Simulation of liquefaction of unsaturated soil using critical state soil model

Abstract

Summary Several researchers have reported that the mean effective stress of unsaturated soils having a relatively high degree of saturation gradually decreases under fully undrained cyclic loading conditions, and such soils can be finally liquefied like saturated soils. This paper describes a series of simulations of fully undrained cyclic loading on unsaturated soils, conducted using an elastoplastic model for unsaturated soils. This model is a critical state soil model formulated using effective stress tensor for unsaturated soils, which incorporates the following concepts: (a) the volumetric movement of the state boundary surface containing the critical state line owing to the variation in the degree of saturation; (b) the soil water characteristic curve considering the effects of specific volume and hydraulic hysteresis; and (c) the subloading surface concept for considering the effect of density. Void air is assumed to be an ideal gas obeying Boyle's law. The proposed model is validated through comparisons with past results. The simulation results show that the proposed model properly describes the fully undrained cyclic behavior of unsaturated soils, such as liquefaction, compression, and an increase in the degree of saturation. Finally, the effects of the degree of saturation, void ratio, and confining pressure on the cyclic strength of unsaturated soils are described by the simulation results. The liquefaction resistance of unsaturated soils increases as the degree of saturation and the void ratio decrease, and as the confining pressure increases. Furthermore, the degree of saturation has a greater effect on the liquefaction resistance than the confining pressure and void ratio. Copyright © 2017 John Wiley & Sons, Ltd.

A theory of loosening earth pressure above a shallow tunnel in unsaturated ground

“…From Equations 4 and 5, S r is given as a function of depth, S r ( z ). It is noted that the proposed method can incorporate several factors affecting the SWCC, such as air‐entry suction, 15 hydraulic hysteresis, 28 or the effect of density, 29 by replacing Equation 5 with an advanced SWCC model 11,30 …”

“…The effective stress proposed by Bishop 10 is usually used for unsaturated soils, as it can uniquely arrange the critical state stress ratio of unsaturated soils regardless of suction, s , as shown in Figure 2. By using this, σ z in Equation 2 is given as follows: where χ is the effective stress parameter for which S r is usually applied 11,31–33 . Borja 34 proved that χ is equal to S r based on the principles of thermodynamics.…”

“…The shear strength of unsaturated soils is usually evaluated using two independent stress state variables such as net normal stress and suction 7–9 . The effective stress tensor for unsaturated soils proposed by Bishop has also been applied widely to model the stress–strain behavior of unsaturated soils 10,11 . This tensor can uniquely describe the critical state stress ratio regardless of the degree of saturation 12 .…”

Mechanism of wetting–induced deformation and failure of unsaturated soils

“…19 Models have been proposed that incorporate both hydraulic hysteresis and density effects in their SWCC models. [20][21][22][23][24][25] The SWCC models considering the effect of specific volume have been successfully implemented in the mathematical framework based on the principle of thermodynamics to study the fluid flow in unsaturated soils in the finite deformation range. 26,27 In the subloading surface model used by Kikumoto et al 21 and Komolvilas and Kikumoto, 22 volumetric movement of the normal consolidation line due to the variation in the degree of saturation was considered.…”

“…[20][21][22][23][24][25] The SWCC models considering the effect of specific volume have been successfully implemented in the mathematical framework based on the principle of thermodynamics to study the fluid flow in unsaturated soils in the finite deformation range. 26,27 In the subloading surface model used by Kikumoto et al 21 and Komolvilas and Kikumoto, 22 volumetric movement of the normal consolidation line due to the variation in the degree of saturation was considered. The wetting-induced collapse of unsaturated soils has been simulated [e.g., 19,21,28,29 ], but the effects of density and confining pressure, especially under anisotropic stress conditions, were not evaluated.…”