Study of Soil Compaction Using X-Ray Computed Tomography

The maximum dry density and optimum moisture content obtained from the laboratory compaction curve have been used customarily to characterize the field behavior of compacted soils. It is well known, however, that the microstructure of compacted soils is dependent on the method of compaction. The structure has an important influence on the engineering behavior of compacted soils. Therefore, in order to provide a better description of compacted soils, methods that can quantify the changes in microstructure are needed. In this study, compacted specimens at various densities and water content were scanned using X-ray Computed Tomography (CT). It has been found that there is direct correspondence between the CT numbers, soil dry density and moisture content. The scanning observations showed also the development of shear planes parallel to the surface of the compacted soil, and changes in structure of the soil towards a more uniform arrangement around the point of optimum moisture content. Compaction of the soil beyond the optimum moisture content appears to disperse soil particles with an overall uniform structure

Multi-slip gradient formulation for modeling microstructure effects on shear bands in granular materials

This paper presents a higher order gradient multi-slip formulation to model the effect of inhomogeneous deformation in granular materials. The effects of heterogeneity and porosity anisotropy within the multi-slip formulation are taken into consideration through the modification of the mobilized friction. The mobilized friction is assumed to be a direct function of either the gradient of the porosity distribution or the fabric tensor. The formulation with two active slip planes was implemented into a finite element code and used to simulate biaxial shear tests on dry sand. The analysis quantifies most of the shear band characteristics observed by past experimentation. It is shown that the localization and shear band characteristics in granular materials are very much dependent on the initial fabric and slip system arrangement

Stress distribution in granular heaps using multi-slip formulation

SUMMARY Many experiments on conical piles of granular materials have indicated, contrary to simple intuition that the maximum vertical stress does not occur directly beneath the sand-pile vertex but rather at some distance from the apex resulting in a ring of maximum vertical stress. Some recent experiments have shown that the observed stress dip is very much dependent on construction history. A multi-slip model has been proposed to investigate the stress dip phenomenon in granular heaps. The double-slip version of the model was implemented into ABAQUS and used to study the vertical stress distribution along the base of a granular pile. The numerical simulations show that plastic deformation is confined within the localized region around the apex while the rest of the pile is in an elastic state of deformation. Within the plastic region the stress distribution differs significantly depending on the initial active slip orientation. The results show that for homogenous state of granular materials such as those produced by a raining procedure the vertical stress profile along the base reached its peak at the apex (i.e. no dip was observed). On the contrary, granular heaps constructed by the use of a localized source such as a funnel resulted in a significant reduction in the stress distribution within the ring with the minimum attained beneath the peak (i.e. a dip). Therefore, we believe that the initial microstructure and thus the initial slip orientation resulting from sand deposition is the source of the stress dip phenomenon