Dynamic shear modulus plays an important role in the seismic assessment of geotechnical systems. Changes in the degree of water saturation influence dynamic soil properties because of the presence of matric suction. This paper describes the modification of a suctioncontrolled cyclic triaxial apparatus to investigate the strain-dependent shear modulus of unsaturated soils. Several strain-and stress-controlled cyclic triaxial tests were performed on a clean sand with various degrees of saturation. Suction in unsaturated sands increased the shear modulus in comparison with the ones in dry and saturated conditions for different shear strain levels, with a peak modulus in higher suction levels. Also, shear modulus decreased with an increase in the shear strain for specimens with similar matric suction. The normalized shear moduli of the unsaturated sand specimens followed a similar trend to the ones predicted by the available empirical shear modulus reduction functions but showed lower values. The modulus reduction ratios of unsaturated sands shifted up as a result of higher effective stress and suction-induced stiffness. These trends were consistent for both strain-and stress-controlled tests.

Dynamic properties of soils including small-strain shear modulus (G max), shear modulus reduction function (G/G max), and damping (D) are affected by changes in the degree of saturation. Inter-particle suction forces in partially saturated soils result in higher effective stress values, which in turn, vary the dynamic soil properties. These alterations could lead to different wave propagation mechanisms, acceleration amplification patterns, and seismically induced settlements. This paper aims to identify the challenges involved in nonlinear seismic site response analysis of partially saturated soils by looking at the response of 10-m sand and silt layers with different constant suction profiles. A set of frequency domain equivalent linear and nonlinear site response analysis under scaled Northridge earthquake motion was performed. A modified version of Bishop's effective stress equation for partially saturated soils has been utilized to calculate the dynamic soil properties (i.e. G max , G/G max , and D). Specifically, surface-to-base intensity amplifications (Peak Ground Amplifications and Arias Intensities), spectral accelerations, and lateral deformation profiles of the sand and silt layers with different suction profiles were generated and compared. The insight gained from this study was used to plan and design more complex nonlinear Finite Element site response analysis.

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Seismic response of unsaturated soil layers may differ from that of saturated or dry soil deposits. A set of centrifuge experiments was conducted to study the influence of partial saturation on seismic response of sand layers under scaled Northridge earthquake motion. Steady state infiltration was implemented to control and provide uniform degrees of saturation profiles in depth. The amplification of peak ground acceleration at the soil surface was inversely proportional to the degree of saturation, especially in low period range. The cumulative intensity amplification of the motion was also higher in unsaturated soils with higher suctions. The lateral deformation and surface settlement of partially saturated sand with higher stiffness were generally lower than that in dry soil. Although neglecting the effect of partial saturation in sand layers might be conservative with respect to seismic deformations, it may result in underestimating the surface design spectra.

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