Experimental Investigation of Unsaturated Soil Stiffness

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Soil stiffness is a strain dependent non-linear parameter. Stiffness is the measure of soil deformation under a particular working load. It can used for predicting ground deformation in engineering earthworks such as highway embankments and foundations. Non-linear analyses for ground behavior have been widely used for developing models to predict the small –strain characteristics of the soil (Atkinson, 2000). This thesis provides a new insight into the behavior of soil stiffness. We used the wave propagation technique to determine the stiffness and term it as the small strain shear modulus (Gmax) of the soil. The objective of the thesis was to predict the influence of wetting/drying cycles on unsaturated soil shear stiffness. Low-plasticity Silty Clay (CL) was tested to determine the variation of soil stiffness along its wetting/drying path using a one dimensional soil water characteristic curve apparatus and an unsaturated triaxial cell. The pore water pressure of the soil was zero as it was open to atmosphere. Soil samples that were compacted to the optimum moisture content using standard Proctor test were used in the experiments. Tests were conducted using the axis translation technique and it was observed that the stiffness of the soil increased with increase in soil matric suction. Significant hysteresis was found in the behavior of stiffness along the wetting path and its value was more than the value of the stiffness along the drying path at the same pressure. Soil samples compacted at 2, 4 , 6% dry of standard Proctor optimum were used and subjected to wetting to find the variation of stiffness with moisture content. Stiffness was found to decrease with increase in moisture content both along the dry and wet sides of optimum. Influence of consolidation on stiffness was investigated. Stiffness was found to increase with increase in consolidation pressure. Stiffness was found to increase with the undrained shear strength, tested using the unconsolidated undrained (UU) triaxial test. Statistical analysis was used to develop a statistical fit model of the results, within fixed limits of matric suction and the soil stiffness.