Study On Microstructural Effects On Collapsibility Of Loess And Its Evaluation Methods

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.