Analysis Of The Impact Of Differential Settlement Of Foundation Over Superstructure Under The Pile Foundation Underpinning And Experiment Study

Underpinning technology is the key technology in constructing the underground space of the existing buildings. This process makes existing buildings foundations differential settlements; consequently, the additional internal forces affect structure safety. Numerical models of three-dimensional finite element analysis based on Ansys code are established to study the the influence of foundation differential settlement for RC frame structures, using the model experiment to simulate and analyse the influence of foundation differential settlement for the superstructure. A limit value of foundation differential settlement was proposed.Two condations were studyed through numerical analysis on the the influence on the superstructure of a three-layer and three-span frame structure of foundation differential settlement:one is stelar column settlement,the other is limbic column settlement. Establish the three-dimensional solid model of the frame structure, by defining the different settlement of column foot to achieve the uneven settlement of foundation. The results show that as to stelar column settlement, internal forces are higher at the border pillar support of frame; as to limbic column settlement, nternal forces are higher at the median pillar support of frame. Vertical displacement of each floor beam has little difference, but stress vary greatly, the second floor is slightly larger than the first floor, the top floor is the minimum. And then establish two-dimensional model of the framework under differential settlement to study the the specific values of additional internal forces, obtained axial force of ground floor columns is the maximum. On balance, the underlying framework is the location of the maximum internal forces caused by unequal settlement and should be monitored strictly.The model experiment is designed based on the numerical analysis to test the reliability of the numerical results and get the differential settlement limits of frame structure under the process of underpinning.A single-storey frame model with the similarity ratio is 1:3 is designed. Specimen is divided into two groups, one group is through lifting the limbic column to simulate the settlement of side column; the other group simulates the stelar column settlement. Analysis showed that as to limbic settlement, the main cracks appeared at the underarm parts of beam-column joints of side columns, the top of beam-column joints of middle columns, and the top of middle span of middle beams; finally, the RC members failed due to cracks transfixion of the beam-column joints of side columns; the cracks appeared when the tangent value of beam turn angle reached 1/250, and the bearing capacity limit state was achieved when the tangent value of beam turn angle reached 1/50. As to stelar settlement, the main cracks appeared at the top of side columns near corresponding side beams, the bottom of middle span of middle beams, and the bottom of joint part of middle span beams; ultimately, the RC members breaked for the top concrete compression failure of middle span beams; the cracks appeared when the tangent value of beam turn angle reached 1/1250, and bearing capacity limit state was achieved when the tangent value of beam turn angle reached 1/40. Therefore, as to the limbic settlement, the top of beam-column joints of middle columns, and the top of middle span of middle beams are the reinforced weak posion; as to stelar column settlement, the bottom of middle span of middle beams, and the bottom of joint part of middle span beams are the reinforced weak posion. These sites should be focused on monitoring and reinforcement when uneven settlement of foundation occoured during the foundation underpining.