Study of the undrained static response of sandy soils in the critical state framework
The thesis reports on experimental research on the static undrained response of silty and clayey sands. The purpose of this testing program was to understand the stress-strain response of sands with small percentages of fines. With this objective in sight, the testing program involved understanding and delineating the effect of four variables: density of the soil, percentage of fines in the sand matrix, plasticity of the fines, and the mode of deformation only under isotropically consolidated, and static loading triaxial conditions. Four characteristic states were identified in the undrained response of the soils investigated: the undrained instability state, quasi-steady state, phase-transformation state and critical state. The experimental data was analyzed in the context of the critical-state framework. The critical state was found to be independent of the initial fabric and of the pre-shear stress history of the sand. The critical-state friction angle increased slightly with the addition of small percentages of silt. The effect of silt on the characteristic states of undrained shear was evaluated and ways to relate the critical state with these states of behavior were proposed. The concepts used in constitutive modeling of sands can be extended to silty sands using the suitable input parameters. The plasticity of the fines had an impact on the response of the sand. The variables conventionally used in understanding the behavior of sands with small amounts of fines were evaluated, and the validity of the different variables was assessed. It was found that extension of modeling concepts of clean sands to clayey sands cannot be done directly. Comparison between triaxial extension and triaxial compression tests conducted on 0 and 5% silty sands showed that the behavior under triaxial extension conditions was more contractive than under triaxial compression for both clean and 5% silty sand. This experimental program aims to provide a well designed and complete data set for systematic understanding of the static behavior of sands and sand-fine mixtures. This data set can also be used to calibrate constitutive models designed for sandy soils.