Pile-soil Bearing Behavior Subjected To Soldier Piles Supported Excavation For Underground Storey Supplement Beneath Existing Piled Buildings

Underground-storey supplement of existing buildings involves complex problems such as the change of bearing capacity of existing foundation piles and the interaction of newly-added and existing supporting structures.At present,researchers as well as engineers are still unclear about the above problems.Most of them follow the traditional design theory for the new excavation process.On the other hand,the risk of underground excavation reconstruction is very high,it is therefore crucial to reveal the evolved performance of existing foundation piles and existing/new retaining piles during excavation.This dissertation takes the underground-storey supplement of piled building as the research background,and carries out comprehensive research by means of model test theory analysis and numerical simulation.The research results are expected to provide support for the engineering design of underground-storey supplement.(1)Using large-scale physical model test platform,a double-row pile support structure model was established to investigate the influence of row spacing between the two pile layers on the pile displacement,the pile force and the earth pressure acting on the front-row piles.The analyses show that as the row spacing decreases,both the displacement and the bending moment decrease in the existing piles but increase in the supplementary piles,leading the supplementary piles transferred to be the principal bearing elements.Based the analysis and fitting of the active earth pressure data of each test,a prediction method for the active earth pressure on the existing piles is therefore established with considerations of the spacing of pile rows,the soil friction angle,the pile-soil interface friction angle as well as the soil arching effect.(2)Referring to the indoor model test setup of the double-row pile support,the corresponding calculation model was established by using Plaxis finite element software.The effects of the pile length ratio of the two rows and row spacing on the two-stage support force sharing and the failure mode of the retaining structure were studied.The strength reduction method was used to analyze the influence of the above parameter changes on the safety factorof excavation pit.Finally,the optimal row spacing and pile length ratio under specific excavation depth were studied through comprehensive evaluation.It is found that the distance between the two-stage support piles and the length ratio of the two-stage support piles has a great influence on the safety factor of the foundation pit and the force sharing of the two-row structure;When the predetermined depth of digging is reached,the difference between the maximum bending moment of the existing and new two-stage supporting piles,the sum and the safety factor of the foundation pit are comprehensively evaluated through optimization analysis.Found when the row spacing is 4 times the pile diameter and the ratio of the two-layer piles is 3/2,the layout scheme of the new support piles is optimal.(3)Through indoor model experiment,attempt was made to reveal the load-bearing characteristics of foundation piles subjected to excavation-related lateral movement of the passive soil.Focus was paid on the influences on the pile's bending moment and shear force of a few factors such as the spacing between the retaining and foundation piles,the excavation depth,the axial loading level and the elevation of pile cap.With reference to the test,the Plaxis finite element software was used to back-analyze the obstruction and back pressure of the existing piles in the pit to support structure and passive soil.For the pile foundation be within the influence range of the passive zone,as the excavation depth increases,the bending moment and shearing force of the pile foundation increase,the closer the distance to the supporting structure,the greater the bending moment and shearing force of the pile body,and the maximum bending moment of the pile in the pit is located in the shallow soil layer under the excavation face.Compared with the influence of the additional internal force of the soil in the passive zone on the engineering pile,the influence of the engineering pile on the working behavior of the soil and supporting structure in the passive zone is slightly smaller.The parameters such as the pile spacing of the engineering piles in the pit and the horizontal distance of the distance support have a great influence on the back pressure and the blocking effect.