| The study on engineering properties of soil based on its structure has been a new direction in the development of soil mechanics. The collappsibility of loess caused by its special structure brings on the tremendous risks in the large number of constructions on the collapsible loess foundation. Collapsible loess in natural state has relatively high strength and low compressibility, but when it is soaked with the water, its structure will be damaged rapidly and collapsed suddenly, at the same time its strength losts a lot, so collapsibility has great harmfulness. Under the support of project"Microstructure and collapsibility of loess in China"funded by National Nature Science Foundation (Project No. 50478096), this study is conducted and completed. In this study, typical collapsible loess samples were taken from different loess regions in China. After a series of tests to determine their physical and mechanical properties, the samples were observed under the scanning electric microscope. The images of loess were analyzed from both qualitative and quantitative ways. Based on the structural properties, the collapse mechanism and the law of deformation were studied, the main structural factors causing loess collapse were suggested, and the relationship between collapsibility coefficients and microstructure properties of loess specimens were concluded. There is an important positive meaning to provide the theoretical basis for evaluating the loess collapsibility correctly and controlling the loess deformation effectively.The main achievements in this study are as follow:1) According to the grain size distribution tests, the content of coarse silt particle is the largest portion in loess specimens, and the content of clay particle is rather high in general. To a certain extent, the content of clay particle increasing can decrease the collapsibility of loess. But it is impossible to judge the collapsibilty based only on the content of clay particle. Though the contents of clay particle in some specimens are more than 30%, they still have strong collapsibility. Using scanning electron microscope to observe the structure of these specimens and found that the most of clay particles existed inside the aggregates, while little clay particle existed among the aggregates to cement them, the aggregates were in loose arrangement and the specimens had the high collapsibility. The high content of clay particle doesn't make a difference on the collapsibility of loess. So to determine the effect of clay particle on the loess collapsibility must consider not only the content of clay particle, but also the clay particle exsiting state in the loess, later one maybe of more importance.2) A set of sampling method for collapsibility loess microstructure analysis, the materials for specimen impregnation and their ratio were determined after lots of repeated tests. By this method, large numbers of clear undisturbed microstructural images were obtained what provided the foundation of the loess structure research.3) A reasonable classification of loess structure was proposed based on the former research of loess. The basic unit element in loess microstructure composite can be summed up as granular unit and aggregated unit; the link between the basic unit elements can be classified as contact link and cement link; the arrangement of basic unit elements leading to various void in loess can be divided into 4 types, ie, void inside the unit, embedded void, open void and large void.4) The voids in loess can be divided into large void (>500μm), middle void (500-50μm), small void (50-5μm) and micro void (<5μm) at their size. This classification is associated with the void form divided by the arrangement of basic unit element. Most of voids inside the unit are mico voids; the embedded voids correspond to the small voids; the open voids include middle voids and large voids with loose void wall. It is possible to quantitatively analyze the void with different structure based on their specific size range.5) Contrasting the analysis result of loess structure in different regions to their collapsibility, the conclusion can be drawn that the arrangement of basic unit elements is the most important part in three structural characters. The open void formed by loose arrangement of basic unit elements is the primary cause of loess collapsibility.6) The void area ratio of specimen was decided to be the main measurement parameter in the structure quantitative analysis of loess on the basis of the SEM image's feature of two-dimension. Based on the analysis of data got from IPP image processing software, the method of calculating the void area ratio was determined. The void area ratio of each specimen can be calculated by choosing the image with the magnification of 1000 and selecting the gray scale threshold as the value corresponding to the left foot of the rear peak on the gray distribution curve.7) The void aera ratio of specimens in different regions calculated by the method mentioned above was verified by contrasting with the porosity of specimens resulted from the soil tests in laboratory. It is certified that the method of calculating void area ratio is reasonable, and the statistical data has the great representation for the structure of loess.8) The quantitative analysis results of specimen image show that the regularity of vertical plane structure of specimen is better than horizontal plane. The void area ratio of vertical plane decreases with the sample depth increases, and increases with the collapsibility coefficient increases. The relationship equations are set up, such as the linear regression equation of void area ratio and depth, the logarithm regression equation of void ratio and collapsibility coefficient.9) By the study on the relationship between physical properties and collapsibility of loess, the conclusion can be drawn as follow: with the increasing of water content, unit weight, dry unit weight and saturation degree, the collapsibility coefficient of specimen reduces, the loess collapsibility declines; with the increasing of void ratio and porosity, the collapsibility coefficient of specimen increases, the loess collapsibility enhance. Besides, some physical property indexes are compared and discussed for non-collapsible loess and strong collapsible loess are concluded. The work is significant for evaluating the loess collapsible correctly.10) From the consolidation tests of loess with different initial water content, the conclusion can be drawn that with the initial water content and the initial saturation degree increase, the compression of specimen increases, while the collapsibility coefficient decreases, the collapsibility of loess decreases.
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