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.     

黏土地基中能量桩力学特性数值分析

能量桩是将地源热泵系统中的换热管埋置在桩体内部,桩同时起到承载和换热的作用,是一种新型的基础型式。为了合理分析黏土地基中能量桩的力学特性,需要了解能量桩运行过程中桩和地基土的温度响应,并考虑温度变化对土体力学性能的影响。基于有限元软件ABAQUS建立了能量桩传热分析三维有限元模型,把能量桩的传热简化为换热管内液体与管壁之间的对流传热、桩体中的热传导和地基中的热传导,将计算结果与常规理论和实测数据进行了对比验证。对热力耦合边界面本构模型进行了二次开发,通过算例验证了模型对土体压缩和剪切性状温度效应的模拟能力。利用所提出的能量桩传热分析方法和热边界面模型,考虑不同的桩顶工作荷载水平,对正常固结黏土地基中能量桩单桩的长期性能进行了研究,分析了温度循环对桩顶沉降、桩侧摩阻力和桩身轴力的影响。结果表明,工作荷载越高,温度循环次数越多,桩顶累积沉降越大。     

Experimental and numerical studies of rock salt strain hardening

The work presented in this paper comes as part of a research program dealing with the thermomechanical behaviour of rock salt. It aims to study laboratory and in-situ long-term behaviour by means of creep tests with deviator and temperature changes. The laboratory results, using a triaxial multi-stages creep tests, highlighted the strain hardening character of rock salt. Furthermore, the in-situ results, using a borehole dilatometer multi-step creep test, have shown that the drilling is carried out in a weakly stressed pillar. The interpretation of the laboratory results, using the J.LEMAITRE law, did not indicate full agreement with all the test results. As a result a ‘double’ J.LEMAITRE model, which takes into account a double strain hardening variable, has been put forward. The validation of this model on the laboratory creep tests is very satisfactory. Furthermore, the activation energy seems satisfactory to represent the influence of the temperature. The in-situ behaviour modelling is clearly more complex than the modelization based on laboratory tests. In fact, it seems that if the rock salt behaviour is maintained by J.LEMAITRE law, it is necessary to vary with the stress, at least, one of the parameters assumed constant in the basic law.     

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.     

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 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.     

温度对粘性土介质力学特性的影响

分析了温度效应对粘性土介质若干基本力学特性的影响。对粘性土的热固结问题、温度对粘性土介质渗透特性的影响、粘性土中的热传导规律及热阻抗特性、土-水体系在温度效应下的作用机理、温度作用下粘性土的本构规律等进行了深入研究。特别就温度诱致的孔隙水压力的变化机理及不同性质土类粘性土体积变化的可逆性问题进行了探讨。     

工作荷载下温度循环对桩基变形与应力的影响分析

目前针对工作荷载下温度循环对桩基承载力特性的研究相对较少,基于室内模型试验方法,对饱和砂土中工作荷载下桩体在多次冷热循环作用时的承载特性和传热特性进行研究,测得温度循环作用下桩体和桩周土体温度、桩周水平土压力、桩体应变以及桩顶位移随时间的变化规律。试验结果表明,施加温度循环作用后桩体及桩周土体温度变化不大,桩周水平土压力也能基本恢复到初值,但在桩体内部则会产生较少残余应变,桩顶下沉并随着循环次数的增加不断累积,从而影响结构的整体承载能力。     

U型、W型和螺旋型埋管形式能量桩热力学特性对比模型试验

能量桩是一种可以节省地下空间和施工埋管费用的新技术,目前针对其在供暖和制冷过程中土体和桩基之间的相互作用机制研究却相对较少。基于模型试验方法,系统地研究了饱和砂土中单U型（绑扎和预埋形式）、W型和螺旋型等4种不同埋管形式情况下的能量桩热力学效应、传热和承载特性,测得桩体和桩周土体温度、桩端阻力、水平土压力、桩顶位移以及桩体应变随时间的变化规律。试验结果表明,在相同输入功率情况下W型埋管形式桩体的温度、土体压力、应力和桩顶位移均较螺旋型和U型埋管形式的情况大。     

地下含水层储能两阶段热量运移数值模型研究

为了简单而准确地预测地下含水层储能情况,考虑了地下含水层储能过程时间跨度大、储能保温分阶段的实际特点,分析了地下含水层流动和换热模型物理参数的对比特点,以热平衡和热扩散原理为基础,建立分成两个连续阶段的地下含水层储能数值计算模型.模型求解中,采用控制容积法,以全隐格式进行热扩散方程的离散化,然后应用Jacobi方法迭代求解,模拟结果和实际观测数据吻合很好.该模型还分析了含水层储能循环采灌过程中抽出储能水的温度变化的一般特点.     

Modelling of the physical behaviour of clay barriers close to water saturation

The physical properties of bentonite-based buffer materials for nuclear waste repositories have been investigated by a number of different laboratory tests. These tests have yielded a material model that is valid for conditions close to water saturation and is useful for describing: (a) the stress, strain and volume change behaviour; (b) the pore pressure and flow of water; and (c) the thermal and thermomechanical response.

The material model is based on the Drucker-Prager Plasticity model and a Porous Elastic Model. The effective stress concept and Darcy's law are applied and the swelling/consolidation and thermomechanical processes are coupled according to the separate mechanical properties of the pore water, the solids and the clay skeleton. The model can be used by the finite-element program ABAQUS.

The model has been tested in several laboratory and field verification tests. Comparison between measured and calculated behaviour shows that the general behaviour is described properly and several calculations of different scenarios have been made for the Swedish KBS 3 concept. However, certain processes, like the hysteresis effect at consolidation/swelling, the curved stress-strain relation at shearing, and the curved failure envelope, are not modelled in a perfectly accurate way and an improved material model is proposed here. It combines the behaviour of the Cam-clay model on the wet side with the more relevant plastic behaviour of a modified Drucker-Prager model with a curved failure envelope and the possibility to introduce strain softening after failure.

The paper presents some laboratory results that are the basis of the first model. It also shows the application of the model to finite-element calculations of some laboratory tests. Comparisons between the calculations and measured results expose some disadvantages of the model and a concept for an improved model is suggested.     

A new bounding surface model for thermal cyclic behaviour

To accurately predict soil volume changes under thermal cycles is of great importance for analysing the performance of many earth structures such as the energy pile and energy storage system. Most of the existing thermo‐mechanical models focus on soil behaviour under monotonic thermal loading only, and they are not able to capture soil volume changes under thermal cycles. In this study, a constitutive model is proposed to simulate volume changes of saturated soil subjected to cyclic heating and cooling. Two surfaces are defined and used: a bounding surface and a memory surface. The bounding surface and memory surface are mainly controlled by the preconsolidation pressure (a function of plastic volumetric strain) and the maximum stress experienced by the soil, respectively. Under thermal cycles, the distance of the two surfaces and plastic modulus increase with an accumulation of plastic strain. By adopting the double surface concept, a new elastoplastic model is derived from an existing single bounding surface thermo‐mechanical model. Comparisons between model predictions and experimental results reveal that the proposed model is able to capture soil volume changes under thermal cycles well. The plastic strain accumulates under thermal cycles, but at a decreasing rate, until stabilization. Copyright © 2017 John Wiley & Sons, Ltd.     

Experimental study of thermal effects on the mechanical behaviour of a clay

The paper presents the results of an experimental study of thermal effects on the mechanical behaviour of a saturated clay. The study was performed on CM clay (Kaolin) using a temperature-controlled triaxial apparatus. Applied temperatures were between 22 and 90°C. A comprehensive experimental program was carried out, including: (i) triaxial shear tests at ambient and high temperatures for different initial overconsolidation ratios; (ii) consolidation tests at ambient and high temperatures; and (iii) drained thermal heating for different initial overconsolidation ratios. The obtained results provide observations concerning a wide scope of the thermo-mechanical behaviour of clays. Test results obtained at 90°C were compared with tests performed at ambient temperature. Based on these comparisons, thermal effects on a variety of features of behaviour are presented and discussed. Focus is made on: (i) induced thermal volume change during drained heating; (ii) experimental evidence of temperature influence on preconsolidation pressure and on compressibility index; (iii) thermal effects on shear strength and critical state; and (iv) thermal effects on elastic modulus. Thermal yielding is discussed and yield limit evolution with temperature is presented. The directions of the induced plastic strains are also discussed. Several remarks on the difference in the mechanical behaviour at ambient and high temperatures conclude the paper. Copyright © 2004 John Wiley & Sons, Ltd.     

能量桩单桩工作特性简化分析方法

基于荷载传递法,建立了热力耦合作用下能量桩单桩工作特性的简化分析方法。该方法中将桩-土荷载传递函数取为双曲线,采用曼辛法则模拟温度循环过程中桩-土界面的卸载和再加载特性,通过再加载过程中刚度的折减近似考虑塑性变形的积累。利用矩阵位移法求解控制方程组后可直接得到任意温度-力学组合作用下的桩体变形、桩身轴力、桩侧阻力和桩端阻力,无需事先假设温度位移零点的位置。通过与试验数据的对比分析,验证了所提方法的可靠性。结合算例,研究了能量桩的长期工作特性。结果表明,温度循环会造成自由桩的桩顶沉降增加,固定桩的桩顶应力减小,温度循环的影响与桩顶静力荷载水平和土体刚度的衰减程度密切相关。     

深层含水层地下储热技术的发展现状与展望

深层含水层储热是一种利用深度>500 m的深层含水层作为储热介质的储热技术,储热对象通常为50~150 ℃的热水。它通过地下水井从深层含水层中抽取和灌入地下水,实现热能储存和回收。深层含水层储热技术是弥补能源供需时空分布的不平衡,综合利用多种可再生能源,实现节能减排的有效途径,是国内外研究的前沿和热点。文中首先阐述了深层含水层储热系统在世界范围内的历史发展,归纳储热系统的热工性能,在总结前人研究工作的基础上分析影响其热回收效率的关键参数,并对各个参数对热回收效率的敏感性做了综述。在此基础上,本文还讨论了限制深层含水层储热系统发展的技术瓶颈,并针对系统的经济效益和市场潜力做了预测和展望。     

Estimating apparent thermal diffusivity of soil using field temperature time series

Reliable estimates of soil thermal properties such as heat capacity, thermal conductivity, and diffusivity are important in analysis of heat transmission through soils in applications such as shallow geothermal applications, buried electrical conduits, and in general heat/fluid flow analyses. A number of analytical, numerical and experimental methods are available to determine the soil thermal properties. In this paper, the analytical and numerical methods developed on the basis of one-dimensional heat conduction equation are used to estimate the apparent thermal diffusivity of soil. Three of the four analytical methods, Amplitude, Phase, and Arctangent provide explicit equations for the apparent thermal diffusivity. Two methods, Harmonic and Numerical, make use of large number of temperature measurements to implicitly solve for the apparent thermal diffusivity. The temperature time series data monitored at different depths in two field sites in Melbourne, Australia for more than 2 year period were used to estimate the apparent thermal diffusivity of soil down to 2 m depth. The apparent thermal diffusivity was calculated using all five methods and compared with laboratory experimental results. The effectiveness of each method in predicting the thermal diffusivity was compared and observed discrepancies were discussed. Finally, the observed soil temperature data for a 12 month period are used to model the temperature variation in the ground analytically using Harmonic method and the model prediction for the following 12 month was compared independently with the field measurements. The analytical model prediction is found to be in good agreement with the field monitored data.     

Local scale heat advection

Surface processes play an important role in the simulation of desertification and climate change. The present study shows that enhancement of evaporation at a given place is due to surface temperature and wetness inhomogeneities compared to its surroundings. Between any adjacent dry and wet strips the induced advective heat transport changes the available net radiation between sensible and latent heat fluxes. It also accommodates a redistribution of surface energy between two adjacent inhomogeneous surface strips. A combined model of Tarpley (1994) and Ya Guo and Schuepp (1994) about the advective heat transport over dry and wet strips has been tested over Andhra Pradesh. Taking the Ananthpur district as a relatively drier region, the induced transport over the rest of the districts is estimated. This study has been done for two monsoon seasons of 1990 and 1991. Attempts are made to identify an indexing criteria based on this study to estimate the magnitude of the heat transport.