Study On Long-term Working Behavior Of Concrete-filled Steel Tube Rock-socked Piles Under Horizontal Cyclic Loading

Concrete filled steel tube rock-socked pile is a kind of deep foundation used in the engineering construction of important port wharf and bridge in the upper reaches of the Yangtze River.Under the action of long-term horizontal cyclic load such as ship impact force,hydraulic force and other long-term horizontal cyclic loads,the steel-concrete interface strength and structural stiffness of concrete-filled steel tube rock-socketed pile are weakened,which leads to the weakening of bearing capacity of pile foundation and large accumulative deformation.Numerical simulation with theoretical research are combined in the paper to analyze the failure mode of concrete-filled steel tube pile-rock structure system under different foundation strength and to obtain the failure mode discrimination method.The working behavior of concrete-filled steel tube rock-socketed piles under long-term horizontal cyclic loading is analyzed,and the related factors affecting the deformation characteristics of concrete-filled steel tube rock-socketed piles are discussed.The main research work and achievements of the paper are as follows:(1)A numerical calculation model of horizontal bearing capacity of pile foundation considering the weakening of structural stiffness and weakening of the steel-concrete interface is established.The main reason for the rapid decline of the horizontal ultimate bearing capacity of pile foundation under horizontal cyclic load is obtained.It is due to the degradation of structural stiffness,the weakening of the strength of the steel-concrete interface and the weakening of the constrained capacity of the pile due to the degradation of structural stiffness.(2)With the change of foundation strength,there are three types of failure modes of pile-rock structure system under horizontal cyclic load.When the foundation strength is strong,the concrete tensile yield zone and shear yield zone of pile are connected to each other,thus the pile is destroyed.When the foundation strength is medium,the concrete of pile appears a large area of tensile yield zone,and the plastic zone of the foundation at the section of the pile passes through,which makes the pile and the foundation to be destroyed together.(3)The failure mode of pile-rock structure system based on the relative stiffness ratio index of pile-rock with Concrete filled steel tube rock-socked piles is put forward.And the failure mode of pile-rock structure system based on pile foundation is judged.The calculation results show that when the relative stiffness ratio index of Concrete filled steel tube rock-socked pile-rock is less than 1.44,the failure of pile body mainly occurs.When the relative stiffness ratio index of Concrete filled steel tube rock-socked pile-rock is between 1.66 and 3.11,the pile and foundation are mainly destroyed together.When the relative stiffness ratio index of Concrete filled steel tube rock-socked pile-rock is greater than 3.11,the common failure of foundation occurs mainly.(4)Concrete filled steel tube rock-socked piles have the cumulative damage effect of stiffness under horizontal cyclic load.The amplitude of cumulative horizontal displacement curve along the direction of pile increases with the increase of the number of cycles(the increase of stiffness damage).Based on the existing experimental data,an empirical formula for fatigue life of Concrete filled steel tube rock-socked piles with rock socketed piles based on the deformation of piles is obtained.It is found that there is a critical value between 3% and 4.9% of the displacement ratio of the pile body at the rock surface.When the displacement ratio is less than the critical value,the fatigue life of the pile foundation is longer.The fatigue life of Concrete filled steel tube rock-socked pile decreases rapidly as the displacement ratio of the pile is gradually larger than this critical value.