【作者】 刘海松；

【导师】 倪万魁；

【作者基本信息】 长安大学 ， 地质工程， 2008， 博士

【摘要】 本文以洛川黄土标准剖面为研究对象,根据前人对黄土与环境研究所取得的举世瞩目的成果,对黄土物理力学性质沿深度的变化差异进行系统分析;以结构特性及其强度研究为桥,将黄土与环境的研究与黄土力学特性研究有机地结合起来;对考虑结构性的黄土压缩性、抗剪强度以及考虑结构性的黄土湿陷性进行了研究,并建立了考虑结构性和应力历史的黄土本构关系。属于基础性的研究,但是对黄土研究的领域具有一定的补充和扩展作用,同时会促动环境土力学研究的发展。在黄土力学研究领域的理论意义显而易见,会给黄土地区的高边坡工程、公路隧道、铁路隧道以及地下建筑工程设计施工中参数的选取提供一定的理论支持。对黄土的工程力学研究具有重要的理论指导和实践意义。主要取得了以下成果:(1)洛川标准剖面黄土-古土壤的主要物理、力学性质以及湿陷特性,均沿深度出现了幅度不等的波动性变化,而且波峰、波谷出现于古土壤以及上覆或下伏黄土层中。静力触探原位测试曲线更清晰地印证这种现象,说明黄土与古土壤、黄土层内以及不同时代的古土壤均存在着明显的结构性差异。(2)结合前人在黄土与环境方面的研究成果,通过分散及不分散处理的颗粒分析试验,得出了黄土中碎屑颗粒明显多于古土壤而粘土颗粒少于古土壤以及古土壤中集粒含量远远大于黄土的认识;结合电镜扫描图片分析,探讨了洛川标准剖面黄土-古土壤序列结构以及结构性差异的形成过程和原因,指出了干冷和温湿的古气候旋回是黄土与古土壤结构差异形成的最关键因素。(3)通过高压固结试验分析,发现标准剖面的黄土-古土壤均呈超固结状态。这种现象是由黄土正常堆积后的环境变化和固结历史造成的。即在黄土的初始结构形成后,在不同的应力作用时间和温湿、干冷气候旋回条件下,黄土及古土壤的结构得到不同程度的加强,存在“后期加固结构强度”。(4)通过高压固结、直接剪切以及三轴剪切试验对比,分析了后期加固结构强度的求取方法可行性和适用性。发现直接剪切试验受试验压力的影响,在求取后期加固结构强度以及抗剪强度指标时均具有局限性,而三轴剪切和高压固结试验获取的后期加固结构强度比较接近,是可行的方法。(5)建立了环境因素以及应力历史与后期加固结构强度的定量关系,然后定量地研究了后期加固的结构强度与黄土-古土壤序列的物理、力学性质的关系,将环境因素以及应力历史引入到黄土力学研究领域内。(6)分析了原状土样完全饱和的实质,是后期加固结构强度的完全丧失和初始结构强度的部分破坏。因此,通过原状、重塑和饱和三种试样的高压固结试验可以得出黄土及古土壤结构对水的敏感程度。(7)从环境和应力历史角度分析了黄土湿陷的原因,可以由内因和外因两方面探讨。内因是黄土遇水后结构强度的降低以及受古气候演化、环境变迁和应力作用时间等控制形成的差异初始含水量和结构水敏度。古土壤形成于温湿的环境,受水影响时间较长,初始含水量大,结构水敏度较小,湿陷性较小;而黄土形成于干冷的环境,受水影响时间较短,初始含水量小,结构水敏度较大,湿陷性较大。而外因则包括水及荷载的变化。在饱水条件下,外加荷载大到足以破坏后期加固结构强度时,黄土及古土壤便开始发生湿陷变形。(8)通过对摩尔强度包络线及应力-应变特性的分析,探讨了后期加固结构强度的发挥特征。认为包线初始段即弹性变形阶段,对应着初始结构强度的发挥;包线中间段即弹塑性变形阶段,对应着后期加固结构强度的发挥到完全丧失;包线最后的水平段即塑性变形阶段,后期加固结构强度完全丧失,土体进入塑性流动状态。(9)考虑土体在受荷过程中的后期加固结构强度的发挥特征,引入可以表达其发挥程度的参数,对邓肯-张模型进行修正,使得它具有了更广泛的适用性。由于邓肯-张模型及其各种修正模型均不适用于强硬化型应力-应变关系,又提出了考虑黄土结构损伤的对数模型,很好地描述了强硬化型的应力-应变关系。更多还原

【Abstract】 Taking Luochuan loess standard profile as the object of study, differences of physical mechanics properties of loess which vary with depth are analyzed systematically on the basis of the remarkable achievements on loess and environment study by former researchers. Study on Loess and environment is organically combined with research on Loess mechanics properties based on taking structural behaviors of Loess and its strength properties. The compressibility and shear strength of the loess with considered the structural property and collapsibility of Loess with considered structural property are taken on study, and stress-strain relation of loess with structural properties and stress history were considered is established. These belong to basis study, but it is a certain kind of supplement and expandedness to the field of loess study and also a driving force to the development of study on environmental soil mechanics. Its academic meaning in the field of loess mechanics study is obvious, and it will provide a certain kind of academic support to the choice of parameter in the design and construction of projects such as high slope engineering, highway tunnels, railway tunnels and underground constructions in loess area. It is also an important guide and practical significance to the study on loess engineering mechanics. The main achievements obtained in the thesis are as follows:(1)The major physical, mechanical properties and collapsibility of loess-fossil soil of Luochuan standard profile all appear different wavy changes with depth. The wave crest and trough appear in the fossil soil, overlying or underlying loess layers. The curve of Cone penetration test on-site confirmed this phenomena clearly, and it also showed that there exist obvious structural differences between loess and fossil soil, among the fossil soil layers formed in different times and the loess layers formed in the same times.(2)Combined with the achievements obtained by former researchers on loess and environment, and through the analysis tests of particles which are treated by the way of disperse and non-disperse, the new understandings of the content of clastic particles in loess is obviously more than in fossil soil while the content of clay particles is on the contrary, and the content of grain in fossil soil is far more than that in loess are obtained. Combined with the analysis of electric mirror scanning pictures, the formative process and reason of the sequence structure of loess-fossil soil and structural differences of Luochuan standard profile are discussed, and paleoclimate gyration with character of dry-cold and warm-wet is pointed out to be the most crucial actor for the formation of the structural differences between loess and fossil soil.(3)By the analysis of high pressure consolidation test, it is discovered that loess-fossil soil of the standard profile are all over-consolidated. This phenomena is caused by the environmental change and consolidation history after the normal accumulation of loess, namely. After the formation of initial structure of loess, structures of loess and fossil soil were strengthened in different degrees, where exists“structural strength of later reinforcement”, under the conditions of different stress affecting time and climate gyration with character of warm-wet and dry-cold.(4)Compared with the tests of High Pressure Consolidation, Direct Shearing and Triaxial shear, the feasibility and applicability of the method for obtaining structural strength of later reinforcement are analyzed. It is found that the direct shearing test is affected by the test pressure, and there exists limitation in obtaining the structural strength of later reinforcement and the index of shear strength. But the tests of High Pressure Consolidation and Triaxial shear are feasible methods because the structural strength of later reinforcement obtained from them agrees with each other.(5)Quantitative relation between environmental factor, stress history and structural strength of later reinforcement is established. Then the relation between structural strength of later reinforcement, physical and mechanical property of sequence of loess-fossil soil is quantitatively studied. Environmental factor and stress history are introduced to the research field of loess mechanics.(6)The entire saturation essence of undisturbed soil sample, are the totally loss of structural strength of later reinforcement and partly damage of initial structural strength. Therefore, the sensitive degree towards water of the structure of loess and fossil soil can be concluded by the high pressure consolidation test with undisturbed soil sample, remolded soil sample and saturated soil sample.(7)The causes of loess collapsibility are analyzed from two aspects: environment and stress history, which could be explored from internal cause and external cause. The internal causes are the structural strength of loess will be reduced when meets water, and the differential initial water content and structural water sensitivity formed by the changes of paleoclimate and environment and the time of stress-effects. The fossil soil formed in the warm and humid environment, had a relatively long effect by the water, a large initial water content; the structural water sensitivity and collapsibility are relatively small. Loess formed in the dry and cold environment, had a relatively short effect by the water, a small initial water content; the structural water sensitivity and collapsibility are relatively large. The external causes include the changes of water and loading. Under the condition of saturation, loess and fossil soil will begin to appear collapsible deformation when the applied load is large enough to destroy the structural strength of later reinforcement.(8)Through the analysis of Mohr strength envelop and stress-strain property, the character of exertion of the structural strength of later reinforcement is discussed. It is believed that the initial section of Mohr strength envelop is the elastic deformation stage which is correspond to the exertion of initial structural strength; the middle section of Mohr strength envelop is the elastic-plastic deformation stage which is correspond to the variation from exertion to completely lost of structural strength of later reinforcement;the last horizontal section of Mohr strength envelop is the plastic deformation stage which is correspond to the completely lost of structural strength of later reinforcement, and the soil comes into the plastic-flow state .(9)Considering the exertion character of the structural strength of later reinforcement in the process of bearing loading and introducing the parameters which can show the exertion-degree of the structural strength of later reinforcement, the Duncan–Chang model is modified, has a more extensive applicability. But because the Duncan- Chang model and all of its modified models are all not suitable for the most harden type stress-strain relation, the logarithmic model considering the loess structural damage is brought forth, which, describes this stress-strain relation very well.更多还原