Study On Pile Group Effect Of Bridge Pile Foundation Based On Stress Superposition Principle

Pile foundation has always been one of the main forms of foundation used in bridge structure.Because of its high bearing capacity,good stability,small uneven settlement and other advantages,it has been used more and more frequently in bridge construction.In order to meet the needs of bridge safety and stability,more and more pile foundations such as double piles,four piles and pile groups have been adopted.Although the specific calculation method and test method for determining the bearing capacity of some pile groups are given in the Code for Design of Foundation and Foundation of Highway Bridges and Culverts（JTG D63-2007）^[1],the calculation method for pile groups with pile spacing less than 6 times of pile diameter in frictional pile groups is too general and single,especially for pile groups with pile spacing of3-6 times of pile diameter which is direct to calculate the vertical bearing capacity of pile groups as a whole,ignoring the specific degree of pile group effect on pile group bearing capacity.It is necessary to analyze the vertical bearing capacity of bridge friction pile group considering pile group effect coefficient.Through theoretical analysis and deduction,field test data research and finite element analysis,this paper makes a comparative study on the calculation methods of vertical bearing capacity of bridge friction pile groups.The main contents are as follows:（1）Through data collection and literature reading,the load characteristics,pile size and calculation method of vertical bearing capacity of pile foundations in civil and industrial construction,port and bridge are understood.In different industries,the location of pile foundation construction and the types of loads different,which leads to certain differences between the calculation methods of pile foundation size and vertical bearing capacity in different industries.（2）By using the field test data of single pile in loess and the simulation data of finite element analysis,the bearing behavior of single friction pile is compared with that of theoretical analysis.The distribution law of pile side frictional resistance obtained by finite element method is basically the same as that of theoretical research and field test results.Before the pile side frictional resistance reaches its limit state,it increases with the depth of pile foundation into soil in a quadratic parabola,and is inversely proportional to the distance from the point of action of pile side frictional resistance to the center of pile.The limit value of pile side frictional resistance is based on the strength of pile-soil interface and the shear strength of soil.In the triangular pile side friction distribution model,the calculated pile side friction will cause reaction force in the soil,and the vertical stress values of different points in the soil obtained by the reaction force are close to the actual value.（3）The calculation method of pile group effect coefficient of bridge friction pile group foundation is obtained by assuming that the pile side frictional resistance is triangular distribution and the vertical load of single pile is all borne by the pile side frictional resistance.Based on the field model test data of pile group in complex soil and the calculation data of pile group model under various working conditions simulated by finite element method,the calculation theory of pile group effect of bridge friction pile group foundation is validated.The analysis and verification show that in pile group foundation with 2,4,9 piles and multiple pile diameters,the difference between the vertical bearing capacity of pile group calculated by theoretical formula of pile group effect coefficient of bridge and the test results and finite element simulation data is very small,which verifies the reliability of the calculation theory.In this paper,the calculation method of pile group effect coefficient of bored frictional pile group of bridge in loess is obtained by comparing and analyzing the above methods,which is suitable for the calculation of bearing capacity of bored frictional pile group foundation with pile spacing of 3-6 times pile diameter in bridge.