Localized failure in unsaturated soils under non-isothermal conditions

Fluctuations of temperature and degree of saturation have considerable influence on the mechanical, hydraulic and retention properties of unsaturated soils. Localized failure is a ubiquitous feature of geomaterials. Major research on localized failure of geomaterials has been focused on geomaterials under the isothermal condition. In this article, we study the localized failure of unsaturated soils under non-isothermal conditions. In particular, we derive the isothermal and adiabatic bifurcation conditions from a homogeneous deformation at the constitutive level under a locally drained condition. A recently proposed meso-scale constitutive model for thermal unsaturated soils is used to derive the isothermal and adiabatic acoustic tensors. We present the spectral form of the consistent tangential elasto-plastic operator from a local material integration algorithm. The numerical simulations at the material level are conducted to study the impact of temperature on localized failure of unsaturated soils under the plane strain condition. The numerical results show that the timing and the critical angle of bifurcation are dependent on temperature.     

Thermo-elastoplastic constitutive model for unsaturated soils

This study presents a simple approach to modelling the effect of temperature on the deformation and strength of unsaturated/saturated soils by using the average skeleton stress and degree of saturation. The concept of thermo-induced equivalent stress is introduced to consider the influence of temperature on the pre-consolidated stress. A skeleton stress–saturation framework is applied to enable the model to describe the thermo-elastoplastic behaviour of both unsaturated and saturated soils, as the skeleton stress can smoothly shift to Terzaghi’s effective stress if saturation changes from the unsaturated to the saturated condition. The new model only employs seven parameters, of which five parameters are the same as those used in the Cam-Clay model. The other two parameters can be easily determined by oedometer tests and simple thermo-mechanical tests. Numerical simulations of isotropic loading tests and triaxial shear tests under different conditions are conducted to illustrate the performance of the proposed model. By comparing with experimental temperature controlled oedometer tests and triaxial tests, it is confirmed that the proposed model is able to capture the thermo-mechanical behaviour of unsaturated/saturated normally and over-consolidated soils with a set of unified parameters.     

A hierarchical thermo‐hydro‐plastic constitutive model for unsaturated soils and its numerical implementation

Unsaturated soils are highly heterogeneous 3‐phase porous media. Variations of temperature, the degree of saturation, and density have dramatic impacts on the hydro‐mechanical behavior of unsaturated soils. To model all these features, we present a thermo‐hydro‐plastic model in which the hydro‐mechanical hardening and thermal softening are incorporated in a hierarchical fashion for unsaturated soils. This novel constitutive model can capture heterogeneities in density, suction, the degree of saturation, and temperature. Specifically, this constitutive model has 2 ingredients: (1) it has a “mesoscale” mechanical state variable—porosity and 3 environmental state variables—suction, the degree of saturation, and temperature; (2) both temperature and mechanical effects on water retention properties are taken into account. The return mapping algorithm is applied to implement this model at Gauss point assuming an infinitesimal strain. At each time step, the return mapping is conducted only in principal elastic strain space, assuming no return mapping in suction and temperature. The numerical results obtained by this constitutive model are compared with the experimental results. It shows that the proposed model can simulate the thermo‐hydro‐mechanical behavior of unsaturated soils with satisfaction. We also conduct shear band analysis of an unsaturated soil specimen under plane strain condition to demonstrate the impact of temperature variation on shear banding triggered by initial material heterogeneities.     

考虑毛细滞回效应的非饱和多孔介质渗流与变形耦合本构模型

在Wheeler本构模型框架的基础上,提出了一个水力与力学耦合的本构模型。该模型中的土-水特征曲线采用毛细滞回内变量模型,能够更好地描述水力历史变化下毛细滞回现象对非饱和多孔介质变形的影响,同时也可描述非饱和多孔介质变形对渗流的影响。非饱和土的强度不仅与吸力有关,而且受到饱和度的影响。相同的吸力下,土样经过吸湿和脱湿路径的饱和度不同,因此,非饱和土的强度也不同。此模型以体积含水率的塑性变化和体变的塑性变化为硬化参数,不仅能描述基质吸力对非饱和土的强化作用,而且考虑了饱和度对强度及变形的影响。试验结果与模型预测基本吻合,证明该模型能够模拟非饱和土的主要特性。为了简化,此模型是在各向同性荷载下推得的,有待于推广到一般的应力状态     

干湿路径影响粉土小应变剪切模量的试验研究

天然土尤其是地表浅层土常处于非饱和状态,其小应变剪切模量是预测地基变形及土工结构物动力反应的一个重要参数。通过对非饱和压实粉土三轴样进行弯曲元试验,研究了吸力和干湿路径对其小应变剪切模量的影响。试验结果表明,非饱和压实粉土样的小应变剪切模量各向异性忽略不计;小应变剪切模量G0(vh)、G0(hh)和G0(hv)均随吸力增大而非线性增大;同一吸力下不同干湿路径上的土样,饱和度不同,其小应变剪切模量随饱和度升高而减小,主要原因是土的平均骨架应力和土中毛细水的作用。根据试验结果对非饱和土小应变剪切模量的半经验公式进行了改进,同时考虑了吸力与饱和度的作用。     

Simulation of liquefaction of unsaturated soil using critical state soil model

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.     

Time‐scale analysis in unsaturated porous media under external wave loads

Extreme waves caused by tsunamis and storm surges can lead to soil failures in the near‐shore region, which may have severe impact on coastal environments and communities. Multiphase flows in deformable porous media involve several coupled processes and multiple time scales, which are challenging for numerical simulations. The objective of this study is to investigate the roles of the various processes and their interactions in multiphase flows in unsaturated soils under external wave loading, via theoretical time‐scale analysis and numerical simulations. A coupled geomechanics–multiphase flow model based on conservation laws is used. Theoretical analysis based on coupled and decoupled models demonstrates that transient and steady‐state responses are governed by pore pressure diffusion and saturation front propagation, respectively, and that the two processes are essentially decoupled. Numerical simulations suggest that the compressibility of the pore fluids and the deformation of the soil skeleton are important when the transient responses of the media are of concern, while the steady‐state responses are not sensitive to these factors. The responses obtained from the fully coupled numerical simulations are explained by a simplified time‐scale analysis based on coupled and decoupled models. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical Evaluation of the Mechanical Behavior of Unsaturated Soils

An evaluation method for the mechanical behavior of unsaturated soils is studied in this paper. Although the mechanical behavior of unsaturated soils is complicated, a simple modeling is preferable in practice. This is because the soil properties are not homogeneous and ground data is limited when structures are being designed. In addition, in order to evaluate the reliability of the design, the physical meanings of the parameters applied in the prediction model should be clear. Firstly, the authors study the relationship between compaction curves and compression indexes in the unsaturated state that is used in the proposed constitutive model. Based on the constitutive model, the stress paths for constant volume shear tests are formulated under a constant void ratio condition and the stress paths for undrained shear tests are calculated under a constant water content condition. In the case of unsaturated specimens, the volume of these specimens changes with the shear deformation and the stress paths depend on the initial degree of saturation. The results of the calculation qualitatively describe the test results by considering the changes in effective confining pressure in the undrained condition and the water retention curves.     

非饱和土中圆柱形衬砌隧道的瞬态动力响应

在以往关于圆柱形衬砌隧道的瞬态动力响应中,衬砌周围土体大多假定为弹性介质或饱和介质。然而,自然界中的土体大多为非饱和介质。考虑土体与衬砌结构的动力相互作用及动荷载引起的附加质量密度的影响,研究了瞬态荷载作用下非饱和土中无限长深埋圆柱形衬砌隧道的动力响应。基于多孔介质混合物理论和连续介质力学理论,建立了非饱和土中圆柱形衬砌隧道受到瞬态荷载作用时衬砌及周围土体的控制方程,利用Durbin数值反演法得到了衬砌及土体在时间域的动力响应。数值分析了饱和度对瞬态荷载下径向位移、径向应力、环向应力和孔隙水压力的影响。结果表明：饱和度对衬砌及周围土体的瞬态响应影响显著;饱和度对径向位移沿径向的衰减影响较小,对环向应力和孔隙压力沿径向的衰减影响较大。     

环境湿度可控的土体小应变刚度试验系统

为研究脱湿过程中饱和度改变对土体小应变刚度的影响规律,自主研制了一套非饱和土小应变刚度试验装置。该装置由3部分构成：相对湿度控制系统、变形测试系统和波速测试系统。相对湿度控制系统可分级提供环境室内相对湿度,通过称重模块记录土样质量变化;变形测试系统利用图像采集结合数字图像处理技术获得土样体变;波速测试系统采用压电陶瓷元件制作的波速传感器,可同时测试土样脱湿过程中压缩波及剪切波波速。试验结果表明：该装置能够分级稳定控制相对湿度环境、间接测量土样体积变化及实时测试弹性波波速,进而研究波速表征的小应变刚度随饱和度的变化规律;土样剪切波速及小应变剪切模量随饱和度的降低逐渐增加,压缩波速及小应变体积模量随饱和度的降低先减小后增加。该装置结构简单,便于操作,为研究非饱和土小应变刚度特性提供了科学有效的测试方法与试验手段。     

非饱和土毛细滞回与变形耦合弹塑性本构模型

在修正剑桥模型的基础上,提出了一个非饱和土毛细滞回与骨架变形耦合的弹塑性本构模型。该模型考虑了基质吸力与饱和度对屈服应力的影响,可以同时描述非饱和土的弹塑性变形特性与毛细循环滞回效应。根据塑性体变的产生使非饱和土进气值增大的特点,建立了变形对土-水特征曲线影响的数学描述。该模型有效地考虑了饱和度对前期屈服应力的作用,准确地反映了土体在不同土-水状态条件下（脱湿和吸湿过程）强度特性的变化,而且还可以有效地描述水力循环历史对土体变形的影响。通过与试验数据对比,证明了该模型能够模拟非饱和土的主要力学特性。     

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

A theory is proposed to evaluate the loosening earth pressure (vertical earth pressure after excavation) acting on a shallow tunnel in unsaturated ground with an arbitrary groundwater level. The theory is developed based on the limit equilibrium theory, combining soil–water characteristic curves, Mohr–Coulomb failure criteria, and effective stress for unsaturated soils. The proposed theory is applied to predict the vertical distribution of loosening earth pressure in unsaturated ground, which shows a significant difference from that in saturated ground. In unsaturated ground, suction contributes to the increase in effective loosening earth pressure and shear resistance. The remarkable effects of groundwater depth, soil type, and scale of overburden height and trapdoor width on loosening earth pressure are also revealed. Based on the soil–water characteristic curve, the degree of saturation decreases, which causes wet density to decrease and the total and effective loosening earth pressures to have contrary tendencies. Moreover, effective loosening earth pressures vary with soil type as the degree of saturation varies. The total loosening earth pressures are, however, very similar regardless of soil type, because wet density and shear resistance have similar tendencies. The proposed theory provides a valid model for loosening earth pressure in unsaturated ground that will be useful for shallow tunnel excavations.     

Influence of Lode angle‐dependent failure criteria on shear localization analysis in sand

Geotechnical experiments show that Lode angle‐dependent constitutive formulations are appropriate to describe the failure of geomaterials. In the present study, we have adopted one such class of failure criteria along with a versatile constitutive relationship to theoretically analyze the effects of Lode angle on localized shear deformation or shear band formation in loose sand for both drained and undrained conditions. We determine the variation in the possible stress states for shear localization due to the introduction of Lode angle by considering the localized deformation as a bifurcation problem. Further, similar bifurcation analysis is performed for the stress states along a specific loading path, namely, plane strain compression at the constitutive level. In addition, the plane strain compression tests have been simulated as a boundary value finite element problem to see how Lode angle affects the post‐localization response. Results show that the inclusion of a Lode angle parameter within the failure criterion has considerable effects on the onset, plastic strain, and propagation of shear localization in loose sand specimens. For drained condition, we notice early inception of shear localization and multiple band formation when the Lode angle‐dependent failure criterion is used. Undrained localization characteristics, however, found to be independent of Lode angle consideration.     

非饱和红黏土的不排气、不排水三轴剪切试验研究

非饱和土中由于有孔隙气的存在而使得非饱和土的力学性质变得非常复杂。气体对非饱和土体强度和变形会产生重要影响。根据12组不同饱和度、不同干密度云南红黏土土样的三轴不排气、不排水剪切试验结果,讨论了气体对非饱和土土体强度和变形的影响。黏聚力在不排气剪切情况下随着饱和度的变化不是单调的,内摩擦角则随着饱和度的增大而减小。干密度增大则黏聚力和内摩擦角都增大。在三轴不排气剪切过程中,试样出现了明显了的剪胀性,且随着干密度的减小,饱和度的增大剪胀性逐渐消失。试样最后的破坏形态与是否发生剪胀可能也有联系。     

A bounding surface model for unsaturated soils considering the microscopic pore structure and interparticle bonding effect due to water menisci

The interparticle bonding effect due to water menisci plays an important role in the hydromechanical coupling properties of unsaturated soils. This paper presents an unsaturated hydromechanical coupling model that considers the influence of matric suction, degree of saturation, and microscopic pore structure on the interparticle bonding effect. The enhanced effective stress and bonding variable are selected as constitutive variables. The bonding variable is correlated with the ratio between unsaturated void ratio and saturated void ratio. The deformation characteristics of unsaturated soils are described based on the bounding surface plasticity theory. A soil–water characteristic model that considers deformation and hydraulic hysteresis is integrated into the constitutive model to achieve hydromechanical coupling. The proposed model can effectively describe the hydromechanical coupling characteristics and the meniscus bonding force of unsaturated bimodal structure soils; the model parameters can be easily obtained through routine experiments. The experimental results of unsaturated isotropic compression, the wetting/drying cycle, and unsaturated triaxial shear tests are used to validate the capability of the proposed model.

     

Continuum deformation and stability analyses of a steep hillside slope under rainfall infiltration

Rainfall weakens an earth slope in a number of ways. It increases the degree of saturation of the soil, thereby breaking the bonds created by surface tension between the soil particles. When the volume of infiltrating water is large enough to mobilize fluid flow inside the soil matrix, the fluid exerts a downhill frictional drag on the slope, creating a destabilizing effect. When excess fluid can no longer infiltrate the slope due to increased saturation in the soil, it is discharged as a surface runoff and erodes the slope. In this paper, we present a physics-based framework for continuum modeling of a hydrologically driven slope failure similar to what occurred in a steep experimental catchment CB1 near Coos Bay, Oregon. We quantify the rainfall-induced slope deformation and assess the failure potential of the slope using finite element modeling that couples solid deformation with fluid pressure in an unsaturated soil. Results of the studies suggest that for a steep hillside slope underlain by a shallow bedrock similar to the CB1 site, failure would occur by multiple slide blocks with the failure surfaces emerging on the slope face. These results suggest that an infinite slope mechanism would be insufficient to represent the failure kinematics for a slope similar to CB1.     

基质吸力对非饱和重塑黄土抗剪强度的贡献

非饱和土抗剪强度随饱和度降低、基质吸力增大而提高.然而,不同类型非饱和土中,基质吸力对抗剪强度的贡献却显著不同.本文通过室内试验,探讨了基质吸力对非饱和重塑黄土抗剪强度贡献的基本特征.研究结果表明,非饱和重塑黄土抗剪强度随着基质吸力的增大,表现为非线性的和明显的4个阶段性增、降和逐渐稳定特征,且分段特征值与土-水特征曲...     

Thermoplasticity and strain localization in transversely isotropic materials based on anisotropic critical state plasticity

Geomaterials such as soils and rocks are inherently anisotropic and sensitive to temperature changes caused by various internal and external processes. They are also susceptible to strain localization in the form of shear bands when subjected to critical loads. We present a thermoplastic framework for modeling coupled thermomechanical response and for predicting the inception of a shear band in a transversely isotropic material using the general framework of critical state plasticity and the specific framework of an anisotropic modified Cam–Clay model. The formulation incorporates anisotropy in both elastic and plastic responses under the assumption of infinitesimal deformation. The model is first calibrated using experimental data from triaxial tests to demonstrate its capability in capturing anisotropy in the mechanical response. Subsequently, stress‐point simulations of strain localization are carried out under two different conditions, namely, isothermal localization and adiabatic localization. The adiabatic formulation investigates the effect of temperature on localization via thermomechanical coupling. Numerical simulations are presented to demonstrate the important role of anisotropy, hardening, and thermal softening on strain localization inception and orientation. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of degree of saturation on mechanical behaviour of unsaturated soils and its elastoplastic simulation

The two stress-state variable approach has been widely used in interpreting unsaturated soil behaviour. However this approach cannot take into account the effect of degree of saturation or water contents on the stress–strain behaviour and strength of unsaturated soils. The triaxial test results presented in this paper show that even if the same path of net stress and suction is followed, the stress–strain relation and strength are different due to different degrees of saturation. When other conditions are the same, the higher the degree of saturation for the soil sample is, the higher the stress ratio corresponding to a given axial strain will be. This effect can be modeled by using an elasto-plastic constitutive model coupling hydraulic and mechanical behaviour of unsaturated soils. Comparisons between the predicted and measured results are presented, which demonstrate that the model can quantitatively simulate the influence of the degree of saturation on stress–strain behaviour and strength of unsaturated soils.     

On the use of effective stress in three-dimensional hydro-mechanical coupled model

In the last decades, a number of hydro-mechanical elastoplastic constitutive models for unsaturated soils have been proposed. Those models couple the hydraulic and mechanical behaviour of unsaturated soils, and take into account the effects of the degree of saturation on the stress–strain behaviour and the effects of deformation on the soil–water characteristic response with a simple reversible part for the hysteresis. In addition, the influence of the suction on the stress–strain behaviour is considered. However, until now, few models predict the stress–strain and soil–water characteristic responses of unsaturated soils in a fully three-dimensional Finite Element code. This paper presents the predictions of an unsaturated soil model in a Three-dimensional Framework, and develops a study on the effect of partial saturation on the stability of shallow foundation resting on unsaturated silty soil. Qualitative predictions of the constitutive model show that incorporating a special formulation for the effective stress into an elastoplastic coupled hydro-mechanical model opens a full range of possibilities in modelling unsaturated soil behaviour.