Empirical Methodology to Estimate Seismically Induced Settlement of Partially Saturated Sand

Abstract

Settlement of soil layers during and after earthquake shaking is a major cause of damage to buildings and geotechnical structures. The available empirical design methods to consider seismically induced settlement focus on sands in dry or water-saturated conditions, and there is currently a gap in the basic understanding of the mechanisms of seismically induced settlements of partially saturated sands. An effective stress-based empirical methodology is proposed to estimate the seismically induced settlement of a free-field layer of sand in partially saturated conditions. This approach estimates the settlement by separately considering the volumetric strains caused by compression of void space during strong shaking (seismic compression) and dissipation of excess pore water pressures generated during earthquake shaking (postcyclic reconsolidation). A parametric evaluation of the methodology indicates that the small strain shear modulus, the parameters of the modulus reduction curve, the approach to estimate the upper bound on volumetric strain during liquefaction, and the pore water pressure generation parameter can have significant impacts on the predicted settlement. The model predictions were validated using results from a newly developed centrifuge physical modeling system that involved the use of steady-state infiltration to maintain a uniform degree of saturation with depth in the sand layer. Both the model and experimental results show a nonlinear trend in surface settlement with degree of saturation, with a minimum value obtained for sand at a degree of saturation between 0.3 and 0.6.

Procedure to Estimate the Seismic Settlement of Partially Saturated Soils

“…Based on the procedure developed by Ghayoomi et al [6], a flow chart was developed to highlight the step-bystep methodology that should be followed to predict the effect of the initial degree of saturation on the seismically induced settlement of a sand layer. The flow chart is presented in Figure 1.…”

“…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.…”

“…Ghayoomi et al [6] adapted and combined a set of available procedures for prediction of the seismic settlement of dry and saturated soils to develop an empirical methodology for estimation of the seismically induced settlement of partially saturated soils. Specifically, they combined empirical relationships for the volumetric seismic compression of soil layers due to collapse of air voids during shaking v-compression, with empirical relationships for the post-shaking consolidation due to dissipation of excess pore water pressure v-consolidation.…”

“…The methodology comprised of synthesizing the recorded data from the literature in an uncoupled formulation using an iterative approach based on that of Seed and Silver [1]. Then, the predicted settlements were verified using the results of centrifuge physical modelling tests [5,6]. The purpose of this technical note is to delineate the steps in this methodology in a simple and organized manner using a flow chart so that they can be readily applied in practice, and to show how the results of this methodology for sand layers having various initial densities and degrees of saturation.…”

“…This includes its effect on the collapse of voids space, pore pressure generation, and liquefaction resistance. Ghayoomi et al [6] used the liquefaction resistance data [3,11] and simplified linear void space relations to address these issues. However, further investigations, experimental data, and analytical developments are needed to fill this gap and provide unique relations for unsaturated soils.…”