Shear induced evolution of structure in water-deposited sand specimens

Sand specimens reconstituted in the laboratory by water pluviation are considered to best simulate the structure of soil masses deposited through water in the natural environment. However, while this was conceptually acknowledged, no significant data or evidence was presented to support this postulate. In this study, apparatus and procedures for water pluviation were developed to produce uniform water-deposited sand specimens. As-consolidated water-deposited specimens of different densities were preserved by epoxy resin impregnation technique and their microstructure was quantified and analyzed based on the images captured at different locations of a hardened specimen using digital image processing and analysis system. The quantification of soil structure was performed in terms of particle orientation, local void ratio distribution and spatial void ratio distribution. Entropy, a statistical parameter, was introduced to characterize distributions of particle orientation and local void ratio. The structure of naturally deposited sand (CANLEX) specimens was also studied using the same methodology of specimen preservation. It was found that the structures of naturally deposited specimens and reconstituted water-deposited specimens are comparable in terms of particle orientation and local void ratio distribution.; To study the evolution of structure during shearing tests, water-deposited sand specimens were subjected to axial compression loading under constant confining pressure and the tests were then terminated at different levels of axial strain along the stress strain curve. After the shearing tests, the structure of strained water-deposited specimens was preserved and quantified following the same procedures as unstrained specimens. The evolution of structure was investigated accordingly. It was found that soil particles in water-deposited specimens reorient horizontally as shearing progressed. The standard deviations of local void ratios in dilated regions of strained water-deposited specimens were found to be smaller compared to those of as-consolidated specimens for a given void ratio. The dilation of water-deposited specimens during shearing tests was found to expand uniformly toward both ends of the specimens due to the restricting effect of end platens. Together with the corresponding data of air-pluviated and moist-tamped specimens obtained from previous studies, both as-consolidated and strained, a comprehensive comparison between these specimens was conducted. Consequently the effects of specimen preparation method on initial structure and shearing behavior of reconstituted sand specimens were identified.