非饱和膨胀土抗剪强度的试验研究

膨胀土是一种特殊的非饱和土，经典的土力学理论在膨胀土问题中己显得无能为力。因此，用非饱和土力学理论来研究膨胀土问题在理论和实际两方面都具有重大意义。在非饱和土抗剪强度理论中，吸力的量测在工程实际中仍没有一种简单易行的方法。基于这种实际情况，试图通过其他间接的途径来代替吸力的量测，以确定非饱和土的吸附强度。对于膨胀土这种典型的非饱和土，膨胀力是其很重要的性质之一，它的大小受含水量的影响很大；另一方面，膨胀土的抗剪强度也随含水量的变化而不断地变化。进行了大量的膨胀力试验和抗剪强度试验，以确定膨胀土的膨胀力与吸附强度是否有一定的关系。通过对黑山土和梅山土的重塑试样试验得到的试验数据分析发现：膨胀力和含水量之间存在良好指数关系；粘聚力的对数和内摩擦角均随含水量的增大线性减小；非饱和膨胀土的吸附强度与膨胀力之间存在较好的线性关系，并在此基础上优化了非饱和膨胀土抗剪强度公式。     

粘性土抗剪强度研究的现状与展望

在土力学计算分析工作中,抗剪强度（土的内摩擦角和粘聚力）是其中最重要的计算参数。能否正确地测定土的抗剪强度,往往是设计质量和工程成败的关键所在。本文综合论述抗剪强度研究工作中四个方面的重要问题：（１）应力、应变和加荷速率对抗剪强度的影响;（２）非饱和土的吸力和膨胀力与吸附强度的关系;（３）内摩擦角和粘聚力的特性及其变化规律;（４）从局部剪切发展到整体失稳过程中的应力变化和强度变化。岩土工程师需要对这些问题有全面的认识,才能对抗剪强度的试验资料作出正确的判断与应用。     

非饱和土强度公式的比较研究

 非饱和土力学的基本理论已经确认了基质吸力的存在有利于非饱和土的强度提高,但是对于强度提高的机制到目前为止没有统一的结论,非饱和土强度理论中存在着单应力状态变量公式和双应力状态变量公式2种基本的理论表达式,并在此基础上产生了许多实用的强度公式。通过珞珈山土样的土–水特征曲线试验和非饱和三轴剪切试验对非饱和土强度公式进行比较研究。结果表明,珞珈山土样的进气值和残余含水量分别为237.7 kPa和4%;珞珈山土样非饱和强度在低吸力范围内,与吸力基本成线性增长关系,验证了非饱和土双变量强度公式的适用性;珞珈山土样的非饱和抗剪强度参数约为20.7°;现有的非饱和土实用公式精确性较差,非饱和土抗剪强度理论的应用有待进一步的研究与完善。     

从粒间吸力特性再认识非饱和土抗剪强度理论

首先对非饱和土中吸力进行了综合分类并对各类吸力的性状进行了系统阐述 ,认为非饱和土中土颗粒间的总吸力由本征结构吸力、可变结构吸力、有效基质吸力、湿吸力和牵引力组成。基于各吸力的特性和Mohr -Coulomb强度准则 ,从理论上直接给出了非饱和土的总吸力与抗剪强度间的关系 ,这不仅从形式上统一了Bishop和Fredlund的强度公式 ,而且在内容上明确了其中各状态变量的物理意义。文中分析了非饱和土抗剪强度的组成及其影响因素。对Donald在非饱和土试验中发现的砂性土抗剪强度与基质吸力间呈山坡形的曲线关系及Gan、Escario、Fredlund等的粘性土抗剪强度和基质吸力 (含水量 )间呈非线性正比关系 ,以及对同一非饱和土因具不同初始孔隙比而具不相同的抗剪强度现象给出了令人满意的理论解释。广义抗剪强度公式客观地反映了非饱和土的强度特性     

非饱和黄土的强度特征

以初始基质吸力为条件，根据大量的试验测定和分析，得到了非饱和黄土的粘聚力与基质吸力的相关关系；阐述了非饱和黄土的抗剪强度随基质吸力的变化特征；给出了临界基质吸力的计算式。为进一步采用控制吸力试验，揭示非饱和黄土抗剪强度的基本规律作了初步探索     

基于SQP和上限法的非饱和土条形基础极限承载力计算

综合考虑有效内摩擦角、有效黏聚力、基质吸力等非饱和土强度参数以及基底以下基质吸力随埋深增加而变化的工程实际(以水土特征曲线表征),采用多刚性块上限分析法对非饱和土条形基础竖向极限承载力进行研究.假定非饱和土抗剪强度公式是Mohr-Coulomb饱和土抗剪强度公式的延伸,根据线性破坏准则和相关联流动法则,构建一个承受竖向荷载的非饱和土地基承载力二维机动破坏模式.根据外力功率与内部耗能相等原理获得非饱和土地基极限承载力的目标表达式,并把其转化成一个求含有非线性约束的极限承载力上限解最小值计算模型.对建立的计算模型采用序列二次规划法进行优化求解.研究结果表明:非饱和土抗剪强度参数取值对极限承载力量值具有非线性影响;土中基质吸力存在所引起的附加抗剪强度使非饱和土地基承载力较饱和土得到提高;土体内基质吸力分布方式和地下水位高低对条形基础极限承载力影响也较大,地下水位升高导致基质吸力降低,极限承载力减小;非饱和土过渡为饱和土时,同类方法相比本解答是较优上限解.     

基于非饱和土理论的简化强度判断公式研究

非饱和土的抗剪强度理论研究已有近40年的历史,目前研究者提出了多种抗剪强度公式。但是这些公式大多都将基质吸力作为单独一个变量考虑,而实际工程中基质吸力的测试是非常困难的。笔者在研究分析前人提出的抗剪强度公式基础上,进一步归纳出基于区域性土水特征曲线数据库的＂无基质吸力项＂的非饱和土强度判断公式。     

非饱和土强度理论的分析研究

对非饱和土进行了介绍,分析研究了主要的非饱和土的理论公式和经验公式,指出非饱和土抗剪强度与饱和土抗剪强度不同,非饱和土的抗剪强度由三部分组成即凝聚力产生的强度、摩擦强度和吸附强度。     

非饱和土抗剪强度公式分类及总结

将众多非饱和土抗剪强度公式分为结合土-水特征曲线、数学拟合、分段函数、总应力指标及其他形式5类,分析了当前非饱和土抗剪强度公式的特点及研究不足.结果表明:有效应力抗剪强度公式和双应力状态变量抗剪强度公式的力学本质不同,吸附强度表达式的不同导致了抗剪强度公式的多样性,急需加强非饱和土真三轴试验研究,建立符合工程实际受力状况的非饱和土强度理论,完善非饱和土的理论基础,并加快非饱和土强度理论的工程实践与应用进程.     

非饱和土抗剪强度研究

对不同基质吸力下的非饱和土抗剪强度进行实验研究,分析了抗剪强度与基质吸力的关系,得出抗剪强度公式,并讨论了抗剪强度随平衡时间变化的规律,以期为非饱和土抗剪强度进一步研究奠定基础。     

非饱和土的抗剪强度与膨胀压力

非饱和土的抗剪强度有三个组成部份:真凝聚力、摩擦强度和吸附强度。本文分析论述这三个组成部份的来源、特性及其变化规律。 根据一系列研究试验的结果,笔者提出以下三种相互关系的计算公式并讨论了有关的概念:①粘土内摩擦角与塑性指数的相互关系;②吸附强度与膨胀压力的相互关系;③膨胀压力与吸力的相互关系。     

非饱和土条形地基梅耶霍夫极限承载力统一解

基于非饱和土的抗剪强度统一解,综合考虑中间主应力、基质吸力和强度非线性,建立非饱和土条形地基的梅耶霍夫极限承载力统一解,对比文献滑移线法解答、上限法解答验证所得统一解的正确性和适用性,讨论与太沙基极限承载力的差异及基底粗糙程度效应,并得出中间主应力、高/低基质吸力以及有效强度参数的影响规律。研究结果表明:太沙基极限承载力将旁侧土的抗剪强度简化为均布荷载,夸大了旁侧土的真实作用,计算结果偏大;基底粗糙程度对承载力系数的确定起关键性作用,梅耶霍夫极限承载力随中间主应力效应的增大而显著增加,不考虑中间主应力影响的Mohr-Coulomb解答过于保守;基质吸力具有双重影响,即在低吸力范围内极限承载力线性增加,而在高吸力范围内却逐渐减小并趋于稳定,这是由非饱和土的强度非线性造成的;有效强度参数的影响亦很重要,且有效内摩擦角较有效黏聚力的影响更明显。由于考虑了中间主应力效应、非饱和特性与旁侧土抗剪强度等工程实际情况,本文结果可为地基优化设计与施工提供有益的参考。     

Upper bound seismic limit analysis of geosynthetic-reinforced unsaturated soil walls

The assessment of the internal stability of geosynthetic-reinforced earth retaining walls has historically been investigated in previous studies assuming dry backfills. However, the majority of the failures of these structures are caused by the water presence. The studies including the water presence in the backfill are scarce and often consider saturated backfills. In reality, most soils are unsaturated in nature and the matric suction plays an important role in the wall's stability. This paper investigates the internal seismic stability of geosynthetic-reinforced unsaturated earth retaining walls. The groundwater level can be located at any reinforced backfill depth. Several nonlinear equations relating the unsaturated soil shear strength to the matric suction and different backfill type of soils are considered in this study. The log-spiral failure mechanism generated by the point-to-point method is considered. The upper-bound theorem of the limit analysis is used to evaluate the strength required to maintain the reinforced soil walls stability and the seismic loading are represented by the pseudo-dynamic approach. A parametric study showed that the required reinforcement strength is influenced by several parameters such as the soil friction angle, the horizontal seismic coefficient, the water table level, the matric suction distribution as well as the soil types and the unsaturated soils shear strength.     

非饱和黄土等效吸力的研究

作为土力学基本框架的重要组成部分,饱和土的强度理论己经基本趋于成熟,而非饱和土在实际工程中分布更为广泛,工程特性更为复杂,但其强度理论尚不完善。在非饱和土力学中,应力理论和强度理论是非饱和土变形稳定性分析的重要基础。非饱和土力学的研究水平目前还未进入实用阶段,主要原因在于吸力量测的困难。鉴于此,提出用等效吸力代替真实吸力的试验方法并进行验证。以常规静三轴试验得到的原状饱和及非饱和黄土的应力–应变曲线为基础,根据在相同σ1-σ3下产生同一轴向应变ε1的变形等效的原则,确定与对应饱和度Sr相当的围压即为等效吸力。建立饱和度与等效吸力之间的非线性关系,并验证该数学公式的正确性。根据试验结果分析含水率对非饱和土抗剪强度的影响,建立非饱和土的实用抗剪强度公式。该研究为非饱和土理论应用于工程实践提供了一种新的实用方法。     

Application of fractal theory to unsaturated soil mechanics

The mechanical properties of unsaturated soils are a function of the saturation degree or matric suction, and can be obtained based on currently available procedures. However, each procedure has its limitations and consequently, care should be taken in the selection of a proper procedure. The fractal approach seems to be a potentially useful tool to describe hierarchical systems and is suitable to model the structure and hydraulic properties of unsaturated soils. In this paper, the soil-water characteristics, unsaturated hydraulic conductivity function, unsaturated shear strength, swelling deformation and compression were derived from the fractal model for the pore-size distribution, and were expressed by only two independent physical parameters, the fractal dimension and the air entry value. The predictions of the proposed soil-water characteristics, unsaturated hydraulic conductivity, unsaturated shear strength, swelling deformation and compression were in good agreement with published experimental data. Comparisons between the experimental results of unsaturated hydraulic conductivity and the predictions of the both fractal model and the van Genuchten-Mualem model were also performed, and it was found that the predictions of the fractal model were better than that of the van Genuchten-Mualem model. __________ Translated from Chinese Journal of Geotechnical Engineering, 2006, 28(5): 635–638 [译自: 岩土工程学报]     

Scherfestigkeit durch Kapillarität in unzementierten ungesättigten bindigen Böden

Capillary strength of uncemented and unsaturated cohesive soils in direct shear. Shear strength of unsaturated soils is larger than in the saturated state due to capillary forces. This affects stability of geotechnical constructions to an often unknown amount. Capillary‐induced stresses may be calculated by the pF‐curve (saturation‐suction‐relation). In this paper, a method for calculation of shear strength is given, which is useful for the whole range of suction. Maximum capillary shear strength may be calculated, which is a minimum value of the whole shear strength and has to be supplemented by the strength of other binding mechanisms. As an example, the method is shown with 6 soils. The critical height of an unsupported vertical slope and the crack‐closing load of a mineral sealing layer is calculated and the possibility of large suction values is discussed.     

Elasto-Plasticity of Unsaturated Soils: Laboratory Test Results on a Remoulded Silt

Current models of the elasto-plastic behaviour of unsaturated soils contain important underlying assumptions that have not been tested due to a lack of adequate experimental data. To address this issue, the objective of this paper is to provide a comprehensive set of experimental data. An extensive experimental program has been performed on a remoulded unsaturated silt. To characterise its elasto-plastic behaviour, samples were taken through various stress paths, including wetting, drying and compression. Experimental results were analysed to provide (1) evidence of suction-induced preconsolidation; (2) dependence of cohesion and shear strength at failure on suction; (3) stiffness in relation to suction and (4) uniqueness of the critical state line.