Characteristics And Dynamic Response Of Composite Foundation About Long And-Short-Pile Under Flexible Load In Collapsible Loess Area

Yinchuan-Xi'an Railway is located in the western region,as a high-speed railway which connecting Yinchuan and Xi'an,it is of great significance to the construction of high-speed railways in China.Because the crossing area is mostly in the loess area,therefore,it is proposed to use cement-modified soil to improve the filling property of the embankment.At the same time,it is proposed to adopt the foundation treatment method of CFG long pile + cement soil compact pile short pile in Gansu-Ningxia section of Yinchuan-Xi'an Railway based on relevant theoretical resea rch and engineering examples.A reinforced cushion is placed inside the embankment fill,so that the upper load on the top of the pile is in the form of a flexible load.The main research contents are as follows:(1)This project relies on the subgrade treatment project of Gansu-Ningxia section of Yinchuan-Xi'an Railway,for its collapsible loess area,through the cement-modified soil compressive strength test and the cement-modified soil field test,the relationship between the cement mixture ratio of the bottom bed embankment,the embankment under the bed,the improved cushion,the optimum moisture content of the filler and the maximum dry density is determined.Also,the relationship between the optimum moisture content of the filler and the compaction coe fficient is determined.And explored whether the incorporation of cement can effectively improve the physical and mechanical properties of collapsible loess.Through the test,the compaction coefficient of the pile body,the compaction coefficient of the s oil between the piles,the bearing capacity of the composite foundation and the collapsibility coefficient of the soil between the piles are used to judge the treatment effect of the foundation.The economics,feasibility and effectiveness of CFG long pile + cement soil compact pile short pile foundation treatment method are also explored.(2)The finite element model of CFG long pile + cement soil compacted pile short pile composite foundation is established and static analysis is carried out.The effect of composite foundation treatment and the embankment filling property and pile properties caused by different cement content on the properties of composite foundation are studied.The parameters of the stress and settlement characteristics of the composite foundation are analyzed,and the degree of influence on the treatment effect of the composite foundation is analyzed to determine the actual optimization direction of the fit project.(3)The train track force is simplified and the finite element dynamic mod el is established to analyze the dynamic response of the composite foundation under train load.The variation law of settlement and dynamic stress of long-short pile composite foundation under train load is studied in detail to judge the safety and comfort of the train during operation.