Research On Settlement Characteristics And Bearing Capacity Of Long-short Pile Reinforced Loess Foundation For High Speed Railway

With the extensive application of composite foundation in loess foundation treatment, there has been a long-short pile composite foundation witch combined with different pile materials and different pile length, it achieved a good social and economic benefits and had a perfect prospects; Because of complex working mechanism,defects appeared more in study, in theory, there were no accurate and reliable calculation methods of settlement and bearing capacity for long-short pile composite foundation under flexible load of embankment, in addition, its method was same to the method under rigid foundation, indoor model test research was less, and the boundary conditions, the pile-soil contact and selection of cushion modulus for numerical simulation were no reliable basis.The paper combined with field test section of the Lanzhou-Xinjiang second double line, using the means of indoor model test and numerical simulation, and studied the load transfer mechanism and the settlement deformation characteristics of long-short pile composite foundation under flexible load of embankment, the main work were here:(1)Summarized the research results of long-short pile composite foundation under flexible load of embankment in side of theoretical research, numerical simulation, engineering applications and indoor model test and so on at home and abroad; Analyzed the defects and the insufficiencies that existed in the theory research of long-short pile composite foundation,and the paper proposed the research methods and research content.(2)Summarized several kinds of pile arrangement of long-short pile composite foundation, introduced the concepts of area replacement ratio, pile soil stress ratio and load sharing ratio and so on; Analyzed the mechanism of long pile,short pile and cushion under long-short pile composite foundation; discussed the calculation method of settlement for long-short pile composite foundation under embankment load and summarized the advantages and disadvantages of each.(3)Based on the similarity theory and combined with roadbed prototype in high-speed test section, a set of indoor model test equipment were designed and the testing scheme that studied the impact that different pile length and pile spacing to the bearing and deformation characteristics of long-short pile composite foundation were made.(4)The effect that different pile length and pile spacing to the settlement,stress of pile, pile soil stress ratio and load share ratio of long-short pile were studied through the means of load test under flexible load of embankment, the load transfer mechanism and the changing regular of settlement deformation characteristics for long-short pile composite foundation were achieved under embankment load.(5)Combining with test section of practical engineering situation, using the finite element analysis software MIDAS/GTS, the characteristics of deformation and stress of long-short pile reinforced loess foundation were analysised under the embankment load; According to the construction stage of field test section, construction stage of numerical simulation was designed; the settlement value of the field test section of subgrade under the heap loads and the settlement value of numerical simulation were compared to verify the validity of the finite element model and parameter; The influence that different pile length, pile spacing, layer stiffness on settlement and load-bearing characteristics of long-shore pile composite foundation were studies under embankment load.Through the above studies, the settlement and load-bearing characteristics of long-short pile reinforced loess foundation for high-speed railway were systematically understanded, the view was put forward that when designs the long-short pile of high-speed railway, these factors such as length of pile(pile length ratio),pile spacing(area replacement ratio), bottom layer stiffness and pile stiffness and so on should be taked into consideration; there are best value or the most economic value on above factors that could be referenced in engineering design and application in the future.