Research On The Bearing Mechanism Of Pile Foundation Underpinning Subject To Basement-addition Under Existing Buildings

Basement-addition under existing buildings are in the ascendant,and the core of basement-addition under existing buildings is soil excavation and pile foundation underpinning.The load on the upper part of the pile foundation underpinning is gradually transferred to the new underpinning pile beam system.After the pile foundation is underpinned,it is necessary to excavate the soil under the bottom of the building,which will inevitably cause the disturbance and removal of the soil at the side of the pile and the pile tip,which will reduce the ultimate bearing capacity and stability of the pile.In some cases,there will be negative friction on the pile body,which will affect the bearing capacity,differential settlement and stability of the original pile foundation.Based on theoretical analysis and numerical simulation,this paper analyzes the difference between the Mindlin stress solution and numerical simulation of the pile ultimate shaft resistance under the traditional formula method and rectangular load,and compares the influence of different excavation depths and excavation widths on the pile ultimate shaft resistance.Through finite element analysis the load transfer and deformation characteristics of the "two-support-one" underpinning system,and discuss the stiffness of the underpinning system,complex working conditions and key construction processes,etc.to provide references for the design optimization and settlement control of the underpinning system.The main contents of the paper are as follows:(1)Taking the basement-addition under Zhejiang Hotel as the background,the traditional formula is derived and compared with the Mindlin stress solution of pile ultimate shaft resistance formula under rectangular area load.The result shows that the traditional formula method is conservative and have a big deviation compared to the Mindlin stress solution and finite element simulation soultion.The Mindlin stress solution under rectangular load and the numerical simulation under the three-dimensional stress are relatively close,and the overall difference is about 5% ～ 10%.(2)Case calculations show that with the progress of soil excavation in the underground layer addition,the load sharing of the pile tip at an excavation depth of 0 ～ 20 m only decreases by 3%.As the excavation depth increases,the pile ultimate shaft resistance decreases approximately linearly.The excavation of the upper soil body not only causes the loss of the pile ultimate shaft resistance of this layer,but also reduces the vertical effective stress of the lower pile side,which weakens the original pile ultimate shaft resistance of the unexcavated soil layer.With the increase of the excavation width,the pile ultimate shaft resistance reduces the marginal effect.When the excavation width increases to a certain value,the influence of the excavation width on the pile ultimate shaft resistance can be ignored.(3)Using the load transfer function of the load transfer method,the calculation formula for the settlement of the pile foundation of the existing building’s underground increase was deduced.The Mindlin stress solution under the rectangular load was used to calculate the hyperbolic function of the pile side and pile tip,and the MATLAB software was used to program iteratively to calculate the pile top load-settlement curve.Based on the analysis of the pile top load-settlement curve based on different excavation depths and excavation widths,the results show that at the initial stage of load application,as the pile top load increases,the pile side soil begins to enter the plastic stage,and the P-S curve is a steep-down type.Under the same pile top load,the excavation depth and width show a positive correlation trend with the settlement.In contrast,the excavation depth has a greater influence on the settlement of the pile foundation.(4)Aiming at the supporting project,the stiffness matrix of the "two-support-one" pile-beam underpinning system is established,and the finite element method is used to establish the upper structure of the upper frame and its lower pile-soil system analysis.After the original pile is cut,the load received by the adjacent underpinned pile is the largest,and the farther the distance is,the less load transfer and the smaller the impact of settlement.Increasing the underpinning beam section size,reducing the underpinning beam length and increasing the underpinning pile diameter can increase the load transfer rate.Among them,changing the underpinning beam length has the greatest impact on the effective load transfer efficiency.(5)For the supporting project,analyze the upper load transfer and settlement characteristics under complex working conditions,select different pile foundation underpinning sequences,geological unevenness and asymmetric underpinning piles,etc.,and analyze the unfavorable conditions and key construction processes that may be encountered in the animation building of the supporting project.Then it provided guiding opinions for the design of the scheme.