Structural Analysis Of Underpinning Enginnering Of Pile Foundation

With the development of urbanization, lots of people are rushing into the city, which brings a lot of traffic problems. Therefore, underground transportation is becoming a main method to ease the traffic jam. Inland underground traffic line often coincides with current existing buildings.The demolition of these buildings does not confirm to the concept of green, efficient and economic development. To solve this issues, pile underpinning technology has drawn much attention.Therefore, the pile underpinning engineering of a commercial railway station is selected as the research background. The aim of this study is to analyze the structural issues of pile underpinning engineering.To guarantee the safty of the underpinning structure, three key issues as follows must be clarified: (1) the interface slip between underpinning beam and pile foundation; (2) the bearing capacity and the deformation of underpinning structure; (3) the mechanical response of different cutting pile sequence to the underpinning structure. To answer the problems mentioned above,some investigations as follows are carried out in this article:(1) The bond-slip experiments and corresponding numerical simulation analysis were conducted to understand the effect of the composite connectors those consist of annular plate,stiffening plate and welding nails on the shear behavior of the steel pipe and the surrounding concrete. The results show that the characteristic slip of the steel tube with the composite connectors is more than 6mm. Increasing the ratio of welding nails enhances the plastic anti-slip stiffness and ultimate strength of the interface. The actual ultimate load value of single welding nail agrees with the calculate value of the codes.(2 ) The bond performance between the underpinng beam and the pile was experimentally studied,and the research parameters included groove width, groove depth,and the ratio of inserting reinforcement. The results show that the ultimate anti-slip strength and the anti-slip stiffness of the interface increases with the reduction of the groove width. The groove width has a great impact on the strength and the stiffness of the interface, however the impact of the groove depth is insignificant. The ductility of the joints can be significantly improved by the mean of inserting rebar. The ultimate anti-slip strength increases with increasing the ratio of inserting reinforcement.(3) Experiment and numerical study were carried out to understand the mechanical property of an underpinning structure framework.To provide a reference for the design of underpinning framework, the load-deflection relationship and the strain of the rebar and concrete were acquired and analyzed. The results show that the finite element simulation results of the load-displacement relationship,the steel strain and the concrete strain are in a good agreement with the experimental results; It is reliable to use the finite element simulation to predict the mechanical properties of the underpinning structure. The long axis beam of the frame structure is mainly dominated by the bending deflection, the short axis beam of the frame structure is mainly dominated by the shear deflection.(4) Experiment and numerical simulation were also conducted to clarify the influence of pile cutting on the mechanical response of the underpinning framework. The relationship between the load and pile displacement under the different cutting pile cases about the underpinning framework was acpuired. The results show that the cutting order has some effect on the mechanical response of the underpinning structure: the best way for pile cutting is to cut the pile along the short axis and then the pile along the long axis from the view of deformation controlling.