Study On Mechanical Properties And Key Construction Technology Of Large-diameter Belled Pile With Enlarged Bottom

The large-diameter belled pile has the advantages of high bearing capacity,effective reduction of pile length,saving project cost,etc.However,due to the complexity of its stress,there are still some problems and disputes on the applicability of the calculation theory of bearing capacity and the pile design method,the large-diameter belled pile is not widely used in Bridge Engineering.Therefore,it is of great theoretical and practical engineering significance to study the stress performance,failure mechanism and measures to control pore size of large-diameter belled pile under vertical load.Firstly,the static load of one pile and the self-balanced test of one pile were carried out in this paper,which based on the pile test project of Heilongjiang Bridge Project.The settlement of pile top,axial force of pile shaft and side friction of pile under vertical load were tested.The variation rules of pile top settlement,pile shaft axial force and side friction of pile of largediameter belled pile were analyzed under the axial pressure.Then,the transfer mechanism of load and failure mechanism of pile foundation under the axial pressure were revealed.According to the force-settlement curve of static load test,the vertical bearing capacity of the foundation pile was analyzed using the percentage method and inverse slope method,and compared with the analysis results of the force settlement curve obtained from the test.The reliability of the vertical bearing capacity obtained by the three methods was explored.Furthermore,the determination and calculation method of the self-balanced point were elaborated in this paper and the experimental data measured by the self-balanced method were analyzed.The load-settlement curve measured by the self-balanced method was converted,and the vertical bearing capacity measured by the self-balanced method was calculated and analyzed.Secondly,Boussinesg solution was adopted to solve the load-transfer function for pile base,and hyperbolic pile side load transfer model was employed to calculate side friction of pile.Considering the influence of pile side friction on the pile end settlement,this paper analyzed the behavior of large-diameter belled pile by combining Mindlin solution and load transfer method.The results revealed the load transfer and failure mechanism of large-diameter belled pile.Under the action of various axial pressures,the side friction of pile at the top takes effect first and the side friction of pile at the bottom is not excited due to small load.With the increase of axial force,the side friction of pile is gradually excited from top to bottom.With the increase of axial load,the deformation of the soil around the pile and the soil at the pile end is different,and the soil at the pile end gradually enters the plastic stage.With the further increase of axial load,the plastic area further develops and gradually penetrates,resulting in the damage of the belled pile.This paper takes the pile and the soil around the pile as the research object to form a pilesoil system by adopting Abaqus software.Through the analysis of pile-soil load transfer analysis,pile-side transfer function,pile-end transfer function,and pile-side friction on pileend settlement,the finite element numerical simulation analysis model of pile-soil linkage of large-diameter belled piles was constructed and the validity of the model was verified by loadsettlement curve,pile axial force and pile-side friction.Using the verified three-dimensional finite element numerical model,the vertical bearing behavior of belled piles with different pile length,different pile diameter,different belled pile diameter,different pile-soil friction coefficient and different elastic modulus of concrete were studied by changing the single parameter while other conditions remain unchanged,and the influence law of each parameter on the force transfer performance of large diameter belled piles was revealed.When the load is small,the friction coefficient has little effect on the settlement of the pile because the pile side friction is in a state of constant play.With the increase of load,the influence of friction coefficient increases significantly,the settlement of pile top decreases and the bearing capacity increases.With the increase of pile length,the vertical bearing capacity of pile increases.When the load is small,the Q-S curve changes linearly,and when the load exceeds a certain value,the Q-S curve exhibits nonlinear growth.The longer the pile is,the greater the load required for the bottom pile side friction and the pile tip reaction to be excited.With the increase of belled pile diameter,the bearing capacity of belled pile increases and the settlement of test pile decreases.Finally,based on the actual project of Heilongjiang Bridge,the key construction technology of large-diameter belled pile was studied.Through the ultrasonic test,the relationship between the consistency of the wall protection mud and the transmission speed of the ultrasonic wave in large-diameter belled pile was studied,which provides a basis for the inspection of the belled hole quality.Then,based on the construction experience of belled pile at home and abroad,the reaming accuracy of reaming size was measured,and a three-stage reaming construction scheme was proposed,which can meet the dimensional accuracy and design requirements of the reaming part of the pile and provide the basis for the size control of the reaming construction of belled pile.