考虑温度影响的非饱和土本构模型

首先建立了反映温度对非饱和土前期固结压力影响的加载-升温(loading thermal, LT)屈服线,而后基于真强度概念并结合潜在强度的确定方法推导出不同温度下非饱和黏土临界状态应力比的理论计算公式;将LT屈服线与反映常温下吸力对土体前期固结压力影响的加载-湿陷(loading collapse, LC)屈服线结合,建立了加载-升温-湿陷(loading thermal collapse, LTC)屈服面,该屈服面综合考虑了温度和吸力对土前期固结压力的影响;最后在临界土力学框架内建立了一个考虑温度影响的正常固结非饱和土本构模型,并将其扩展到超固结非饱和土。与巴塞罗那模型相比,所提出的模型仅增加了一个参数来反映非饱和土的前期固结压力随温度升高而降低的特性。模型能够综合描述温度、吸力以及不同超固结程度对土应力、应变的影响,方便于数值计算和有限元分析。模型预测和试验分析表明,某一固定吸力下升温会使正常固结非饱和土强度提高;对于超固结非饱和土,升温或湿化均会破坏土体超固结,降低软化和剪胀的效果。     

A fully coupled simple model for unsaturated soils

Different phenomena influence the strength and volumetric behavior of unsaturated soils. Among the most important are suction hardening, hydraulic hysteresis, and the influence of volumetric strain on the soil-water retention curves. Fully coupled hydro-mechanical models require including all three phenomena in their constitutive relationships. Among these phenomena, suction hardening is the most influencing as it determines the apparent preconsolidation stress, the position of the loading-collapse yield surface, and the shift of both the isotropic consolidation and the critical state lines. In this paper, a fully coupled hydro mechanical model is presented. It is based on the modified Cam-Clay model but includes a yield surface with anisotropic hardening that takes account of the shift of the critical state line with suction. For highly overconsolidated materials, the sub-loading surface concept has been included in order to increase the precision of the model for these materials. The shift of the retention curves produced by volumetric strains is simulated using a hydraulic model based on the grain and current pore size distribution of the soil.     

非饱和黏土状态相关本构模型的数值实现

基于双重塑性机制,即剪切滑移和加载湿陷机制,及毛细滞回和变形间的耦合作用,提出了一个适用于非饱和黏土的状态相关本构模型。建立了该模型的隐式积分算法,并处理了双重固相屈服面交点处的应力积分问题。考虑了吸力与应力间的关系,推导了本构关系的一致性切线模量。最后,利用该模型预测了一组非饱和黏土的三轴试验,以此反映模型的描述能力;对比了不同应变步长下应变控制试验的计算结果,并以此验证算法的收敛性和准确性;利用有限元程序平台U_DYSAC2计算了二维情况下非饱和土地区地基固结问题,从而验证所得一致性切线模量的有效性。     

A mathematical model for predicting coupled heat and water movement in unsaturated soil

An investigation was conducted to develop a comprehensive moisture model for predicting non-isothermal moisture conditions in soils. An extensive literature review indicated that a model based on the Philip and de Vries equations for non-isothermal moisture movement and heat conduction would give the best results. By using numerical methods, the implicit finite difference approximations to the moisture movement and heat-transfer equations were programmed for computer solution of water content and temperature in the soil with time. Validation studies indicate that the moisture model can be used to predict accurately moisture conditions in the soil. The model was validated by using hydraulic data from laboratory studies conducted on soil columns compacted with AASHO A-3 and AASHO A-4 soil. The application of the moisture model to the study of non-isothermal moisture movement in the field is demonstrated. The influence of parameters such as water table depth, precipitation, and soil hydraulic properties on soil moisture content are shown by use of the moisture model. The model is shown to be applicable to a wide range of boundary conditions and that it predicts the moisture-temperature regime with time in soils utilizing climatic input data.     

循环荷载下非饱和结构性黏土的弹塑性双面模型

非饱和黏土的结构性能够显著影响其力学特性。基于非饱和土经典模型BBM（Barcelona basic model）和一种可描述循环塑性的硬化法则,引入体积破损率的作为标准土体结构破损的参数,建立了一个描述常吸力下非饱和结构性黏土静态及动态力学特性的弹塑性双面模型。模型在应力空间中包含与重塑非饱和土屈服面几何相似的结构性边界面和加载面,采用径向映射法则和可移动的记忆中心原理,通过结构性边界面和加载面在应力空间中的演化来反映循环加载过程中非饱和结构性黏土的循环塑性特征和结构损伤过程。通过与相关非饱和黏土控制吸力试验数据的比较,表明该模型能够较好地反映静态加载下非饱和结构性黏土的力学特性,而模型预测的循环荷载下的应力?应变特征也具有一定的合理性。     

A thermo-chemo-electro-mechanical framework of unsaturated expansive clays

Prediction of the coupled chemo-mechanical behaviours of porous media is an important problem in many areas, i.e., expansive clays in geotechnical and petroleum engineering, engineered barriers in the underground storage of nuclear wastes, and biological tissues in biological engineering. A further complex condition occurs when the voids are not fully saturated by liquid, such that capillary effects cannot be neglected a priori and might play an important role. Focusing on the modelling of expansive clays in the geotechnical field and based on the modified mixture theory, the work presented in this paper contributes to the construction of a theoretical framework used to model such complex coupling behaviours. The Clausius–Duhem inequality, which governs the dissipation associated with mechanical work, phase transformation, mass transport and thermal transport, is rigorously derived. Based on this theoretical framework, a chemo-poro-elastic unsaturated model is developed. The model is subsequently used to simulate the salt solution infiltration process through an unsaturated expansive clayey soil that induces changes in the mechanical and hydraulic field quantities. The logical tendencies are obtained and provide a preliminary demonstration of the capabilities of the newly developed theoretical framework.     

A hypoplastic constitutive model for clays

This paper presents a new constitutive model for clays. The model is developed on the basis of generalized hypoplasticity principles, which are combined with traditional critical state soil mechanics. The positions of the isotropic normal compression line and the critical state line correspond to the Modified Cam clay model, the Matsuoka–Nakai failure surface is taken as the limit stress criterion and the non‐linear behaviour of soils with different overconsolidation ratios is governed by the generalized hypoplastic formulation. The model requires five constitutive parameters, which correspond to the parameters of the Modified Cam clay model and are simple to calibrate on the basis of standard laboratory experiments. This makes the model particularly suitable for practical applications. The basic model may be simply enhanced by the intergranular strain concept, which allows reproducing the behaviour at very small strains. The model is evaluated on the basis of high quality laboratory experiments on reconstituted London clay. Contrary to a reference hypoplastic relation, the proposed model may be applied to highly overconsolidated clays. Improvement of predictions in the small strain range at different stress levels is also demonstrated. Copyright © 2005 John Wiley & Sons, Ltd.     

结构性粘土的堆砌体模型

天然粘土一般都具有结构性, 其变形过程必然伴随着结构的破坏。 提出了一种新型的堆砌体模型, 用以描述这种结构破坏现象。 这一模型把变形过程中的结构性土看作不同大小土块的集合体, 总的变形将由土块的弹性变形、土块之间滑动引起的塑性变形和土块破碎引起的损伤变形三部分组成。 塑性变形常用屈服函数描述, 损伤变形则可以引入一种类似的损伤函数加以描述。推导了相应的应力应变关系式并提出了模型参数的测定方法。     

A constitutive thermomechanical model for saturated clays

The structural deformation in clays results from microscopic phenomena involving the mechanical contact-stress change, the physico-chemical variation of repulsive forces in expansive clays, and thermal dilatancy of macropores. These textural strains are associated to three plastic mechanisms represented by respectively the yield surfacesf_Tm, f_R-A andf_T. Under a thermal cycle, the sizes of interlamellar spaces between clay platelets are not modified, hence the temperature cycle is expected to have no effect on repulsive forces and thus the second mechanism is not affected by temperature changes.

This paper suggests a formulation of a model of thermo-elasto-plastic behaviour of non-expansive saturated clays characterised by two plastic mechanisms. The mechanical yield surfacef_Tm of the contact-stress mechanism is based on a modified cam-clay model; the thermal softening yield surfacef_T is a plane separating two thermal domains. In normally consolidated conditions, the resulting response to an increase of temperature is compressive. However, in highly overconsolidated conditions, a small irreversible dilative volumetric strain is observed when the temperature is above a threshold value. In intermediate conditions, the material starts with an expansion and tends to a compression.

The constitutive model combines thermo-mechanical hardening, predominant in normally consolidated states (NCS) and absent in overconsolidated states (OCS) where the thermal softening occurs. The characterisation of the model requires information about rheological parameters obtained from oedometric and triaxial paths. Lastly, some numerical simulations of thermo-mechanical tests onremoulded Boom, ‘Bassin Parisien’ andPontida clays are presented, which show satisfactory agreement between experiments and model predictions.     

A critical state model for overconsolidated structured clays

This paper presents a generalised critical state model with the bounding surface theory for simulating the stress–strain behaviour of overconsolidated structured clays. The model is formulated based on the framework of the Structured Cam Clay (SCC) model and is designated as the Modified Structured Cam Clay with Bounding Surface Theory (MSCC-B) model. The hardening and destructuring processes for structured clays in the overconsolidated state can be described by the proposed model. The image stress point defined by the radial mapping rule is used to determine the plastic hardening modulus, which varies along loading paths. A new proposed parameter h, which depends on the material characteristics, is introduced into the plastic hardening modulus equation to take the soil behaviour into account in the overconsolidated state. The MSCC-B model is finally evaluated in light of the model performance by comparisons with the measured data of both naturally and artificially structured clays under compression and shearing tests. From the comparisons, it is found that the MSCC-B model gives reasonable good simulations of mechanical response of structured clays in both drained and undrained conditions. With its simplicity and performance, the MSCC-B model is regarded as a practical geotechnical model for implementation in numerical analysis.     

结构性粘土的边界面砌块体模型

把天然粘土的变形看作是由结构的破坏引起的。总的变形由结构性粘土的弹性变形、结构面的滑移塑性变形和结构体破损引起的的损伤塑性变形组成。把滑移屈服面看作可以扩大、旋转的运动硬化面,并作为边界面,通过内插塑性模量来描述滑移塑性变形。采用损伤边界面,通过规定加荷或反向加荷产生损伤变形,卸荷不产生损伤变形来描述损伤塑性变形。建立了结构性粘土的边界面砌块体模型,并用试验进行了验证。     

A dual-porosity model for ionic solute transport in expansive clays

A microstructure model of dual-porosity type is proposed to describe contaminant transport in fully-saturated swelling clays. The swelling medium is characterized by three separate-length scales (nano, micro, and macro) and two levels of porosity (nano- and micropores). At the nanoscale, the medium is composed of charged clay particles saturated by a binary monovalent aqueous electrolyte solution. At the intermediate (micro) scale, the two-phase homogenized system is represented by swollen clay clusters (or aggregates) with the nanoscale electrohydrodynamics, local charge distribution, and disjoining pressure effects incorporated in the averaged constitutive laws of the electro-chemo-mechanical coefficients and the swelling pressure, which appear in Onsager’s reciprocity relations and in a modified form of Terzaghi’s effective principle, respectively. The microscopic coupling between aggregates and a bulk solution lying in the micropores is ruled by a slip boundary condition on the tangential velocity of the fluid, which captures the effects of the thin electrical double layers surrounding each clay cluster. At the macroscale, the system of clay clusters is homogenized with the bulk fluid. The resultant macroscopic picture is governed by a dual-porosity model wherein macroscopic flow and ion transport take place in the bulk solution and the clay clusters act as sources/sinks of mass of water and solutes to the bulk fluid. The homogenization procedure yields a three-scale model of the swelling medium by providing new nano and micro closure problems, which are solved numerically to construct constitutive laws for the effective electro-chemo-hydro-mechanical coefficients. Considering local instantaneous equilibrium between the clay aggregates and micropores, a quasisteady version of the dual-porosity model is proposed. When combined with the three-scale portrait of the swelling medium, the quasisteady model allows us to build-up numerically the constitutive law of the equilibrium adsorption isotherm, which governs the instantaneous immobilization of the solutes in the clay clusters. Moreover, the constitutive behavior of the retardation coefficient is also constructed by exploring its representation in terms of the local profile of the electrical double layer potential of the electrolyte solution, which satisfies the Poisson–Boltzmann problem at the nanoscale.     

超固结粘土的二元介质模型

为了分析超固结粘土边坡的变形和稳定的需要,在岩土破损力学的框架内建议了一个适用于这类土的二元介质模型。该模型考虑了土体内在的不均一性,把它看作由结构块和结构带组成的复合体,两者共同分担外荷载,模型包含10参数,并拟定了这些参数的测定方法。通过模拟三轴试验的计算表明,该模型可以反映London粘土的应力-应变特征。     

A coupled poroplastic damage model accounting for cracking effects on both hydraulic and mechanical properties of unsaturated media

Damage induced by microcracking affects not only the mechanical behaviour of geomaterials but also their hydraulic properties. Evaluating these impacts is important for many engineering applications, such as the safety assessment of radioactive waste disposal facilities. This paper presents a new constitutive model accounting simultaneously for the impact of damage on hydraulic and mechanical properties of unsaturated poroplastic geomaterials. The hydro‐mechanical coupling is formulated by means of the thermodynamic framework for partially saturated media, extended by taking into account isotropic damage and plasticity. State and complementary laws are governed by the so‐called plastic effective stress and equivalent pore pressure. Assuming a bimodal pore size distribution for cracked porous media, the hydraulic part (water retention curve and hydraulic conductivity) is modelled using phenomenological functions of damage variable. The participation of damage on both mechanical and hydraulic part enables this model to describe bilateral couplings between them. This coupled model is then validated against a number of experimental data obtained from Callovo‐Oxfordian argillite, which is the possible host rock for a radioactive waste disposal in France. Parametric studies are also carried out to check the consistency and to better demonstrate the bilateral couplings in the model. Copyright © 2015 John Wiley & Sons, Ltd.     

A double hardening thermo-mechanical constitutive model for overconsolidated clays

On the basis of a double hardening model for clays and available experimental results, a new thermo-elasto-plastic constitutive model for saturated clays is proposed to describe the effects of temperature and overconsolidation ratio on the mechanical properties of saturated clays. Two hardening parameters are introduced: s_c^￠ {\sigma}_{{\rm c}}^{\prime} and α. The proposed model is then applied to simulate the relevant important features of saturated clays with different overconsolidation ratios under different temperature and loading conditions. The model predictions are compared with available experimental results to demonstrate its accuracy and usefulness.     

A monotonic bounding surface critical state model for clays

This study investigates a simple constitutive model based on the critical state framework and bounding surface (BS) plasticity that is suitable for reconstituted clays over a wide range of overconsolidation ratios under monotonic loading. For heavily overconsolidated (OC) clays, rather than using the conventional Hvorslev line, an empirical surface is introduced into the model formulation based on two image points on the BS. The peak strength and the dilatancy of heavily OC clays can thus be predicted satisfactorily. Comparisons with triaxial test data show that the model well captures the peak strength and the dilatancy of heavily OC clays under monotonic loading.     

A constitutive model for unsaturated soil interfaces

A constitutive model based on the disturbed state concept is presented to describe the behavior of interfaces in unsaturated soil. The model is an extension of an existing model developed for a sand–steel interface. As opposed to the original model, the modified model incorporates two independent stress variables, which are the net normal stress and matric suction. The saturated and dry state of the interface can be modeled as a special case using the constitutive model presented in this paper. The modified model is capable of capturing the main features of unsaturated interfaces observed during laboratory testing, including increasing shear strength and strain softening with increasing suction and net normal stress and increasing dilatancy with increasing suction. Laboratory tests were carried out on unsaturated interfaces in a modified direct shear test apparatus. The observed behavior of interfaces between unsaturated soil and steel plates (rough and smooth) is presented in comparison with model predictions. Copyright © 2008 John Wiley & Sons, Ltd.     

A two‐surface thermomechanical model for saturated clays

This paper presents an advanced thermomechanical model – TEAM in the framework of two‐surface plasticity for saturated clays, with emphasis put on some important thermomechanical features of natural clays evidenced experimentally such as the limited thermomechanical elastic zone, the smooth transition from elastic to plastic behavior. Two plastic mechanisms are introduced in the model: one is to reproduce the thermoplasticity involving thermal expansion and contraction observed at high over‐consolidation ratios and the second one describes the temperature effect on the yield behavior. The model adopts additional yield surfaces, namely inner yield surfaces that are associated with the two proposed plastic mechanisms to account for the plastic behavior inside the existing conventional thermomechanical yield surface namely yield surfaces. The general expressions of the yield surfaces and plastic potentials in p′–q–T space are introduced. A progressive plastic hardening mechanism associated with the inner yield surface is defined, enabling the plastic modulus to vary smoothly during thermomechanical loadings inside the yield surfaces. Several tests on natural Boom clay along different thermomechanical loading paths have been simulated by TEAM, and results show its relevance in describing the thermomechanical behavior of saturated clays. Copyright © 2015 John Wiley & Sons, Ltd.