| Zhengzhou-Xi'an Passenger Dedicate Line (PDL) crosses a loess area with complex geological conditions such as collapsible loess ground, and the formation is mostly made up of new loess accumulation horizon. When some PDL large section tunnels are constructed on such a kind of ground, the cement-soil compacted pile technique is proposed to treat this kind of special foundation. However, as a collapsible loess foundation treatment used for PDL loess tunnel, the cement-soil compacted pile is applied in high railway foundation reinforcement in our country for the first time.In this thesis, the Pandong tunnel at DK303+495～530 of Zhengzhou -Xi'an PDL is mainly studied. The main works done in this paper include field survey and exploration, laboratory soil test, field pile test, theoretic analysis and finite element analysis by using PLAXIS2D. Then based on the results of field pile test in which different construction methods and pile spacings are adopted and PLAXIS2D finite element analysis in the Pandong tunnel entrance, the application of cement-soil compacted pile is discussed in large cross section loess tunnel. The pore-forming and pile-forming process of single pile are simulated by finite element method, and the influence scope of this kind of single pile is obtained. Furthermore, the superposition effect of multiple piles in filed pile testing is evaluated. The reasonable pile spacing is determined according to discussions mentioned above.The main conclusions of this thesis are as follows:By filed tests and laboratory tests, the optimum mixture ratio of cement-soil is determined, and the relationship between the strength of pile and mix proportion at natural moisture and saturated state are also obtained respectively.The superposition effect of multiple piles with different construction methods and spacing is analyzed by filed pile testing and the influence area of single pile is discussed by finite element analysis. In addition, the reasonable diameter and spacing of cement-soil compacted pile are determined. On the basis of above analysis, the cement-soil compacted pile reinforcing method is proposed to apply in the collapsible loess foundation treatment in Pandong tunnel entrance project.In this paper, the pore-forming and pile-forming processes of cement-soil compacted pile are systematically studied by laboratory test, field pile test and finite element method. It provides reference data and technology supports for the design and construction of high-speed railway on loess area. And it also provides reference value for the foundation treatment of railway tunnel under similar conditions.
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