Research purposes:Soft soil is broadly distributed and deep-buried in the northwest region of Yunan province，with many sorts and thick soils. The research was done on the cause of formation and the characteristics of soft soil to summarize the characteristics of the soft soil in fault basin and the principle of geological alignment of railway was presented for providing the reference to the research on similar soft soil.
Research conclusions:(1)The control of the regional geological structure on the distribution of the soft soil represents: Firstly，the neotectonics movement is the essential cause of forming for the soft soil. Secondly，the regional fault structure controls the distribution of soft soil in fault basin.(2)The soft soil in this fault basin has eight characteristics. These characteristics include the high natural water content，big crude void rate，strong compressibility，good plasticity， low intensity，low consolidating coefficient，low water permeability and poor homogeneity.(3)The railway alignment in the area of thick soft soil should pass round the area of deep and thick soft soil area based on considering the investment. The alignment principle should be determined on the basis of fully researching the influence of regional structure on the distribution of soft soil. (4)The research result is available for the engineering geological investigation for the railway in the active faults area in the northwest region and in the soft soil developing area.
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