifurcation point (i.e., around the peak strength), different local responses in the sample...[ Read more ]

Part I of this thesis investigates the behavior of cemented sands (i.e., bond breakage and stress strain response) with three bond strengths by numerical simulations using the discrete element method (DEM). Bond breakage at different axial strains follows a trend similar to the stress-strain curve trend. Concentrated patterns of bond breakage produce a pronounced local weakness and therefore a persistent shear band in the samples with strong and intermediate bond strengths; the shear-band thickness decreases with increasing bond strength or strain softening rate. In the weakly cemented sample, bond breakage and associated local weaknesses are always randomly formed as shearing proceeds. After the bifurcation point (i.e., around the peak strength), different local responses in the sample with strong cementation become distinct. Strain softening and volumetric dilation are observed inside the shear band while the region elsewhere undergoes elastic unloading and an associated rebound in axial and volumetric strains. Such a definite difference in the local strain responses ultimately leads to strain localization in the sample.

In the second part, experimental studies using true triaxial apparatus, and numerical simulations based on the DEM were jointly used to study the stress- and fabric-induced shear-stiffness anisotropy in soils at small strains. Verified by experiments and DEM simulations, the shear modulus is relatively independent of the out-of-plane stress component, which can be revealed by the indistinctive change in the contact normal distribution and normal contact forces on that plane in the DEM simulations. Results also demonstrate that the shear modulus is equally contributed by the two principal stress components on the associated shearing planes. Fabric-induced anisotropy, i.e., the highest G_xy or G_hh, can be explained by simulation findings that more contact normals prefer to distribute along the horizontal direction. The assumption of transversely isotropic fabric in soils is valid based on the DEM simulation results; however, not supported by the experimental results.