A Compression Model for Unsaturated Collapsible soils

Since most of the soils in their natural state are unsaturated, therefore understanding and description of the compression and failure behavior of unsaturated soils are essential. The compression and failure behavior of unsaturated soils are affected by the interaction of the solid, liquid, and gas phases of the soil. Most of unsaturated soils exhibit a sudden change in their volume due to saturation that is called collapse phenomenon. The compression and collapse behavior of collapsible soils are so complex that can not be explained in the total or net stress spaces. Wetting induced collapse of the collapsible soil has a discontinuous response in net stress space and needs to be described using the matric suction. Calculations of the effective stress using the matric suction shows that the wetting induced collapse response of unsaturated soils is a continuous but a highly non-linear behavior. On the other hand, the compression characteristics of dry and saturated soils are different and change as the moisture content or the degree of the saturation of the soil change. In this research the compression and the wetting induced collapse behavior of unsaturated CL-ML soil have been investigated in the laboratory. Laboratory tests have been conducted by oedometer test device. Compression and wetting induced collapse behavior of unsaturated soil in dry state before collapse, during saturation and collapse state, and fully saturated after collapse state were studied. Pressure plate device was employed to obtain the soil water characteristic curve of the soil. Based on the laboratory results a compression and collapse model has been proposed to capture the compression and collapse behavior of the soil before, during and after wetting induced collapse. Using the soil water characteristic curve, the compression and collapse response of the soil was transferred to effective stress space. The binary-medium model was employed to describe the compression and the collapse behavior of the unsaturated soil based on its responses in two dry and fully saturated states as two reference states. Based on the binary-medium model, soil mass was considered as binary medium including two states of 0 and 1. The state of the soil in dry condition was considered as binary 0 and its state in fully saturated state was considered as binary 1. The state of the soil at any particular state between these two states was interpolated using a state function. An exponential form state function in terms of matric suction and the effective stress was introduced to relate the volume change of the unsaturated soil during the collapse state to its compression behavior in two dry and fully saturated states The state function was derived based on the laboratory experiments. The compression behavior of the soil in dry state was. Using the proposed model, the compression and collapse behavior of unsaturated soil in dry state, collapse state, and saturated state could be described in a single generalized model. The proposed model was verified by the laboratory tests conducted in this study and the available published in the literature. Verifications illustrated the ability of the proposed model in capturing the compression and collapse behavior of unsaturated collapsible soils.