Study On Composite Foundation With Variable Stiffness Composite Pile And Its Working Performance

At present,the types of composite foundation for collapsible loess include CFG pile,lime soil compaction pile,screw pile and rigid flexible long and short pile.However,the engineering practice of high-speed railway in loess area shows that these composite foundations have the problems of insufficient settlement control ability or high pile-soil stress ratio.One of the main reasons for this phenomenon is the lack of pertinence of foundation treatment to engineering quality conditions.Aiming at the engineering geological conditions of loess river terrace,this paper puts forward a type of variable stiffness composite pile composite foundation composed of "upper flexible part" and "lower rigid part" in the reinforcement area,and carries out in-situ static load test,compaction coefficient measurement test,theoretical calculation and numerical simulation,The bearing characteristics,pile-soil stress ratio,soil compaction effect,soil stress distribution,pile stress distribution and failure mode of composite foundation of variable stiffness composite pile composite foundation are studied,and the adaptability of variable stiffness composite pile composite foundation to loess river terrace is discussed.This paper draws the following conclusions:（1）The settlement of variable stiffness composite pile composite foundation in the whole process of loading is less than that of cement soil compaction pile composite foundation,and less than that of long and short pile composite foundation when the load is small,which indicates that variable stiffness composite pile composite foundation has a strong ability to control settlement.（2）The pile-soil stress characteristics of variable stiffness composite pile composite foundation depend on the upper flexible part.Similar to cement soil compaction pile composite foundation,it can effectively reduce the stress concentration,give full play to the bearing capacity of soil between piles,and have good stress and deformation coordination ability.（3）Under the conventional pile arrangement（pile spacing 1.2m）,the compaction coefficient of soil between piles of variable stiffness composite pile composite foundation meets the requirements of the specification,and the collapsibility of loess can be eliminated.（4）In the composite foundation of variable stiffness composite pile,there is obvious soil arching effect,which makes the load transfer from the soil to the pile at the junction of the upper and lower layers of the composite foundation,so as to reduce the load on the lower soil of the composite foundation and better adapt to the engineering geological conditions of the loess river terrace.