冻土与结构接触面循环剪切损伤模型

抗剪强度和压缩体应变是冻土与结构接触面的力学特性指标中最重要的两个参数,对冻土与结构接触面的力学分析至关重要。采用自行研制的大型多功能冻土-结构接触面循环直剪系统,开展了冻土与结构接触面力学性能及变形规律的试验研究,采用损伤力学理论,基于等应变分布假设、零弹性域假设、弹性应变无耦合假设,选择不可逆性体应变与最大不可逆性体应变的比值作为损伤因子,建立了用于描述冻土与结构接触面剪切行为的抗剪强度与压缩体应变损伤模型。通过相关试验对建立的损伤模型进行验证,结果表明,所建模型是准确、合理的。     

Coupling between anisotropic damage and permeability variation in brittle rocks

In this paper, a coupled constitutive model is proposed for anisotropic damage and permeability variation in brittle rocks under deviatoric compressive stresses. The formulation of the model is based on experimental evidences and main physical mechanisms involved in the scale of microcracks are taken into account. The proposed model is expressed in the macroscopic framework and can be easily implemented for engineering application. The macroscopic free enthalpy of cracked solid is first determined by approximating crack distribution by a second‐order damage tensor. The effective elastic properties of damaged material are then derived from the free enthalpy function. The damage evolution is related to the crack growth in multiple orientations. A pragmatic approach inspired from fracture mechanics is used for the formulation of the crack propagation criterion. Compressive stress induced crack opening is taken into account and leads to macroscopic volumetric dilatancy and permeability variation. The overall permeability tensor of cracked material is determined using a micro–macro averaging procedure. Darcy's law is used for fluid flow at the macroscopic scale whereas laminar flow is assumed at the microcrack scale. Hydraulic connectivity of cracks increases with crack growth. The proposed model is applied to the Lac du Bonnet granite. Generally, good agreement is observed between numerical simulations and experimental data. Copyright © 2005 John Wiley & Sons, Ltd.     

花岗岩力学特性的温度效应试验研究

通过实时高温（常温～850 ℃）加载和高温（常温～1 200 ℃）后冷却再加载两种情况下的单轴压缩试验,对不同高温下花岗岩的力学性质进行了研究,分析了两种情况下单轴抗压强度、弹性模量、纵波波速、剪切滑移应变等随温度的变化规律,并研究了热-力耦合效应。研究结果表明：（1）在实时高温加载作用下单轴抗压强度和弹性模量随着温度升高而发生连续劣化;（2）高温作用冷却后再加载,花岗岩在常温～600 ℃区间峰值强度变化不大,800 ℃左右岩样强度突然降低;（3）纵波波速随加热温度的升高而逐渐降低;（4）剪切滑移应变在800 ℃之前相对较小,且变化不大,之后便迅速增大,表现出明显的塑性;（5）提出了热-力耦合因子的概念,并借助其提出了一维非线性热-力耦合本构模型,模型曲线和试验曲线较吻合。     

岩盐弹塑性损伤耦合模型研究

岩盐力学模型是进行能源岩盐储存工程稳定性分析的基础,而损伤和塑性机制并存且相互耦合是岩盐力学行为的基本特点。采用云应岩盐,进行了多组围压条件下的三轴压缩试验,分析了不同围压下岩盐的变形特征。在试验分析的基础上,提出了一种能够描述岩盐特性的弹塑性损伤耦合的模型,该模型描述了岩盐损伤的演化和塑性变形的耦合关系,并引入了一种非关联的塑性流动法则来描述岩盐从塑性体积压缩到膨胀的转化。采用该模型对在三轴压缩下的岩盐应力-应变关系进行了模拟分析,并与试验数据进行了对比,结果表明该模型能够较好地描述岩盐的主要力学和变形特性。     

A method for frost jacking prediction of single pile in permafrost

In this paper, a model is proposed to simulate frost jacking performances of a pile foundation within an axisymmetric pile–soil system through a coupling strategy. We consider three diversified stages for frost heave of adjacent foundation soil below freezing point, where mathematical expressions for the volumetric strain are given in terms of volumetric ice content, negative temperature and porosity. A modified strain-softening model characterizing frozen soil–pile interactions is established based on experimental results, taking into account the effects of normal pressure, negative temperature and moisture content. The proposed computational approach is then illuminated and validated via the numerical example of a simplified bridge pile foundation under natural permafrost condition. Variation of temperature regime, volumetric ice content, displacement and stress over time is analyzed. This model can be further applied to evaluating effects of different countermeasures that mitigate frost jacking hazard of single pile subjected to cold climate.

     

高温-高含冰量冻土原位旁压蠕变试验

为了研究高温-高含冰量冻土的蠕变特性,在青藏高原多年冻土区开展了旁压蠕变试验。试验结果表明：旁压蠕变曲线可以借鉴Ladanyi的工程蠕变理论来表示;每个试验中瞬时弹性应变在各级压力下的总应变中所占比例的平均值为3%～7%,冻土温度轻微的改变都会引起剪切模量的大幅变化;冻土的初始蠕变应变可以采用幂函数形式来计算,其强化系数k=1.87,应变系数与负温绝对值之间呈线性正相关关系。稳定蠕变阶段的应变速率可以用Glen蠕变模型来表示,其中参数n随体积含冰量的增加而线性增加;若体积含冰量外推至100%（纯冰）时,n=3.85,这与前人通过室内试验得出的结果有些差异。温度较低时,蠕变速率较小;当温度趋近于0 ℃时,蠕变速率迅速增大。     

水饱和边坡夹层热-孔隙水-力耦合作用模型及应用

为了研究温度对水饱和边坡夹层力学参数的影响,将水饱和边坡夹层视为固-液两相的线弹性体,建立了水饱和边坡夹层热-孔隙水-力耦合作用的力学模型,并推导了其耦合控制方程;采用物理相似模拟的方法,建立了与边坡原型相似的试验模型,研究温度引起的边坡夹层力学参数的变化特征;通过比较分析理论计算结果与模型试验结果,验证了所建立的耦合力学模型的适用性。研究结果表明：孔隙水压力系数和热压力系数是引起水饱和边坡夹层孔隙水压力增加的关键控制因素;孔隙水压力系数取决于孔隙排水压缩特性和固相介质压缩特性,这两者差值越大,孔隙水压力系数越大;孔隙水热膨胀系数和孔隙体积热膨胀系数是影响热压力系数的主要因素,这两者差值越大,热压力系数也越大;边坡夹层孔隙水压力随温度升高呈现出先缓慢增加而后急剧增加的变化特征,而黏聚力和抗剪强度随温度升高而缓慢降低,且孔隙水压力的理论计算结果与试验测试结果吻合良好。因此,水饱和边坡夹层热-孔隙水-力耦合作用的力学模型能较好地反映孔隙水压力在加热升温过程中的变化特征,为科学地预测和控制类似边坡工程的稳定性提供了参考。     

Fabric dilatancy and the plasticity modeling of granular media

A novel conceptual model of the mechanics of sands is developed within an elastic–plastic framework. Central to this model is the realization that volume changes in anisotropic granular materials occur as a result of two fundamentally different mechanisms. The first is purely kinematic, dilative, and is the result of the changes in anisotropic fabric. There is also a second volume change in granular media that occurs as a direct response to changes in stress as in a standard elastic/plastic continuum. The inclusion of the two sources of volume change results in three important datum states. When subjected to isotropic strains, the resulting stress state in granular materials is not isotropic but lies upon the kinematic normal consolidation line. There exists a state at which the fabric‐induced volumetric strain rate becomes equal to the stress‐induced volumetric strain rate making the total plastic volumetric strain rate equal to zero. Granular response changes from contractive to dilative at this phase transformation line. The third datum state is the one in which the stress‐induced volumetric strain rate is zero. The sand, however, continues to dilate at this state with the difference between stress and dilation ratio a constant as predicted by Taylor's stress–dilatancy rule. These predictions are shown in accordance with experimental data from a series of drained tests and undrained on Ottawa sand. Copyright © 2011 John Wiley & Sons, Ltd.     

高温下红黏土热导率的变化规律试验研究

目前国际上对高温下土壤热导率的试验和模型预测研究比较缺乏,通过KD2 Pro测试两种红黏土在较广温度范围（5~90℃）和含水率范围内的热导率,并选择IPCHT模型预测高温下体积含水率-热导率的变化规律。测试结果表明,两种红黏土的热导率对体积含水率的敏感程度与温度有关,且热导率均随温度的升高而增大,在90℃时热导率最高可达5℃的3~4倍。60~90℃范围内热导率随体积含水率的变化存在明显的临界含水率（对应土壤的塑性指数）,但相同温度、体积含水率下,柳州红黏土中水汽潜热传输效应较桂林红黏土要明显。模型预测研究表明,除粉砂质黏壤土外,高温下IPCHT模型预测效果均不理想,经传质增强因子ξ修正后,柳州红黏土以及细砂的热导率预测值和实测值均相符得较好（RMSE < 30%）,但桂林红黏土的整体预测效果仍较差。     

A Transversely Isotropic Damage Model for Boom Clay

Boom clay can be considered as a transversely isotropic geomaterial. However, due to lack of experimental evidence and data base, it is still difficult to describe the transversely isotropic plastic behavior of this argillaceous rock. In this paper, we present first, by means of an experimental approach, the main features of the mechanical properties of Boom clay. Then, combining the transversely isotropic elastic model and the modified Mohr–Coulomb criterion, a suitable constitutive model is introduced so as to fully describe the mechanical behavior of the studied material, in which, an elastic damage law which takes into consideration, the transversely isotropic effect, a plastic hardening law and a plastic damage law were introduced to describe the nonlinear elastic, hardening and softening behavior of Boom clay. As a preliminary step, the evolution law of both elastic moduli and Poisson’s ratio during the elastic stage was obtained by direct analysis of the test data. The synchronism of the elastic damage in both transversal and axial directions was proved by this method. Some of the parameters of the model in the elastic stage were also determined by direct analysis method and further verified by back analysis. Other unknown parameters in the model were determined by back analysis.     

Experimental and theoretical investigation of the high‐pressure behavior of concrete

The results of an experimental study aimed at characterizing the behavior of concrete for high confining pressures (up to 500 MPa) are reported. The main characteristics of the response under deviatoric conditions are quasi‐linearity in the elastic regime, stress‐path dependency, and gradual change from compressibility to dilatancy under increasing deviatoric stress. By performing the cyclic triaxial compression tests with several load–creep–unload and reload cycles, the time influence on the overall behavior was detected. Further, whether the main features of the observed behavior can be described within the framework of elastic/viscoplasticity theory was investigated. For this purpose, Cristescu's (Rock Rheology. Kluwer Academic Publishers: The Netherlands, 1989) approach was used. It was shown that the proposed elastic/viscoplastic model captures the main features of concrete behavior at high pressures. Copyright © 2008 John Wiley & Sons, Ltd.     

A micro-mechanics-based elastic–plastic model for porous rocks: applications to sandstone and chalk

A micro-mechanics-based elastic–plastic model is proposed to describe mechanical behaviors of porous rock-like materials. The porous rock is considered as a composite material composed of a solid matrix and spherical pores. The effective elastic properties are determined from the classical Mori–Tanaka linear homogenization scheme. The solid matrix verifies a pressure-dependent Mises–Schleicher-type yield criterion. Based on the analytical macroscopic yield criterion previously determined with a nonlinear homogenization procedure (Shen et al. in Eur J Mech A/Solids 49:531–538, 2015), a complete elastic–plastic model is formulated with the determination of a specific plastic hardening law and plastic potential. The micro-mechanics-based elastic–plastic model is then implemented for a material point in view of simulations of homogeneous laboratory tests. The proposed model is applied to describe mechanical behaviors of two representative porous rocks, sandstone and chalk. Comparisons between numerical results and experimental data are presented for triaxial compression tests with different confining pressures, and they show that the micro-mechanical model is able to capture main features of mechanical behaviors of porous rock-like rocks.     

基于微结构失稳假说及广义吸力理论的黄土体变模型

针对原状非饱和及饱和黄土的体积变形问题,提出了一个复合体计算模型。该模型将黄土看作由各自均匀分布于土中的原状部分和扰动部分组成的,这两部分土体的体积比率,可由黄土结构的微观几何模型、孔隙率密度曲线及考虑广义吸力的微结构失稳条件计算得出。假设原状部分仅发生弹性变形,扰动部分符合重塑土的变形规律。复合体的应变增量包括原状部分的弹性、重塑土的塑性及结构崩塌3部分;其中：结构崩塌变形特指微结构失稳时原状土体的孔隙率突然变为重塑土的孔隙率;重塑土的体积应变形采用广义吸力理论计算。采用不同的初始饱和度的黄土结构强度计算出非饱和土微观结构刚度与饱和度的函数,从而获得非饱和土黄土微观结构失稳的判别条件。分别计算了低含水天然非饱和及饱和黄土的压缩曲线,并与试验结果对比说明了模型的合理性。     

粗粒土的一种应变硬化模型

堆石料等粗粒土在经受剪切时不仅其强度和刚度随应力水平而变化,其体积变化特征也和应力水平密切相关。在Mohr-Coulomb模型的基础上,提出一个针对这类粗粒土剪切特征的应变硬化模型。模型以Mohr-Coulomb准则作为屈服准则,但考虑土体强度、刚度和体积变化特征均随应力水平而变化等重要特性。对Rowe剪胀理论的推导及其试验基础进行了深入讨论,指出该理论对于粗粒土的局限性,并在此基础上针对粗粒土的剪胀特性进一步发展给出其机动剪胀角的变化规律。与试验结果的计算对比表明,该模型能较为全面地反映粗粒土的主要剪切特性,特别是剪切时的体积变化特性,且相对简便实用。     

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.     

冻融循环条件下岩石弹性模量变化规律研究

岩体冻融损伤机制为温度降低使岩体中的水发生相变、体积膨胀、产生冻胀力的作用,岩体中的微裂隙在冻胀力的作用下扩展延伸,温度升高时,融化的水进入新的裂隙,冻结成冰再次产生冻胀作用,反复循环使裂隙网络扩展,最终造成岩体的损伤。基于此,从弹塑性力学、断裂力学的角度出发,研究了在冻胀力的作用下单裂隙扩展特性,推导了冻胀力与裂纹扩展长度之间的关系,利用Mori-Tanaka方法建立了岩体宏观损伤量与冻胀力及冻融次数之间的关系式,讨论了岩体弹性模量与冻融次数、冻胀应力以及渗透系数的变化规律,并与试验结果进行了比较分析。结果表明,岩体在冻融循环条件下的弹性模量随冻融次数的增加呈非线性减小;冻胀应力越大,岩体弹性模量衰减越快;岩体的渗透系数越大,弹性模量衰减越慢。     

基于广义塑性理论的尾砂本构模型在ABAQUS中的二次开发

建立尾砂的本构模型是开展尾矿坝数值模拟和安全评价的重要基础,而目前尾砂的本构模型研究多集中于非线性弹性模型,如Duncan-Chang模型,关于弹塑性模型的研究较少。结合尾砂的应力-应变特性,提出了一种适用于描述尾砂力学特性的改进广义塑性模型。基于用户自定义材料子程序UMAT,将提出的模型在ABAQUS中二次开发实现,应力积分采用Runge-Kutta显式积分。通过三轴试验模拟验证,偏差应力曲线表明,有限元计算结果可以反映围压对应力-应变关系的影响,抗剪强度随应变逐渐硬化,达到峰值强度,随后发生应变软化,抗剪强度有所下降。体应变曲线结果表明,广义塑性模型可以很好地描述体应变曲线的体缩-体胀发展过程,与试验曲线吻合较好。同时有限元数值模拟结果和理论值误差很小,且和试验结果拟合较好。该研究成果可进一步用于尾矿坝的应力变形计算和安全评价。     

A theoretical model for gas adsorption-induced coal swelling

Swelling and shrinkage (volumetric change) of coal during adsorption and desorption of gas is a well-known phenomenon. For coalbed methane recovery and carbon sequestration in deep, unminable coal beds, adsorption-induced coal volumetric change may cause significant reservoir permeability change. In this work, a theoretical model is derived to describe adsorption-induced coal swelling at adsorption and strain equilibrium. This model applies an energy balance approach, which assumes that the surface energy change caused by adsorption is equal to the elastic energy change of the coal solid. The elastic modulus of the coal, gas adsorption isotherm, and other measurable parameters, including coal density and porosity, are required in this model. Results from the model agree well with experimental observations of swelling. It is shown that the model is able to describe the differences in swelling behaviour with respect to gas species and at very high gas pressures, where the coal swelling ratio reaches a maximum then decreases. Furthermore, this model can be used to describe mixed-gas adsorption induced-coal swelling, and can thus be applied to CO₂-enhanced coalbed methane recovery.