Improvement Of The Calculation Method Of Displacement And Internal Force Of Pile Bracing Structure Based On Incremental Method

Deep foundation pit pile bracing support structure has mature construction technology and better control of pit deformation,which is widely used in foundation pit projects with large excavation depth such as subway stations.In this kind of support structure,the incremental method is a common method to analyze the displacement and internal force,while the accurate calculation of the load increment becomes crucial.At the same time,the need to consider the increment of reaction force caused by the reduction of horizontal resisting stiffness coefficient of soil below the excavation reference surface is also the focus of research.And it is also important to investigate and analyze the factors influencing the horizontal elastic resistance of soil body.Therefore,this paper relies on a deep foundation pit project of a subway station of Jinan rail transit line 4,and uses theoretical calculation,finite element simulation,indoor triaxial test and field measurement to study the calculation method of displacement and internal force of pile support structure,and the influencing factors of soil elastic horizontal resistance,mainly as follows:（1）Taking the theory of incremental method as the starting point,with the help of winkler vertical foundation beam model and using spring equivalent to replace the action between pile,soil and support,the deficiencies of load increment calculation in incremental method are analyzed,and the type of earth pressure acting on the pile body is clearly non-limiting earth pressure,meanwhile,it is proposed that the reaction force released by the soil body below the excavation base due to the reduction of horizontal resistance stiffness coefficient and the pre-added axial force exerted by steel support should be taken into account when calculating the load increment,and the improved incremental method of deformation and force of pile support structure is proposed.（2）The K₀ consolidation lateral unloading triaxial test was conducted on the powder clay soil at a subway station pit project site in Jinan to study the deformation characteristics of the soil in the active zone around the pit under the unloading stress path.The test results show that the stress-strain relationship of the lateral unloading stress path test is approximately hyperbolic under K₀ consolidation with different surrounding pressures,and the stress-strain relationship of the powdered clay in Jinan area under this stress path has more significant normalization characteristics,which can be normalized by using the average consolidation pressure to obtain the calculation parameters a and b required for the displacement earth pressure theory considering the influence of unloading.（3）Relying on a deep foundation pit project of a subway station of Line 4 of Jinan City Rail Transit,we use Fortran language to write the corresponding calculation procedure according to the calculation parameters of the project example,and compare and analyze the theoretical calculation results,numerical simulation results and field measurement results,and the results show that the improved incremental method proposed in this paper is more reasonable and accurate.（4）The control variable method and finite element simulation are used to explore and analyze the important influencing factors of soil horizontal resistance,namely,cohesion force c and internal friction Angleφ.The results show that with the increase of c orφvalue,soil elastic resistance gradually increases,and the horizontal displacement of pile body gradually decreases.Compared with the influence of c on the horizontal displacement of supporting pile,the change ofφhas a greater influence on pile displacement.