4Soil deposits may experience different hydraulic states in terms of suction (ψ) and degree of 5 saturation (S r ) while subjected to large strain caused by dynamic loading during earthquakes. 6 This phenomenon is investigated by considering the effects of initial ψ, initial S r , sample 7 preparation method, drainage condition, and cycle number (N) on dynamic properties. Shear 8 modulus (G) and damping ratio (D) are specifically studied for an unsaturated sand using a 9 modified cyclic simple shear device for all zones of soil water retention curve (SWRC). 10Results revealed the relative significance of both ψ and S r in relation to SWRC on observed 11 trends in G and D. Through analyzing some results from the literature, it was shown that most 12 of the data follow a sigmoid function in the plane of normalized stiffness (G/G sat ) versus 13 suction normalized to the air entry value. It was also revealed that minimum damping is not 14 in conjugation with maximum stiffness due to the contribution of other mechanisms in 15 damping like inter-particle water lubrication. Comparison between results of two testing 16 methods indicated a notable influence of initial fabric on G. Furthermore, the overall trends in 17 G and D against N are marginally influenced by initial ψ and S r .

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Shear modulus (Gmax at very small strain and G at large strain) and constraint modulus at very small strain (M) are important soil parameters for static and dynamic analysis in geotechnical applications. However, these dynamic properties of unsaturated soil are rarely reported. In this study, a cyclic simple shear apparatus was newly-modified for allowing both the shear and constrained moduli at both very small and large strains to be measured. Benders or ultrasonic sensors were embedded in an unsaturated soil sample for transmitting/receiving shear- and pressure-wave, respectively. Two very-small-strain tests were conducted to determine the Gmax, M and soil damping ratio of a sand for a wide range of suction covering from the boundary-effect, transition and residual zone of the water retention curve of the sand. In addition, six large-strain cyclic simple shear tests were carried out to investigate G. The test results showed that Gmax and M were approximately constant before reaching the air-entry value, but there was a significant increase in Gmax as the sand dried further. Yet, M dropped within the transition zone, and interestingly when the suction was beyond the residual value, M increased. M along the wetting path was higher than that along the drying path. The damping ratio, on the other hand, first reduced before reaching the air-entry value, but it increased at the transition zone and then decreased within the residual zone. At large strain, G/Gmax also increased as suction increased until reaching the residual zone, beyond which the normalised value show substantial decreased.