Determination of Stress Strain Characteristics of Sand Packs Under Uniform Loads by the Use of Computer Assisted Tomography and Finite Element Modelling

ABSTRACT

The local changes in the porosity of dry sand packs as a function of the imposed overburden were determined using computer Assisted Tomography. Pressure cycles were imposed on sands of different characteristics and the porosity was measured at a resolution of 0.075 cm x 0.075 porosity was measured at a resolution of 0.075 cm x 0.075 cm x 0.3 cm. Hysteresis in the porosity was observed. The imposed overburden and pore volume change were translated to stress and strain respectively. The local porosity changes depict the local strain under a given load. porosity changes depict the local strain under a given load. A two dimensional finite element model was developed to predict the microscopic behavior of sand. The model predict the microscopic behavior of sand. The model provided an excellent correlation between measured and provided an excellent correlation between measured and calculated strains. The mean and standard deviation differed by 0.509 % and 0.35 % strain respectively, from the measured data. However, the model had difficulty in predicting dilation and extreme compression (5 to 6 % predicting dilation and extreme compression (5 to 6 % volumetric strain). It was also found that, in terms of elastic moduli, the model indicated that a macroscopically homogeneous sand is highly heterogeneous on the macroscopic scale. Under these conditions, local stresses varied sharply and stress components on an element tended to be non-hydrostatic.

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