【作者】 高凌霞；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2011， 博士

【摘要】 黄土具有湿陷性、结构性及非饱和性等特殊性质,往往对工程具有不利影响,是岩土工程研究的热点问题。为评价黄土地区边坡、路基等土工结构的稳定性和地基稳定性等问题,有必要结合非饱和土力学理论、土结构性理论及土工测试技术,深刻认识和科学评价黄土的湿陷变形特性。黄土的湿陷特性受吸力、净应力水平及其路径,土微结构特征(包括颗粒大小、级配、形状、定向性、堆积方式、密实程度、矿物成分等)等多种因素影响。本文首先研究了非饱和黄土湿陷性和结构性与其基本物性指标之间的关系,并基于微结构理论对黄土湿陷性和微结构特性两者之间的相互关系进行了定性和定量的研究。其次,建立了湿陷性指标与其微结构参数之间的函数关系。本文主要研究内容及所取得的研究成果包括下列方面。1.对黄土结构性的表现形式、组成要素及影响因素进行了较为全面的分析。在此基础上进一步分析了黄土结构强度的测试方法,研究了黄土结构性与湿陷性之间的关系,同时对黄土湿陷系数与压力和含水量之间的关系进行了研究。2.研究了击实功和击实含水量两种因素对非饱和重塑黄土渗透性的影响。采用常规压力板仪测试了重塑黄土在三种不同击实功和五种不同击实含水量下的土水特征曲线,并用变水头法测量了相应的饱和土渗透系数。结合土水曲线和饱和渗透系数,基于Van Genuchten模型和Fredlund模型,探讨了非饱和重塑黄土在不同击实条件下的渗透性。结果表明：重塑非饱和黄土的渗透性与击实含水量和击实功密切相关。在较低基质吸力条件下,渗透性随击实含水量的增加而降低,同时随着基质吸力的增大,不同击实含水量条件下的渗透性差异逐渐减小,当吸力达到1000kPa时,渗透系数与击实含水量基本无关；在轻型击实和简化轻型击实条件下,饱和状态与非饱和状态的渗透系数基本相近,对于重型击实功,在饱和状态时,土体的渗透系数约为轻型击实条件下渗透系数的l／100,随着土基质吸力的增大,渗透性差异逐渐减小。3.采用BP人工神经网络方法建立了湿陷系数的计算模型。西安地区黄土的实测资料作为网络模型学习、训练样本和测试样本,对网络模型的计算结果与实测值进行了对比分析。结果表明,该方法能较全面考虑黄土湿陷性的影响因素,与其它计算方法相比BP人工神经网络法快捷、准确性高和适用范围广。在一定程度上,可替代湿陷试验。4.基于非饱和土的粒问吸力理论及黄土的微结构特征,研究了黄土的湿陷机理。研究表明,非饱和黄土的湿陷性由其微结构特征控制。西安黄土的微结构特征表现为集粒、团粒问以点接触为主,且胶结物为黏土矿物,胶结程度较弱。黄土的湿陷性是由其特殊的微结构特征和粒间吸力共同作用所致。黄土的孔隙性为其变形提供了空间,而随含水量变化的粒间吸力作用则为湿陷变形提供了驱动力。5.建立了基于黄土物性指标的湿陷系数预测模型。湿陷系数是表征黄土湿陷变形特征的重要指标之一,该指标的获得一般是通过室内浸水压缩试验确定。黄土的组成复杂,种类繁多,试验结果的离散性很大,并且试验中难以避免会出现误差,因而很难从土的工程性质方面对土的物理指标和湿陷系数间的关系进行严格的理论分析。通过对黄土湿陷性影响因素的综合分析,认为湿陷性影响因素可以用宏观物理指标含水量(w)、孔隙比(e0)和塑性指数(IP)表示。通过多元线性回归分析,得出湿陷系数与物性指标间的定量关系式,并对该分析模型得到的湿陷系数计算值与实测值进行了对比分析。分析表明该定量预测模型较为合理,特别是对中、强湿陷性的黄土,计算值与实测值较为接近。6.将多元统计方法引入到对黄土微观结构参数的研究之中,在进行了参数标准化和相关性检验之后,给出了适合于黄土的主成分分析方法和谱系聚类分析方法。采用图像处理软件系统对黄土SEM照片进行了图像分割并提取了相关信息,对不同固结压力作用下所获取的参数进行了详细研究,进而基于多元统计方法,构造了主成分等黄土的合成微结构参数。对一般常规微结构参数的研究表明：随着固结压力增大颗粒体所占面积比逐步增大,第一主成分也近似线性增大；对于不同的固结压力,黄土的微观样品具有比较明显的聚类特征,类之间的距离也随着固结压力的增大有增大的趋势。另外,分析了灰度阈值对微结构参数的影响。进一步的研究表明：颗粒体面积和周长可以表示为阈值的三角函数；不同观测点颗粒体域微结构参数各主成分随阈值的变化趋势是一致的,且在最大方差阈值附近,第一主成分取值达到最大；不同阈值对应的颗粒体微结构具有非常明显的聚类特征。7.通过改变含水量与固结压力,对西安原状黄土进行了湿陷性实验,由此分析了不同含水量与固结压力条件下湿陷性的差异与联系,同时通过电镜扫描(SEM)获取了湿陷前后黄土试样的微结构照片,进而获取了相应条件下的SEM照片的简单微结构参数。分析了湿陷前后微结构参数的变化,探讨了简单微结构参数间相关关系,基于主成分分析法构建了合成微结构参数。研究表明：黄土湿陷前后,其简单微结构参数发生了相应的变化,合成微结构参数,即第一主成分近似线性增大,黄土湿陷系数与其累计主成分呈线性关系。根据这一重要认识,建立了主成分得分的湿陷系数计算方法,进而建立了黄土湿陷性评价方法。该方法可以比较客观地分析和评价黄土地区地基湿陷性。更多还原

【Abstract】 Loess inhabits a special status in geological engineering due to its special characteristics such as collapsibility, constitutive property, unsaturation and its effects on engineering. In order to evaluate the stabilities of earth structures such as slope and subgrade as well as stabilities of ground in loess areas, it is necessary to completely understand and scientifically evaluate collapse deformation behaviours of loess by combined unsaturated soil mechanics, the structural theories and soil test technology. The collapsibility of loess not only is controlled by suction, normal stress lever and path, but also depends on micro structural characteristics which include particle size, gradation, particle shape, directing property, accumulation mode, density and mineral composition and so on. This thesis first studied the relationship between collapsibility and structure and index properties in detail. Then based on theories and research approaches of microstructure, qualitative and quantify studies on relation between collapsibility and structure of loess were carried on. Finally, the function relationship between collapsible index and micro-structural parameters were established. The research contents in this thesis include following aspects.1. The express forms, constituent elements and influence factors of structural characteristics are discussed in detail. Then, the relationship between loess structure and collapsibility is further analyzed. The relationship between collapsible coefficient and consolidation pressure and water content is interpreted qualitatively by graphic method.2. The influences of compactive water content and compactive efforts on permeability of unsaturated remolded soil are studied. Using the pressure-plate extractor, the soil-water characteristic curves under different compactive water content and different compactive efforts are obtained. The corresponding saturated permeability coefficient is gotten by the variable water head method. Combined with the soil-water characteristic curves and saturated permeability coefficient, based of Van Genuchten model and Fredlund model, the permeability of remoulded unsaturated loess under different compactive conditions are discusses. The results show that compactive water content and compactive efforts both have close relation with permeability of unsaturated remolded soil. When the suction is lower, coefficient of permeability decreases with the increase of compactive water content, and the difference reduces with the increase of suction, and permeability coefficient can be assumed identical when the suction reach 1000kPa. The permeability coefficient of samples compacted with standard proctor compactive efforts and reduced proctor compactive efforts are very similar in both saturated and unsaturated states. For modified compaction, the value of permeability coefficient is about 0.01 in saturated state, and increases as the suction increases.3. Based on the analysis of the main factors influencing coefficient of loess collapsibility, model for calculating coefficient of the collapsibility is established by applying the theory of artificial neural network (ANN). A large amount of tested data in Xi’an is used as learning and training samples to train and test the ANN model. The calculated results of the ANN model and the test values are compared and analyzed. The results show that the proposed method can consider the influencing factors on collapsibility of loess more completely, comparing with other theory methods. This method is quick, accurate in calculating and shows flexibility in application. Therefore it is expected to use this method to determine the collapsible coefficient instead of testing.4. Based on inter-particle suction theory and micro structural characteristics of loess, the mechanism of loess collapse was analyzed. The results indicate that the microstructure of unsaturated loess has clear effect on its collapsibility. The silts in microstructure of intact simple coated by cements formed into aggregates. Clay minerals are cemented into clay platelets. Aggregates and clay platelets contacted in a point patter. And if the cements are clay minerals, the structural system will be easily collapse.The analysis results show that the collapse of loess is outgrowth of effects of both micro structure and suction. Structure character provides the space for deformation, while the changes of suction among particles with water content provide driving force for collapse.5. The quantitative relation between loess collapsibility and its physical index are established. Coefficient of collapse, one of important indexes of describing deformation characteristics of loess collapsible, is determined by collapsible test in laboratory. It is difficult to make theoretical analysis of relation between coefficient of collapsibility and soil parameters in engineering properties due to the complex composition, large numbers of types, and dispersion of test results and error of test. On the basis of general analysis of the factors affecting loess collapsibility, it is believed that the physical factors mainly are water content, void ratio and plasticity index. Using mult-variables regression analysis, a quantitative model of loess collapsibility equation of collapsible coefficient (δs) against water content(w), void ratio(eo) and plasticity index (IP) is established. Comparative analysis are carried out between calculated values and measured values. The analysis shows that the proposed model is reasonable for predicting the collapsibility of loess,especial for strong and mediate collapsible loess. 6. Multivariate statistical technique is employed to study the parameters of micro-structure, after standardizing parameters and checking their correlation, principal component analysis and cluster analysis methods, which are tested to be suitable for loess, are proposed. Utilizing image processing system, SEM images of loess are processed and related information are achieved. Micro-structure parameters of SEM photos under different consolidation pressure are studied in detail. Synthetical micro-structure parameters of SEM photos, like principle components, are constructed based on multivariate statistical method. The study results for simple micro-structure parameters indicate that the quantity of particle increase with the increasing of consolidation pressure. With the increasing of consolidation pressure, the first component increases linearly. Under different consolidation pressure, the samples of loess display obvious cluster character. The distance between clusters increases with the increasing of consolidation pressure nonlinearly. In addition, the influences of threshold on micro-structural parameters from SEM photo are studied. The results indicate that grain area and perimeter can be expressed by trigonometric functions of threshold. Principal components of micro structural parameters of grain areas in various observation points have the same trend, and nearby the maximum variance of threshold, the first principal component reaches the maximum value. Micro structures of various grains for different thresholds have obvious convergent character.7. The collapsible tests of the original loess from Xi’an with various water content and consolidation pressure were carried out. The differences and the relations of collapsibility of loess under various conditions are analyzed based on the results of these tests. The Electron Scanning Microscope images are obtained both on loess samples of before and after collapsing, then corresponding micro-structural parameters are obtained. Comparative analysis of simple micro-structural parameters is made. The relationship among these parameters is discussed and the synthetic micro-structural parameters are constructed as well. The results indicate that the proportion of particle and the first principle component increase with the increase of consolidated pressure; there is a linear relationship between collapsibility and the first principle of loess. Based on this understanding, the calculation method of coefficient of collapse is established. The collapsibility of loess can be evaluated objectively using the proposed method.更多还原