Study On The Mechanism Of CFG Pile In The Treatment Of Medium Soft Foundation Of High-Speed Railway By Finete Element Method And Field Experiment

CFG piles are often applied in reinforcing soft foundation of high-speed railway. So it is necessary to analyze the main factors influencing pile settlement and the reinforcing mechanism of composite foundation of CFG piles. In this thesis, the influence of factors such as compression modulus of soil between piles, pile length, spacing, compression modulus of sublayer and foundation is discussed by numerical simulation. On the basis of analysis mentioned above, CFG pile composite foundation in moderate soft ground section of Peking-Shanghai high-speed railway is taken as an example, field test and numerical simulation analysis are done.DK190+100～DK190+300 section of Peking-Shanghai high-speed railway locates between Tianjin super large bridge and Qingcang super Targe bridge, on the left of Liyao rail-laying base. It is a plat and wide terrain, and high fill embankment is adopted in this area, of which the height is about 7.3m. As the ground soil mainly consists of silt and silty clay and the requirement of controlling embankment settlement is very strict in construction of high-speed railway, CFG pile is applied for reinforcing the soft ground. In this paper, field geological surveying, field monitoring, theoretical analysis, and PLAXIS finite element analysis are done. On the basis of the observation results of stress and settlement of CFG piles in different phase of construction, the change law of settlement and pile-soil stress ratio on the top and at the bottom of the embankment is discussed respectively. Numerical simulation and comparative analysis of several type of CFG pile composite foundation such as column-net structure, column-net structure with cap, reinforced concrete slab structure are made. On the basis of analysis mentioned above, the settlements of CFG pile composite foundation in and after construction phase are evaluated. And the mechanism of CFG pile composite foundation is discussed.By deep study in this paper, following conclusions are drawn: The load transfer and settlement deformation laws of CFG pile composite foundation on embankment load are revealed by comprehensive consideration of field monitoring, finite element method and theoretical analysis.Field tests are made, and the reinforcing effect of CFG pile comprehensively evaluated. The results show that it is effective and reliable to treat moderate soft ground in high-speed railway construction, and the technical indexes can meet design requirements, so that the design parameters and construction techniques have guiding significance and reference function on other similar projects.The field monitoring data are analyzed detailedly to study the deformation behavior of CFG pile composite foundation under embankment load. It shows that CFG pile can increase foundation bear capacity and decrease foundation settlement greatly. The pile-soil stress ratio is relatively large, and the emphasis of stress concentration is obvious, so it is revealed that the piles undertake most of the loads.The settlement and pile-soil stress ratio of pile-slab structure, column-net structure and column-net structure with cap are compared, and it shows that pile-slab structure is better to control settlement than the other two type of structures. However, column-net structure with cap can take the advantage of pile better, and the soil arching effect is obvious. If the settlement can be controlled in allowable range and construction cost is taken into consideration, the column-net structure has some advantages.