Horizontal Bearing Capacity Calculation Method Of Concrete-filled Steel Tube Rock-socked Piles Under Cyclic Loading

As a type of deep foundation,the concrete-filled steel tube rock-socked piles are widely used in the infrastructure construction of port wharves,bridges,and high-speed railways.At present,the calculation methods for the bearing capacity of concrete-filled steel tube rock-socked piles are mainly to simplify the calculation model by converting the strength of the steel tube to the strength of the concrete.However,these methods ignore the interaction of the steel casing with the pile body and the rock mass around the pile during the loading process.In addition,the current calculation methods are mostly under static load conditions,and are relatively rare under cyclic loading.Based on the project of National Natural Science Foundation of China which named “Study on steel-concrete interface strength weakening mechanism and long-term working behavior of concrete-filled steel tube”,this paper established the numerical models of concrete-filled tube steel rock-socked piles under cyclic loading to study the failure modes.Based on this,a calculation method for the horizontal bearing capacity of concrete-filled steel tube rock-socked piles considering the interaction between the steel tube and the pile body and the rock mass around the pile is proposed.The main research work and achievements of this paper are as follows:(1)Based on the interface slip stress-strain curve obtained from the cyclic shear test of the steel-concrete interface,a nonlinear spring element is used to simulate the interface slip process.Based on this,a finite element numerical calculation model considering the slippage between steel and concrete is established.The calculated results are basically consistent with the test results,indicating the reliability of the finite element simulation method.(2)A numerical model for the concrete-filled steel tube rock-socked piles which considering the weakening of the interface strength between steel tube and concrete under cyclic loading is established.The analysis of the stress-strain of the pile and the slip process of the steel-concrete interface show that: as the horizontal load increases,the maximum tensile stress area of the pile body expands from the base surface to upper and lower sides;due to the slippage of the steel-concrete interface,the maximum tensile stress of the steel tube occurs approximately one pile diameter above the base surface;as the number of cycles increases,the slip value at the same position of the pile increases,and the maximum slip value appears at the tensile side of variablecross-section.(3)A numerical calculation model for the concrete-filled steel tube rock-socked piles under different working conditions is established.The influence of the number of cycles,the depth of the steel tube embedded in the rock,and the strength of foundation on the failure modes of concrete-filled steel tube rock-socked piles is analyzed.The calculation results show that: failure position of the concrete-filled steel tube rock-socked piles occurs near the variable section;under cyclic loading,the steel-concrete interface weakens obviously at the surface of the foundation,and the embedding effect of the foundation on the pile is weakened,which reduces the horizontal bearing capacity;as the depth of the steel tube embedded in the rock increases,the failure position moves from the variable section to the base surface;the weakening of the foundation strength makes the plastic zone more likely to occur in the rock mass around the pile,thereby reducing the constraint of the foundation on the pile body,and finally leading to the reduction of the horizontal bearing capacity of the structure.(4)According to the failure mode of the variable cross-section and the theory of elastic mechanics and material mechanic,the calculation of horizontal bearing capacity of the concrete-filled steel tube rock-socked piles is derived,which based on the assumption that the pile is fully embedded at the variable cross-section.When considering cyclic loading,bending stiffness weakening coefficient is introduced to simulate the effect of cycle times on structural failure.The calculation method can provide reference for the calculation of the concrete-filled steel tube rock-socked piles in inland terminal depots.