The small strain shear modulus (G max) is an important parameter in geodynamic problems. In order to predict G max of unsaturated soils which are normally subjected to complex drying, wetting processes, effect of hydraulic hysteresis needs to be evaluated. Although several equations have been proposed in recent years, limitations still exist, requiring more research studies in this field. In this study, G max was investigated in a multi-stage test during several drying-wetting cycles and a loading-unloading cycle of net stress. The results revealed four key factors that directly influence the magnitude of G max : the void ratio, the net stress, matric suction and degree of saturation. While variations of the void ratio, net stress, and matric suction cause persistent responses of G max (i.e. if all other factors remain unchanged, G max would then be reversely proportional to the void ratio and directly proportional to the net stress and matric suction) , variations in the degree of saturation result in different responses. A decrease in the degree of saturation may induce a reduction or growth of G max since on the one hand it reduces the effect of matric suction, while on the other hand it increases the total effect of van der Waals attractions and electric double layer repulsions. At the same stress state, a reverse trend, induced by an increase in the degree of saturation, will occur with a growth in the effect of matric suction Void ratio Void ratio function Maximum small strain shear modulus k Material constant Propagation distance
In this study, a weight-control bender element system has been developed to investigate the impact of matric suction equalisation on the measurement of small strain shear modulus (Gmax), during an air-drying process. The setup employed is capable of measuring the shear wave velocity and the corresponding Gmax of the soil sample in either an open system in which the soil sample evaporates freely or in a closed system that allows the process of matric suction equalisation. The comparison between measurements of Gmax in the open and closed systems revealed underestimations of Gmax when matric suction equalisation was ignored due to the non-uniform distribution of water content across the sample cross-sectional area. This study also investigated the time required for matric suction equalisation tse to be established for samples with different sizes. The experimental results indicated two main mechanisms, driving the matric suction equalisation in a closed system during an air-drying process, namely the hydraulic flow of water and the flow of vapour. While the former played the key role when the micro-pores were still saturated at the high range of water content, effects of the latter increased and finally dominated when more air invaded the micro-pores at lower water contents.
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