Numerical Simulation Analysis And On-site Construction Monitoring Of Inner Ring Supporting Structure Of Foundation Pit

Since the reform and opening up,the emergence of a large number of high-rise buildings and the expansion of underground resources and environment,cause foundation pit engineering to face many foreseeable and unpredictable risk challenges.It is more urgent to ensure the smooth operation of foundation pit and the stability of surrounding environment.Annular internal bracing is a new type of deep and large foundation pit,which conforms to the conditions of the times and the development trend of foundation pit.In this paper,the properties of stress and deformation of annular inner bracing structure under different layout schemes are analyzed by using FLAC 3D software,and then a practical deep foundation pit project with annular inner bracing in Taiyuan city is taken as the research object.By means of real-time monitoring data and numerical simulation,the stress and displacement of the bracing structure in the ring of foundation pit are discussed.Finally,the displacement of supporting structure of foundation pit under earthquake action is discussed.The main results obtained are as follows:(1)the internal force ofannular braces and the vibration of soil around the foundation pit are obtained as follows:(1)From the view of the layout of the annular inner bracing plan,the axial force is always the first internal force of the annular bracing,followed by bending moment and shearing force,regardless of whether the annular bracing is tangent to the capping beam or the circumferential capping beam,and the annular bracing is tangent to the girdle capping beam,The inner force and displacement of the whole ring bracing are sensitive to the centrifugal rate,especially the axial force and bending moment of the ring inner bracing,so it is suggested that the centrifuge rate should be controlled below 0.7 in practical engineering;for rectangular foundation pit,It can be considered that the ring bracing can protrude the foundation pit in the direction of the short side of the foundation pit,which is advantageous to the uniform distribution of the internal force and the reduction of the displacement.The maximum value of shear force or bending moment is usually at the point,and the change of the cross-section dimension of the support rod is closely related to the moment and axial force,and the fluctuation of shear force is relatively mild,so the concept of strong ring and weak bracing should be adopted.Strive to improve the geometric structural stiffness to improve the displacement state;(2)During the excavation of foundation pit and earth,the distribution of the internal force and deformation of the bracing inside the ring is continuously changing,and the position of the axial force extreme at different times is alsovery different,and the internal force of the bracing ring beam is greater than that of the supporting beam and crown beam,and the internal force of the supporting ring beam is larger than that of the supporting beam and crown beam.The maximum axial force appears in the southwest and northeast direction of the bracing ring beam,excavation of foundation pit will cause springback of supporting column,the maximum rebound amount obtained from monitoring point is 0.14 depth of foundation pit excavation,and the maximum amount of settlement obtained from surface settlement monitoring point of pit is 0.15 depth of foundation pit excavation.The settlement of the outside surface of the pit is related to the change of groundwater level outside the pit,the excavation of foundation pit will affect the surrounding buildings of the pit,and the west side of the pit will be affected.The four buildings are inclined to one side of the foundation pit,and the average settlement of the building near the center of the foundation pit is larger than that near the edge of the foundation pit.(3)The overall horizontal displacement of the supporting pile is arched,and the position of the maximum displacement is slightly higher than that of the excavated surface,and the surface subsidence of the supporting pile is trench,and the maximum of the settlement is generally between 1 / 3 ~ 2 / 3 of the excavation depth behind the pile.The maximum settlement of the soil outside the pit is slightly larger than the maximum horizontal displacement of the wall,and with the increase of the peak acceleration of the seismic wave in the input bedrock,the displacement of the pile body increases,and the relativedisplacement difference between the top and the bottom of the pile will increase.(4)The displacement time history curve of each monitoring point on the supporting pile is similar in shape and has obvious phase time domain characteristic,and with the increase of the peak acceleration of the input seismic wave,the absolute value of the horizontal displacement of each monitoring point also increases.The increase of peak acceleration can not significantly increase the internal force of annular bracing,and the bracing structure of foundation pit will have a certain constraint on the free vibration of surrounding soil.