Simplified Equivalent Linear and Nonlinear Site Response Analysis of Partially Saturated Soil Layers

Abstract

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.

Centrifuge tests to assess seismic site response of partially saturated sand layers

“…Sand was dry-pluviated in the container to reach the target void ratio of 0.66. Soil Water Retention Curve (SWRC) of this sand measured by Mirshekari and Ghayoomi [49] and Le and Ghayoomi [43] are shown in Figure 3 (b) alongside fitted curves for both drying and wetting paths using van Genuchten's equation [59]. Geotechnical physical and hydraulic properties of the sand are summarized in Table 2.…”

“…This, in turn, yields to different soil dynamic properties including small-strain and strain-dependent shear modulus and damping [32][33][34][35][36][37][38][39][40][41][42][43]. As a result, seismic wave propagation mechanisms may vary in partially saturated soil layers [44] that would lead to a different seismic site response [30,[45][46][47][48][49]. Soils in either dry or fully saturated conditions have been believed to result in more conservative solutions because matric suction in unsaturated soils increases the ground stiffness.…”

“…Therefore, partial saturation has not been directly considered in the state-of-the-practice site response analysis. However, recent investigations on the site response in unsaturated soils showed that this assumption might not be always reasonable [47][48][49]. Further, the influence of the degree of saturation on seismic response analysis is often considered by incorporating the in-situ measured shear wave velocity of shallow unsaturated soil layers.…”

“…Recently, Ghayoomi and Mirshekari [48] and Mirshekari and Ghayoomi [49] numerically studied the seismic response of partially saturated sand and silt layers using site response software DeepSoil [24]. In the absence of any available numerical procedure to account for partial saturation, this influence was investigated by adjusting the soil unit weight and effective stress for any given degree of saturation.…”

Procedure to Estimate the Seismic Settlement of Partially Saturated Soils

“…Although several studies have been conducted in this research area leading to the proposal of several empirical procedures, they have focused on soil layers in dry or water-saturated conditions [1][2][3][4]. However, recent studies have emphasized the importance of understanding the influence of partial saturation on the seismic response of soils [5][6][7][8]. Since soils near the ground surface are typically partially saturated, it is important to have an inclusive methodology to incorporate the degree of saturation in seismic analyses.…”