| China is one of the countries in the world which loess distribution isreally extensive. Its loess area has reached 0.63 million km~2, and 60% ofwhich has different collapsibility. The intensity and compress-resistance ofnatural collapsible loess is satisfied in low humidity, but this character willchange upon wetting and loading, that is that the soil will develop a largeand suddenly settlement and its intensity will also reduce obviously. Suchspecial water-sensitivity of collapsible loess has caused a lot of economicloss for the construction of many districts in China. We can't ignore thesudden change of collapsible loess either in quantity or in quality, and thissusceptibility has been a big problem which will affect the stability ofabove buildings. In order to prevent the foundation unstable accidents, andcarry out project design and construction safely and economically, whenconstructing in the districts of collapsible loess, the evaluation of loesscollapsibility is necessary according to the loess characteristics and projectrequirement.In this thesis, some collapsible loess from some areas in our countrysuch as Shanxi province and Shaanxi Xi'an were studied and the regular of collapsible deformation was discussed primarily. First, odemeter test andthe routine soil tests were carried out not only on the undisturbed samplesbut also on the samples whose water contents were changed eitherincreasing or decreasing. Through these tests, the changes of collapsiblecoefficient as well as collapsible deformation were analyzed, and thecomparison of such changes with the different initial water content andliquidity index was conducted, too. It was indicated that for the samesample, regardless of increasing or decreasing water content, itscompressive deformation gradually increased but the collapsible coefficientobviously reduced along with the initial water content increasing. Second,form the view of liquidity index to analyze the collapsibility of loess is theimportant character of this paper, and also is its contribution to loess study.According to the changes of compressive deformation and the collapsiblecoefficient of the loess samples with the different liquidity index under thedifferent pressure, compressive pressure was divided into three groups ie.low-sensitive pressure, mid-sensitive pressure and high-sensitive pressure.Engineers must pay more attention to the soil with low liquidity indexwhich is under a high-sensitive pressure. In such a case, a little change ofsoil water content may cause a serious deformation of the foundation.Another key point in this thesis was analyzing its collapsiblemechanism from the loess microstructure characteristic. The mineralcomposition and structure characteristics of loess samples from some areaswere analyzed by using the scanning electron microscope (SEM) and X-ray.The microstructure characteristics of loess were qualitatively described,and the types of microstructure and pore were also divided. The pore of soilsample was divided into two groups, visible pore and invisible pore. In turn, the porosity of each group of pore was calculated based on themicro-photos of SEM (×250). After comparison with the water content andcollapsible coefficient, it was drawn a conclusion that the main reasonaffecting the loess collapsibility was the visible pores between aggregatesand crumbs, but not those invisible pores intra-aggregate. Finally, themacroscopic mechanical properties of two collapsible loess samples fromXi'an and Shanxi were compared with each other, and the region changerules of collapsible loess were summarized following previous researchresults, and pointed out that the mineral component, the microstructure andthe porosity characteristic of loess are the basic reasons which causedregional differences on the collapsible characteristics.
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