基于统一强度理论的柱形孔扩张问题研究

考虑了岩土材料的软化和剪胀特性,利用空间轴对称问题的统一强度理论分析了柱形孔扩张问题,求出了岩土材料在柱形孔扩张时的应力场、应变场、位移场和最终扩张压力的统一解表达式。对蒋明镜-沈珠江扩孔公式从不同的方面作了论述。通过算例分析了岩土材料的软化和剪胀特性以及统一强度理论参数对应力场、位移场和扩张压力的影响。已有的关于该问题的Tresca解和Mohr-Coulomb解均为本文的一个特例。     

基于广义SMP破坏准则的柱形孔扩张问题理论分析

采用应力跌落的简化应力-应变模型考虑土的应变软化特性,同时采用简化的体积应变?v与大主应变?1及大主应变?1与小主应变?3之间的相互关系反映土的剪胀特性,根据空间准滑动面(SMP)理论和平面应变轴对称问题的柱形孔扩张基本方程,推导并给出一般黏性土中柱形孔扩张问题的应力场、应变场、位移场、塑性区半径和孔扩张压力。通过算例分析,探讨了土的剪胀因素、软化特性对孔扩张问题的影响程度。为了反映中主应力的影响,将本文解与基于Mohr-Coulomb破坏准则的解答进行了比较。计算结果表明,土的剪胀性和软化特性及中主应力对孔扩张问题的影响是显著的,基于Mohr－Coulomb破坏准则的孔扩张解答往往偏于保守。     

等应力增量比路径下接触面应变软化模型

等应力增量比路径单剪试验条件下,部分土体与结构接触面表现出明显的应变软化和剪胀特性。基于广义位势理论,将土与结构的接触面问题看作应力空间上的二维问题,势函数取法向应力和切向应力,用塑性状态方程取代传统的屈服面,建立等应力增量比路径条件下的土与结构接触面应力-应变软化模型。采用指数函数,拟合单向压缩试验中法向应力与法向应变的关系,采用复合指数函数,拟合应力比与切向应变的关系,采用另一复合指数函数,拟合法向剪胀分量与切向应变的关系。通过对拟合函数进行微分,确定模型中待定系数的求解方法。结合试验结果对模型进行验证,模型拟合效果良好,具有一定的合理性。     

考虑土体剪胀性和应变软化性的K-G模型

在三轴试验中密实无粘性土具有剪胀性和应变软化性,广泛应用于工程计算的非线性弹性邓肯-张模型、常规的非线性弹性K-G模型等都不能反映土体的这些性质。根据紫坪铺面板堆石坝筑坝料的大量三轴试验研究,分析密实无粘性土的剪胀性和应变软化性,提出了考虑这种剪胀性和应变软化性的K-G模型和模型参数的求取方法;通过预测曲线与试验曲线对比说明模型的合理性。模型适应多种应力路径,参数可方便地求取,可以在有限元应力-应变分析中推广运用。     

土工环剪仪的开发及其应用研究现状

在回顾国内外有关土工环剪仪研发及其应用研究工作的基础上,介绍了环剪试验设备的开发历程以及当今国际上对环剪试验系统研制方面的最新进展。总结了环剪仪在土工研究工作中的应用情况以及新的试验方法和测试技术等。着重阐述了利用环剪试验在对土峰后应变软化现象与残余强度分析、土的剪切带形成机制试验研究、地质灾害成因机制分析以及土工合成材料界面抗剪强度测定等方面研究中的主要内容、方法和成果。     

基于能量理论的球孔扩张大变形分析

以常用的摩尔-库仑强度准则模拟弹塑性区的本构关系,研究小孔扩张问题,推导了扩孔压力和能耗理论解。基于球孔扩张的假设,结合能量理论和非关联流动法则,将扩孔过程视为能量转换问题,进行黏土大应变能耗分析。弹性区采用小应变理论分析。考虑塑性区弹性变形和塑性区大应变的影响,得到了扩孔压力、能耗与扩孔半径的关系。通过与已发表结果比较,验证了所提方法的有效性。最后,研究塑性区弹性变形和大应变对扩孔压力和能耗的影响。结果表明,扩孔压力随剪胀角的增大而增大,外力所做的大部分功在塑性区转化为能量。剪胀角对塑性区域发展和扩孔压力演化影响显著,随着剪胀角的增加,塑性区半径和扩孔压力明显增加。该能耗理论解为揭示注浆、土工试验与贯入桩等方面提供了一种新的分析手段和必要的理论依据。     

修正剑桥模型在DPC桩—土结构层中的应用分析

针对DPC桩-土结构层开展大型直剪试验,基于试验分析,在考虑DPC桩-土结构层注浆影响上引入临界应力比,构建了可描述应变软化的修正剑桥模型应力应变方程。直剪试验表明DPC桩-土结构层剪切特性表现为剪切软化,具有明显的峰值强度和残余强度,呈现出明显的结构性,其特征与超固结的黏土的剪切应力应变曲线特性类似。推导出的修正剑桥模型能较好的解释实验结果,所得出的结论对DPC桩的设计有一定的指导意义。      

压实粉土非线性应力-应变关系的试验研究

对京九线路基粉土用GDS三轴仪进行固结不排水剪试验,研究不同围压、含水量、压实度下压实粉土样的应力-应变关系。结果表明:压实粉土在低围压下为应变软化,高围压下为应变硬化,存在一个屈服应力;含水量和压实度对粉土强度和应力-应变关系影响显著,随着含水量的减小、压实度的增大,压实粉土样强度增大,应力-应变曲线上升。对围压小于屈服应力的应变软化曲线,用常规的土的软化关系式进行拟和;对围压大于屈服应力的应变硬化曲线,用邓肯-张模型进行拟和;这二者的结合能描述压实粉土的应力-应变关系曲线,效果良好。     

粗粒土剪胀性大型三轴试验研究

剪胀性是土体显著区别于一般弹性材料的基本特性,与土体的强度和变形特性密切相关。通过4组不同初始密度的塔城砂砾石常规大型三轴试验,研究剪胀性对粗粒土强度和变形特性的影响。试验结果表明,（1）若体变速率（体变和轴向应变均以压缩为正）先从正值减小到负值并达到最小值,随后又有所增大但仍小于0,则应力-应变曲线为软化型,在比值为最小值时土体剪胀性最大,对应于峰值强度,若体变速率从某一正值单调减小并一直大于0,则应力-应变曲线为硬化型;（2）体变变化趋势取决于剪胀性和压缩性的大小,剪切后期若剪胀速率大于压缩速率,则体变先压缩后膨胀,应力-应变曲线呈软化型,反之若剪胀速率小于压缩速率,则体变一直是压缩的,应力-应变曲线呈硬化型。研究结果对于加深认识粗粒土的强度和变形特性具有重要意义。     

考虑水合物填充和胶结效应的深海能源土弹塑性本构模型

水合物的填充效应和胶结效应增大了能源土的密实性和强度,使能源土呈现出类似于密实砂土或胶结土的性质。在黏土和砂土的统一硬化模型(CSUH模型)框架下,总结了能源土的力学性质,引入压硬性参量描述水合物对能源土填充和胶结双重作用下的等向压缩特性,引入黏聚强度修正屈服函数并构建了黏聚强度的演变规律,利用状态参数调整剪胀方程,反映能源土剪胀、软化等特性对密实度的依赖性,从而建立能够描述能源土强度、刚度、剪胀与软化等特性的弹塑性本构模型。编制了模型的测试程序,把模拟结果与能源土室内试验结果进行对比。结果表明:提出的弹塑性本构模型能够较好地描述能源土的应力-应变关系、剪缩硬化和剪胀软化等力学特性。     

Undrained cavity expansion in anisotropic soils with isotropic and frictional destructuration

This paper proposes a semi-analytical solution of undrained cylindrical cavity expansion in anisotropic soils with both isotropic and frictional destructuration. The rigorous derivation based on the general form of the SANICLAY model with destructuration is provided following a standardized solving procedure, and the features of anisotropy and structuration are then invoked in the cavity expansion solution by adopting the non-associated hierarchical model. Cavity expansion tests in both structured and unstructured clays with various overconsolidation ratio are conducted to investigate the evolutions of effective stresses, excess pore pressure, anisotropic parameters and structuration factors during cylindrical expansion. The results show that the effective stresses at the cavity wall are lower after expansion and the cavity excess pore pressure is oppositely higher in structured clays with slightly smaller plastic regions. The evolutions of anisotropy for structured clays appear to follow similar patterns to unstructured cases, whereas the degree of anisotropy is further developed with gradual loss of inter-particle bonds. Finally, the proposed solution is applied to predict the limit pressure of pressuremeter tests in Bothkennar clay, showing its ability for interpretation of in situ testing data in natural structured clays.

     

静压沉桩与CPTU贯入离心模型试验及机制研究

为了探究静压沉桩与CPTU贯入力学机制,开展了饱和黏土中静压沉桩及CPTU贯入的离心模型试验,获得了静压沉桩与CPTU贯入过程中土压力、超孔压和贯入阻力的变化规律。同时,将静压桩和CPTU压入过程视为一系列球孔的连续贯入,应用圆孔扩张解答,建立了静压沉桩和CPTU贯入过程中锥头阻力、侧阻力与超孔压的预测方法。通过离心模型试验和理论预测结果的对比分析表明：随着桩体的压入,桩周土体的超孔压和土压力均逐渐增大,当桩头通过监测点时,超孔压与土压力均达到最大值;在饱和黏土中,CPTU锥头阻力、锥侧摩阻力和锥头超孔压与锥头贯入深度总体上呈线性关系。预测方法估算沉桩和CPTU贯入引起的土压力、超孔压与模型试验结果相符,较好地反映了饱和黏土中静压沉桩和CPTU贯入的力学机制。     

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 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.     

饱和土中劈裂灌浆压力研究

在塑性力学和大变形理论的基础上,分析土体在劈裂灌浆初始阶段的力学机理。将劈裂灌浆的初始阶段视为无限土体中的圆孔扩张问题,并将圆孔周围土体中的应力分布分为两个区域。在弹性区中土体服从小变形假设,在塑性区中服从大变形假设。利用水力压裂理论和圆孔扩张理论,假设孔隙水压力大于或等于初始应力与扩张应力增量之和时,土体将出现拉应力,若拉应力超过土的抗拉强度,土体将产生开裂,此时的灌浆压力就为初始劈裂灌浆压力。当土体在灌浆压力作用下,大小主应力正好换位时,则出现劈裂时的灌浆压力就为二次劈裂灌浆压力值,推导出饱和土体劈裂灌浆压力的理论公式。该理论的计算结果与工程实测结果比较接近,初步证实了该理论的可靠性。     

Two-dimensional elastoplastic analysis of cylindrical cavity problems in Tresca materials

This paper presents analytical elastic-plastic solutions for static stress loading analysis and quasi-static expansion analysis of a cylindrical cavity in Tresca materials, considering biaxial far-field stresses and shear stresses along the inner cavity wall. The two-dimensional static stress solution is obtained by assuming that the plastic zone is statically determinate and using the complex variable theory in the elastic analysis. A rigorous conformal mapping function is constructed, which predicts that the elastic-plastic boundary is in an elliptic shape under biaxial in situ stresses, and the range of the plastic zone extends with increasing internal shear stresses. The major axis of the elliptical elastic-plastic boundary coincides with the direction of the maximum far-field compression stress. Furthermore, considering the internal shear stresses, an analytical large-strain displacement solution is derived for continuous cavity expansion analysis in a hydrostatic initial stress filed. Based on the derived analytical stress and displacement solutions, the influence of the internal shear stresses on the quasi-static cavity expansion process is studied. It is shown that additional shear stresses could reduce the required normal expansion pressure to a certain degree, which partly explains the great reduction of the axial soil resistance due to rotations in rotating cone penetration tests. In addition, through additionally considering the potential influences of biaxial in situ stresses and shear stresses generated around the borehole during drillings, an improved cavity expansion approach for estimating the maximum allowable mud pressure of horizontal directional drillings (HDDs) in undrained clays is proposed and validated.     

饱和膨润土及其与砂混合物的压缩变形特性

对用不同制样方法得到的饱和膨润土及其与砂混合物进行了压缩试验。试验结果表明,饱和膨润土的压缩曲线呈双线性,不同于普通黏土的压缩曲线。压缩试验中量测了侧向应力,由此得到的饱和膨润土的静止侧向压力系数值较一般黏土的数值要大。对膨润土与砂混合物的击实样进行了由非饱和到饱和状态的浸水试验,并得出试验过程中侧向应力的变化规律。由于浸水饱和的试样和抽真空饱和的试样在较高压力时压缩曲线趋于一致,可采用快速抽真空饱和的方法进行试验研究,以缩短非饱和混合物击实样浸水饱和所需时间。引入骨架孔隙比的概念,用来判断膨润土与砂混合物中砂骨架是否形成,得出影响混合物压缩特性的决定因素。     

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.     

K0固结饱和土柱孔扩张问题弹塑性分析

采用K0固结各向异性土体本构模型,将柱孔扩张后周围土体分为弹性区和塑性区,根据柱孔扩张理论和边界条件,推导出K0固结状态下饱和天然土体柱孔扩张问题弹塑性区的应力、塑性区半径以及超孔隙水压力的理论解答。同时,通过算例与修正剑桥模型解答进行对比分析,结果表明,土的不同初始固结状态对柱孔扩张后孔周围的应力和超孔隙水压力产生很大影响,采用考虑K0固结诱发各向异性土体本构模型所得到的应力和超孔隙水压力解答大于修正剑桥模型的解答,但塑性区影响半径却明显小于后者。     

Stress and pore pressure changes in clay during and after the expansion of a cylindrical cavity

The disturbance of a clay mass, due to either the installation of a driven pile or the expansion of a pressuremeter membrane, is often modelled as a cylindrical cavity expansion. In addition, it is usual (and convenient) to assume that the expansion occurs under conditions of plane strain. For this problem a method of analysis is presented which considers the soil to be a saturated two-phase material with a pore fluid which flows according to Darcy's Law. Non-linearity in material behaviour is permitted as long as the effective stress–strain law can be written in an incremental or rate form. The use of a consolidation analysis allows the changes in effective stress and pore pressure to be determined at any stage during both the cavity expansion and the subsequent period of reconsolidation. Expansions may occur at any prescribed rate, including the very fast (undrained) and the very slow (fully drained) case. The technique is illustrated by considering the expansion of a cavity in two different types of elastoplastic soil. It is shown how these solutions may be used to model the disturbance of the soil due to pile driving.