Study On The Working Behavior And Design Method Of The Pile Composite Under The Flexible Foundation

People find there lies different working behaviors between the pile composite ground under the rigid foundation and the flexible foundation,such as embankments of highway and railway, dams,stack yards,oil tanks etc..Applying the design method,which is applicable to the rigid foundation,to the flexible foundation,accordingly a notebale gap between the calculated value and the measured will be induced,and it is apt to insecurity.Therefore,it is of great theoretical significance to study on the working behavior of the pile composite ground under the flexible foundation.The results of study on the design method,which can be used to the engineering practice,have great values of social and economical benefits.Based on an overview of experimental research,analytical method,numerical analysis and design methods,etc.,the principal contents and original work are as follows:1) The flexible foundation-the cushion-the pile composite ground-the underlying strata is considered as an interactive system by bringing in a typical unit model and virtual soil pile model.The Alamgir-type assumption of the soil displacement is unified by bringing in a pile's skin friction distribution function and a relative movement function,which can consider the relative movement between the pile and soils around the pile,and can assume a different settlement at a same original height of soil around the pile.Combining the load transfer mechanism with the ideal elastic-plastic constitutive of the pile-soil interface,the distribution along depth of the exerting degree of nonlinear skin friction is obtained.Based on the foregoing assumptions,the differential quations of the system is presented and solved.The rationality of the presented analytic method is verified by an engineering case.2) Based on the presented analytic method and orthogonal test method,the impact of height and modulus of fill,thickness and modulus of cushion,replacement ratio of pile,pile diameter,pile length,pile modulus,modulus of soil between piles,and the modulus of the underlying strata on the characteristic of the system,such asthe equal settlement plane,neutral point plane,pile-soil stress ratio or pile-soil load share ratio,are studied.3) Through 3D FEM and orthogonal test method,the effects of embankment width,pile group,grid reinforeced cushion,and layered filling are researched.Based on the basic case,the total settlement,the amount of compression of the pile improved area and the underlying area, the amount of penetration at the top and bottom of the pile,the maximum axial stress of the pile, the skin friction of the pile,and the stress of the grid are analyzed.Considering the situations of the engineering practice,the impact of the fill height and width,the layer number of grid,the replacement ratio and modulus of pile,the modulus of the underlying strata on the equal settlement plane,neutral point plane,pile-soil stress ratio or pile-soil load share ratio,are studied. The results could provide some references to the engineering practice.4) The design idea of the pile composite ground under the flexible foundation is presented. And the method to determine the position of the equal settlement plane,the neutral point plane, pile-soil stress ratio,which are relevant to the load transfer mechanism,are provided.Then the method to calculate the bearing capadty,the settlement arid the global stability of the pile composite under the flexible foundation baesd on super-sub structure interaction is put forward.5) In the end,the load transfer mechanism and the method of the pile composite ground under the flexible foundation are applied to retreat a foundation acddent of a stack yard. Comparative bearing capacity of the original composite foundation is calculated based on a rigid foundation and flexible foundation respectively.And according to the load transfer mechanism and comparative calculation,the causes of the accident are analyzed.Accordingly,three comprehensive measures of the semi-rigid cushion,binary composite ground,compound retaining wall were adopted in the retreatment design.The whole process of monitoring verified that the measures worked soundly,for the yard reached its scheduled height smoothly.