| The composite foundation which bears the vertical load by the reinforcement body and the foundation soil together and obtains the high load bearing capacity and the good economy,is a common foundation form in many high-rise buildings.The foundation pit excavation of a new neighbor project will inevitably cause deformation and internal force changes in existing composite foundation while the composite foundation exerts the vertical bearing capacity.The existing theory of composite foundation and foundation pit support is lack of effective guidance for ensuring the safety of composite foundation and supporting structure design.In this paper,centrifugal model test,theoretical analysis and numerical simulation are used to study the formation mechanism of composite foundation and the mechanical properties of supporting excavation,and the theory of side pressure of composite foundation is constructed.The findings are summarized as follows:1.Two centrifuge model tests with the same additional load and different replacement rate have been carried to trace back the formation mechanism of composite foundation.The vertical load transfer law of composite foundation is specified:There is negative friction in the upper part of the pile,and the settlement increases linearly with the load.As the replacement rate increases,the relative settlement of pile and soil slows down,the position with zero friction moves up,the pile force,the side friction resistance,and the settlement all decrease.2.Four centrifuge model tests with the different additional loads and different replacement rates have been carried to study on the joint behavior of composite foundation with lateral excavation and supporting structure excavation:(1)The internal forces of composite foundation and supporting structure:Excavation causes soil unloading and pile axial force increases due to load transfer.The distance from the foundation pit and the top constraint determine the form and size of the pile moment.Due to the uneven settlement of pile and soil near the foundation pit,the upper part of the pile is pushed into the cushion layer and is subjected to horizontal restraint,resulting in the negative bending moment on the upper part of the pile body.The bending moment of supporting structure is simply supported,and its maximum value is about 3m below each excavation depth.(2)The displacement field of composite foundation:The surface settlement is exponentially distributed along the distance from the foundation pit.The foundation settlement and horizontal deformation decrease linearly along the depth of excavation.The displacement vector of soil particles is 45 degrees along the foundation pit and gradually decreases along the diagonal line.Compared with sand foundation,the lateral influence area in the composite foundation,the horizontal displacement of foundation and the bending moment of supporting structure are reduced by 30-40%.All mechanical properties decreased due to the increase of replacement rate and increased as load increased,which indicates that the increase of replacement rate can effectively improve the bearing behavior of composite foundation and support structure.The increase of load is beneficial to the full play of the action of the pile.3.Combined with centrifuge tests and finite element analysis,the lateral pressure variation law of composite foundation is studied:The shielding effect increases linearly with the overlying load,and decreases with the decrease of replacement rate.The maximum influence depth of composite foundation is 2m below the pile bottom.Clay soil is more conducive to the shielding effect of composite foundation than sandy soil.Considering the combined influence of overburden load,replacement rate and pile length on lateral pressure of supporting structure,a simplified formula for calculating lateral pressure along the depth of composite foundation is obtained. |
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