含统一屈服面的非饱和黏性土弹塑性本构模型

首先将非饱和土巴塞罗那基本模型(Barcelona basic model,BBM)中的加载湿陷(loading collapse,LC)屈服面和吸力增加(suction increase,SI)屈服面统一为一个光滑的屈服面,然后基于弹塑性理论导出相应的弹塑性本构模型.统一屈服面的优点是可以进行LC屈服和SI屈服二者之间的耦合分析,即能模拟净应力和吸力同时变化的加载路径.此外,由于所建模型的屈服面没有尖点,因而可以简化数值模拟计算.所建模型共有14个参数,其中12个与非饱和土的物理力学性质和塑性屈服特性有关,另外2个与土的塑性硬化特性有关,所有参数都可通过非饱和土室内控制吸力的三轴等压和剪切试验确定.所建模型的模拟结果与本次试验研究及已发表文献中试验数据的对比表明,二者之间有较好的吻合,说明所建模型的合理性及有一定的实用价值.     

非饱和土等效时间流变模型

为了理论分析流变速率效应力学特性,需要建立非饱和土流变模型.首先把非饱和土屈服面分为LC和SI屈服面。其次根据LC屈服面的压缩蠕变公式,利用等效时间法建立相应的黏塑性体应变速率公式,结合巴塞罗那屈服方程和相关联流动法则,获得LC屈服面的三维流变速率方程,揭示了本文与Gennaro-Pereira提出的LC后继屈服面方程的异同。再次,根据SI屈服面的吸力压缩蠕变公式,利用等效时间法建立相应的黏塑性体应变速率方程.最后,把LC和SI屈服面的流变速率方程与弹性方程相结合,建立非饱和土等效时间三维弹黏塑性模型.当不考虑流变速率效应时,LC和SI流变屈服方程可以退化到巴塞罗那模型的塑性屈服方程.理论分析了非饱和土一维流变特性,并与土工试验数据进行对比,两者较为吻合。三轴加载条件下的流变特性数值分析结果表明,按不同方式施加到同一恒载时,随着恒载维持时间的增长,不同加载方式引起的体积流变趋向一致,但剪切流变有较明显的差别.     

非饱和土的黏弹塑性本构模型研究

非饱和土变形是随时间发展的,为了描述非饱和土随时间变化的长期变形行为,本文建立了非饱和土黏弹塑性模型.在已有的非饱和土率无关模型的基础上,考虑应变率对变形和强度影响,建立了一个非饱和土率相关的黏弹塑性模型.根据试验结果,应变率越大,屈服应力则越大,将屈服应力表示为应变率的函数,给出率相关影响的加载湿陷屈服面.利用一致性条件,给出了一维条件下非饱和土的增量方程.对模型进行数值分析,对模型基本行为进行预测.将模型退化饱和条件和率无关2种情形,利用已有的试验结果对模型进行了验证,结果表明模型可以用于预测非饱和土的蠕变行为.     

重塑非饱和土抗剪强度指标与含水状态变化的关系

土体含水状态的变化对非饱和土强度的影响极为明显,对重塑非饱和粉质粘土抗剪强度进行分析,建立非饱和土抗剪强度的计算模型.通过室内非饱和土快速剪切试验,得到不同含水量的非饱和土抗剪强度指标,根据数值分析,建立重塑非饱和粉质粘土抗剪强度指标与含水量的关系,为工程设计及施工提供依据.     

常吸力下非饱和土水-力耦合循环塑性的模拟

为描述非饱和黏性土在静、动态荷载作用下的应力-应变特征及水-力耦合效应,在塑性增量流动理论的框架内建立一个描述常吸力条件下非饱和黏性土水-力耦合效应的弹塑性双面模型.基于BBM(Barcelona basic model)及Li和Meissner提出的一种塑性硬化准则,并考虑非饱和土的典型土水特征,通过边界面和加载面在...     

根系固土边坡的滑移变形模拟

在平面应变的假定下求解均质无限长土坡的应力平衡方程以得到坡土应力分量。考虑根系固土作用,对沈珠江水滴形屈服面进行了修正。通过拟定坡土的滑移速度和黏塑性应变率的关系式,得出滑移变形速率的显式表达式。经修正黏滞性参数来改进模型的模拟精度。最后通过一个算例,将模型计算值和现场实测滑移数据进行比较,分析表明二者较为一致。     

非饱和土Barcelona模型修正及存在问题探讨

在分析、研究各种修正的基础上,对当前非饱和土Barcelona模型中存在的问题进行了探讨,包括对土体压缩线,LC屈服面和SI屈服面的修正,及流动法则的选用等.研究表明各种修正有很多不吻合之处,除了土体自身特性不同会引起差异外,对机理认识的不同是主要原因.除现有修正外,试验结果表明,模型中还有很多因素尚未考虑,如土体临界状态线斜率;LC和SI屈服面的选择、确定及相互间的耦合运动;滞水特性描述及合适的应力变量选用;土体凝聚力随吸力变化情况,及土体初始结构、沉积历史影响等.研究结果指出两屈服面之间的耦合作用和滞水特性尤为重要,这是实际工程中较高压力和吸力循环变化时非饱和土稳定和变形预测的理论基础.     

灌木对黄土边坡的加固效应

植物对边坡稳定性的贡献源于根系对土体的加劲作用、根系吸水消耗孔隙水压力及其引起的抗剪强度增加.通过对非饱和含根土开展剪切试验建立了根-土复合体的抗剪强度模型,其抗剪强度是根系密度和土壤含水率的函数.通过建立二维根系吸水模型,开展根系吸水条件下的渠道边坡土壤水分运动数值模拟.根据土壤水分动态分布和根-土复合体抗剪强度模型计算了不同气象阶段植被渠道边坡的稳定性.对比相同时刻植被边坡和裸土边坡的稳定性计算结果发现,植物根系通过加劲土壤和吸取土壤水分能够显著增强黄土渠道边坡的稳定性.     

非饱和重塑土的临界状态研究

临界状态模型是非饱和土结构理论中的一个重要的内容。通过非饱和土排水剪切以及不排水剪切三轴试验,结合非饱和土临界状态模型中的四个状态参量,探讨了非饱和土临界状态的特点。     

非饱和土剪切试验颗粒流模拟

实验研究基质吸力对非饱和土的变形及强度特性影响耗时耗力,为减少实验所带来的问题,采用颗粒流理论,以颗粒黏结强度为桥梁,建立非饱和土的PFC3D直剪数值模型,模拟不同固结应力及基质吸力作用下非饱和红土的力学特性,得到其剪切应力与剪切位移的关系。基于摩尔-库伦准则,通过拟合计算得出与基质吸力相关的黏聚力c、内摩擦角φ并与实验值对比;通过黏聚力与基质吸力的关系得到φb角,数值模拟与实验值非常接近,结论验证了模拟的合理性。研究成果为非饱和土性质验证提供了新的思路。     

Coupled hydro-mechanical analysis of slope under rainfall using modified elasto-plastic model for unsaturated soils

Two modifications for the basic Barcelona model(BBM) are present. One is the replacement of the net stress by the average skeleton stress in unsaturated soil modeling, and the other is the adoption of an expression for the load-collapse(LC) yield surface that can match flexibly the normal compression lines at different suctions. The predictions of the modified BBM for the controlled-suction triaxial test on the unsaturated compacted clay are presented and compared with the experimental results. A good agreement between the predicted and experimental results demonstrates the reasonability of the modified BBM. On this basis, the coupled processes of groundwater flow and soil deformation in a homogeneous soil slope under a long heavy rainfall are simulated with the proposed elasto-plastic model. The numerical results reveal that the failure of a slope under rainfall infiltration is due to both the reduction of soil suction and the significant rise in groundwater table. The evolution of the displacements is greatly related to the change of suction. The maximum collapse deformation happens near the surface of slope where infiltrated rainwater can quickly reach. The results may provide a helpful reference for hazard assessment and control of rainfall-induced landslides.     

河北省非饱和土深基坑稳定性研究

为了研究河北省非饱和土深基坑的稳定性,首先采用三轴试验对不同含水量土样的抗剪强度参数进行了测定,探究土样含水量与抗剪强度参数的关系.然后,以河北省非饱和土基坑工程为例,对含水量不同的非饱和土深基坑边坡的稳定性进行了分析.研究结果表明:含水量变化对非饱和土的粘聚力有较大的影响,随着含水量的增加,土的抗剪强度参数降低,使得相同状态下基坑边坡的稳定安全系数呈现出明显的下降趋势.     

Slope Stability Analyses of Outang Landslide Based on the Peak and Residual Shear Strength Behavior

Most of the natural and compacted fine-grained soil slopes that are in saturated or unsaturated condition undergo a large deformation prior to reaching failure conditions. Such slopes should be designed taking account of their strain-softening behavior using the residual shear strength(RSS) parameters. In this paper, the slope stability of a recently reactivated Outang landslide near the Three Gorges Dam in China is analyzed based on the RSS parameters of unsaturated soils. In addition, comparisons are provided in the FOS values of slope using both the peak shear strength(PSS) and RSS parameters. Firstly, a series of site investigations of the hydrologic and geologic conditions, ground surface displacements and cracks were described. The PSS and RSS behaviors of the sliding soils derived from a series of direct shear test results performed on saturated and unsaturated soil specimens are summarized.Secondly, a series of slope stability analysis were conducted considering the precipitation and Yangtze River water level variation within a representative period of 7 months, based on the PSS and the RSS properties. In this study, three different scenarios were considered, which include: i) considering only the precipitation with a constant water level; ii) considering only the decrease in water level without rainfall; iii) considering the combination of precipitation and decrease in water level. In each scenario, four steps were included to calculate the values of factor of safety(FOS) at different times. 1) A steady-state seepage analysis was conducted with a constant total head at 525 m on the left boundary and 175 m on the slope surface below the Yangtze River water level. The initial pore water pressures were simulated in the slope under no precipitation and variation of water level. 2) A specific boundary condition was applied on the slope surface to model the precipitation and Yangtze River water level variation. A transient seepage analysis was conducted to calculate pore water pressures at different times based on the initial pore water pressures. 3) The FOS values at different times were calculated by the Morgenstern-Price method taking account of the variation of pore water pressures at different times, using the peak shear strength(PSS) parameters. 4) The last step was repeated replacing PSS parameters with RSS parameters.The RSS parameters were lower than the peak values from laboratory's direct shear test results for the soils in the sliding zones. The reduction in shear strength from peak to residual state under unsaturated soil condition was greater than that for a saturated soil. The FOS decreased almost linearly with time for the scenario in which only the influence of rainfall infiltration was considered. However,the total reduction in the FOS was relatively small. The FOS decreased rapidly at a linear rate with respect to time with a decrease in water level for the scenario in which Yangtze River water level decrease was considered. The FOS reached to a relatively constant value after Yangtze River water level reached the lowest value. The decrease in Yangtze River water level was the dominant factor that contributed to a reduction in the FOS. The FOS was strongly dependent on the development of the phreatic line after the Yangtze River water level reached the lowest value. The FOS calculated by RSS(i.e. FOSR) is less than unity; they were approximately 16%lower in comparison to that calculated by PSS(FOSP). If PSS parameters were used, the slope would still be stable even under the combined influence of precipitation and Yangtze River water level decrease. These results are inconsistent with the field observations. For this reason, the RSS parameters should be taken into account to evaluate reliably the slope stability of the Outang landslide.     

路基非饱和土抗剪强度的吸力效应

从宏观的吸力控制型直剪试验和微观的土体颗粒结构2方面入手,对路基非饱和土抗剪强度的吸力效应进行了研究。在不同吸力和不同法向应力条件下完成4组剪切试验,并选取2种不同含水量的同类型土样进行结构扫描。研究结果表明,与粘性土不同的是,当粉质砂土含水量逐渐降低时,土体基质吸力对土体抗剪强度的贡献效果并不是一直增加的,而是存在一个峰值。这种现象主要是由于土体微观结构以及所含水分的存在形式所造成的。另外,土体所处的应力环境也会对土体强度的吸力贡献情况造成影响。     

Influence of water content and shear rate on the mechanical behavior of soil-rock mixtures

Soil-rock mixtures (S-RMs) are widely distributed in the nature. The mesoscopic deformation and failure mechanisms as well as the macro-mechanical behaviors of the S-RMs depend largely upon the rate of deformation, water content and particle sizes. In this research, a series of large-scale direct shear tests with different water contents and different grain-size distributions were conducted to study the influence of the aforementioned factors on the mechanical properties of the S-RMs. Due to the effect of the rock blocks’ breakage in the S-RMs, the relationship between the shear strength and the vertical stress of S-RM follows a power law instead of a linear one. It is found that there exists a threshold value for the vertical stress during the shearing process, below which the soil strength is mainly determined by the inter-locking of particles and the re-arrangement of meso-structure, and otherwise large-sized rock blocks are gradually broken into smaller fragments, resulting in a decrease in the soil strength. The shear rate can also significantly influence the degree of particle breakage and the meso-structural rearrangement of the S-RMs, namely, under low shear rate, the particles of the samples are fully broken resulting in enhanced macro-strength. As a result, the lower the shear rate, the higher the macroscopic strength. So under unsaturated conditions, the water content will affect the strength of the S-RMs by reducing the strength of rock blocks. As the water content increases, the soil strength decreases gradually, and assumes a moderate value when the water content reaches 8%. At the same water content, the soil strength increases with the sizes of large rock blocks. For the occlusion, breakage and structure re-arrangement of the oversized rock blocks inside S-RM, which have a huge influence on the mechanical characteristics of the samples.     

考虑多种强化机制土的内时弹塑性本构模型

为建立更符合实际情况的岩土的弹塑性应力应变模型,在考虑变形历史、围压、应变率、偏斜塑性变形对体积塑性变形影响引起的多种强化机制的基础上,合理地定义了广义时间标度,建立了考虑多种强化机制土和体量与偏量相互耦合的内时弹塑性本构关系。结合土的静水压缩载荷对偏斜响应影响小,而偏斜压缩载荷对体积响应影响大的实验事实,给出了模型参数分别用静水压缩实验数据和常规三轴实验数据确定的方法。用所建立的考虑多种强化机制的内时弹塑性本构模型对中密永定河砂的三轴响应进行了预测。由模型预测的砂土应力应变曲线与实验相吻合,并且能够较好的描述永定河砂的剪胀和剪缩现象。     

粒状土与结构接触面多重剪切边界面模型

针对工程中存在大量的土与结构物相互作用,使接触面的力学特性对土体和结构物的受力变形及其相互作用产生重要影响的问题,利用塑性滑移理论,将粗粒土与结构接触面的宏观变形分解为一个宏观法向变形和剪切面内一系列不同方向分布的相互独立虚拟微观剪切变形;通过虚功原理,在粒状土的状态相关和边界面弹塑性理论框架内建立一个粗粒土与结构接触面的多重剪切边界面模型.每个微观剪切变形包含一个微观剪应力-应变关系和一个微观应力-剪胀关系.引入一个与接触面土体密度和法向应力相关的状态参数,用以描述不同状态下土与结构接触面的变形和强度特性.对不同粗粒土与结构接触面在二维或三维应力条件下的单调和循环剪切试验进行模拟计算,模型预测与试验结果吻合良好,说明模型能够合理地描述粗粒土与结构接触面的非线性力学行为.