Load Transfer Model And Settlement Calculation Method Of Super Large Diameter Pile With Variable Cross-section

With the development of railway and highway network,the distribution of karst in China is quite wild-ranging and the problem which can be evaded in the past has become a major challenge for engineering construction now,especially it brings great difficulties to the practical project when the bridge infrastructure inevitably encounter cave formation.In order to effectively solve the technical problem.of bridge pile foundation construction in complex karst stratum with thickness of certain cover,super large diameter pile that has segmentally variable cross-section is produced.Method of not sticking into rock is adopted by the pile to reduce the disturbance of thecave,avoiding various diseases induced by that the cave is forced intruded by the pile and fundamentally solving the difficult problem of complex bridge pile foundation in karst area.It has been applied in a certain degree in engineering field,but theoretical research lags far behind engineering practice.So the load transfer mechanism and settlement method of super large diameter pile with variable cross-section are studied through theoretical analysis and numerical simulation.The conclusions are as follows:(1)Based on disturbed state concept,the model of load-transfer function of super large diameter pile with variable cross-section pile-soil interface is constructed and the influence rules of model parameters arid curve are analyzed by using MATLAB.Meanwhile,the load transfer mechanism of the large diameter pile with variable cross-section is discussed emphatically.Then,the theoretical analysis of the coupling effect between the resistance of variable cross-section and the lateral resistance is carried out.The resistance of variable cross-section of the pile is deduced by using Mindlin solution of the lateral resistance enhancement effect.The results show that:load-transfer function which based on disturbance state theory is widely adapted because it can be used to describe the softening and hardening characteristics of soil.The resistance of variable cross-section of super large diameter pile with variable cross-section has significant enhancement effect on lateral resistance.(2)Based on rheological mechanics theory and perturbation state theory,themodel of aging perturbation load transfer for pile-soil with variable diameter trapezoidal variable cross-section is constructed,the Kelvin model and viscoplastic fluid model are adopted respectively in the relatively complete state and the complete adjustment state.At the same time the calculation principle and method of the disturbance factor are put forward.Therebythe analytic expression of the relationship between displacement and time of soil element is obtained.The influence of various parameters in the model on the settlement time curve of pile top was analyzed by MATLAB software.The engineering example shows that the model can better reflect the secular disturbance process of soil mass transfer around the pile.(3)Based on the load transfer model constructed in this paper,An improved load transfer method is adopted and the settlement calculation methods of homogeneous soil and layered soil medium section pile are deduced respectively;on the basis,the calculation methods are extended to n variable cross-section pile,and the basic differential 'equation of load transfer method is further corrected considering' the weight of the pile,in order to obtain the settlement calculation method of super large diameter pile with variable cross-section;in the meantime,' the reliability and rationality of the' single pile settlement and semi analytical solution is verified combined with the engineering examples.The results show that,due to the larger size of the large diameter pile,the weight of the pile body should be considered in the settlement calculation,and it can be directly calculated by the improved load transfer method.(4)Based on the load transfer method,this paper proposed the pile foundation settlement calculation method with long term secular perturbation,and the method was extended from ordinary uniform pile to variable cross-section pile,as well as from homogeneous soil to layered soil.Considerred the weight of pile body,the long term settlement formula of super large diameter pile with variable cross-section was derived by the improved load transfer method,and the rationality and the accuracy of the theoretical model was verified according to the simulation results of FLAC3D simulation.The research results can provide a theoretical basis for the design and construction optimization of the new type of pile foundation in complex karst area.