Experimental And Numerical Simulation Research On Pile-soil Mechanical Response During Pile Pulling Process

With the rapid development of urbanization and the construction of bridge engineering is being processed like a raging fire of China.In most bridge constructions,steel pipe piles are needed as temporary supports,and a temporary support to form temporary support system with scaffolding and Bailey beams.When the bridge construction had been completed,the temporary support system needed to be removed.The upper Bailey beams and full space supports are easy to remove;however,it is difficult to pull out the steel pipe pile at the bottom.The traditional method requires divers to cut the steel pipe piles above the riverbed underwater,and the steel pipe piles under the riverbed face the problem of being discarded.In order to reflect the construction concept of resource-saving and environment-friendly society,it is particularly urgent to remove the remaining steel pipe piles.This is also the requirement of promoting the recycling and comprehensive utilization of construction and waste materials.It is also an urgent requirement to promote green development,adhere to the priority of saving,and comprehensively improve resource utilization efficiency.In this paper,based on the engineering background of Jianning bridge pile pulling project,the systematic laboratory tests and numerical simulations have been conducted to research the pile-soil effect during pile pulling process and reveal the dynamic characteristics of the surrounding soil response mechanism and discover the regular pattern between pile and soil when pulling pile.The main contents are as follows:（1）The evaluation method of pile-soil interface roughness was proposed,6different roughness test blocks were printed by using Acrylonitrile Butadiene Styrene（ABS）material and 3D printing technology.The mechanical parameters of the pile-soil interface were determined through a series of direct shear tests,which provided data support for the establishment of the numerical model.（2）Combined with the results of direct shear tests with different roughnesses,the micro parameters of the soil were calibrated and the direct shear numerical model with different roughnesses was constructed.The mechanism of the change of macro and micro mechanical parameters such as displacement-stress curve,damage evolution,force chain evolution,shear band,coordination number,porosity,etc.were explained of the pile-soil interface.The results showed that the parameters could effectively reflect the macro and micro mechanism of contact failure,and the response of each shear stage was consistent.（3）Based on the results of direct shear test,the laboratory scale static and dynamic pile pulling test models were established using the similarity theory,at the same time,a series of parameters of pile and soil were monitored and analyzed,and the influence of pile pull under different pile diameters,pile lengths and embedded depths were discussed.The static and dynamic pulling force of piles and the area of influence had been preliminarily determined during pile pulling process.（4）Based on the scale static pile pulling test,the numerical simulation model of static pile pulling was established on the discrete element method（DEM）,meanwhile,the laboratory test,2D model and 3D model were compared to verify the accuracy of the numerical model.The macro and mesoscopic parameters such as soil stress,particle displacement,velocity,coordination number and force chain were measured and analyzed during pile pulling process.The results showed that the major influence zone of the pile pulling on both sides was concentrated within 3d_pileand on the pile bottom is concentrated within 3～4d_pileduring pile pulling.（5）Based on the scale dynamic pile pulling test,a numerical simulation model of dynamic pile pulling was built,the accuracy of the numerical model was verified by comparing the laboratory test with the numerical model.The dynamic response of soil under different vibration loads and different pile pulling heights was explained from the micro perspective.The results showed that the response curve from the top of the pile to the bottom of the pile also conformed to the characteristics of"inverted cone"transmission.The damage of the soil on the top of the pile showed that the particles on the top of the pile uplift with the increase of the height of the pile,and the cracks were radial and continues to spread around the pile.The major influence area of the pile pulling,the 6d_pilehad a greater impact on the surrounding area,which would be greater than of 10d_pileduring the dynamic pile pulling process.In order to study the series of problems during pile pulling process,the direct shear test,static pulling out test and dynamic pulling out test were used to determine the related mechanical parameters of pile-soil.On this basis,a numerical direct shear model of pile-soil interface,a numerical model of static pile pulling and a numerical model of dynamic pile pulling were established.The corresponding macro and micro parameters were obtained,which provided a theoretical and data basis for judging the area affected during the pile pulling process.This research can also provide reference for similar projects such as pulling piles of offshore platforms and abandoned buildings.