| The railway construction in China is developing rapidly in recent years,and the role of passenger railway lines is becoming more and more important.The construction of the Baoji-Lanzhou passenger line is of great significance to the development of the high speed railway in the west of China,and the region where the line passes is mostly collapsible loess region.The collapsibility of loess is extremely unfavorable to the ground treatment.The engineering properties of collapsible loess should be fully considered in the process of foundation treatment,and corresponding foundation treatment measures should be taken.Based on the foundation treatment project of the Tianshui south station of the Baoji-Lanzhou passenger line and combining with the engineering geological conditions,the static analysis of single pile and group piles of several foundation treatment methods apply to collapsible loess region was studied.By studying the characteristics of displacement and stress respectively,the composite foundation which can not only improve the bearing capacity and reduce the settlement of the foundation,but also eliminate the collapsibility of the loess was selected,and the parameters of the selected composite foundation were optimized.The rationality of composite foundation design was verified by field bearing capacity test.Considering the foundation deformation when the train run ning,dynamic response under train load and earthquake was studied.The main research of this paper is as follows.(1)Based on the geological conditions of the Tianshui South Station of the Baoji-Lanzhou passenger line and the properties of different composite foundations,according to the corresponding design specifications,CFG pile composite foundation,cement-soil compaction pile composite foundation,thrusted expanded pile composite foundation and CFG-cement-soil compaction pile composite foundation were selected.By the finite element analysis,the mechanical characteristics of the single pile of CFG pile composite foundation,cement-soil compaction pile composite foundation,and thrusted expanded pile composite foundation were analyzed.Under the embankment load and vertical load,the displacement and stress characteristics of the pile groups of the four kinds of composite foundation were analyzed.According to the analysis result,the most suitable composite foundation type was selected.(2)Due to the conservative design of the primary pile,it should to be optimized in order to reduce costs.The different pile diameter,pile length,pile spacing and mattress layer thickness were selected respectively,and the deformation under load was analyzed in order to select the parameters which can meet the load requirement and economic rationality.Combined with the test of bearing capacity of single pile composite foundation by Wuhan Tiechen Engineering Inspection Co.,Ltd.,the rationality of the selected composite foundation was confirmed.(3)Dynamic response of optimized composite foundation duri ng train operation was analyzed.The dynamic responses of composite foundation under train load,earthquake and train-earthquake were studied respectively in order to obtain the deformation rules and the influence on composite foundation.The results show that:(1)The pile strength of CFG pile composite foundation is larger,and the foundation settlement is the smallest among the four kinds of composite foundations.The foundation treatment can be carried out in the deeper soil layer,but the pile spacing is large,the collapsibility treatment effect of the upper loess is not as good as the cement-soil compaction pile.The pile spacing of the cement-soil compaction pile composite foundation is small,therefore,the effect of eliminating the loess collapsibility is better,but the treatment depth is small and the foundation settlement is the biggest.The treatment effect is not that good in the area with large collapsible loess thickness.The treatment depth of thrusted expanded pile composite foundation is larger than that of the cement-soil compaction pile composite foundation,but is smaller than that of the CFG pile composite foundation.The foundation settlement is larger than the CFG-cement-soil compaction pile composite foundation,and is l arger than that of the cement-soil compaction pile composite foundation.Wit h the large length,in the CFG-cement-soil compaction pile composite foundation,the CFG pile can transfer the upper load to the deep soil,and the pile distance of short pile namely cement-soil compaction pile is small,which can eliminate the collapsibility of the loess better,and the settlement of the CFG-cement-soil compaction pile composite foundation is smaller than that of the CFG pile composite foundation.Aft er comprehensive analysis,CFG-cement-soil compaction pile composite foundation is the most suitable for the design of this project.(2)The design parameters of the CFG-cement compacted pile composite foundation is conservatively.Based on the purpose of settlement control,the pile length,pile diameter,pile spacing and the mattress layer thickness were optimized.The optimized settlement meets the settlement requirement.The bearing capacity test verified that the bearing capacity of the CFG-cement compacted pile composite foundation meets the requirements.When the parameters was optimized,the composite foundation can not only meet the project needs,but also save the cost,therefore,the subgrade treatment of Tianshui South Station is reasonable.(3)In the train working condition,when under the action of train load,earthquake and train load-earthquake,the displacement and stress of the composite foundation under the train-earthquake are the largest,and the displacement and stress under the action of the earthqu ake alone are the smallest.In the three cases,the displacement and stress values of the composite foundation are very small,which will not influence the normal operation of the train.Therefore,safety of the design is safe,and can provide a reference for the selection of composite foundations for similar projects. |
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