Research On Influence Law On Composite Foundation Supported By Excavation Of The Near Foundation Pit

The theory of composite foundation is becoming more and more mature,and it is widely used in multi-tall buildings.As urban development and renovation efforts have increased,a large number of new projects have emerged adjacent to existing composite foundation buildings.The excavation of the newly built foundation pit in the neighborhood will inevitably cause the deformation and internal force changes of the existing composite foundation,which will affect the safety of the existing building and the newly constructed foundation pit.The existing composite foundation and foundation pit support theory is not clear enough to guide the safety of the composite foundation and design the foundation pit support structure.The thesis uses a combination of centrifugal model test,numerical simulation and theoretical analysis to study the effect of overlying load on composite foundation and supporting structure,and discusses the failure modes and safety deformation control standards under different overlying load.The following progress has been made:1.The centrifugal test of composite foundation with three sets of overlying loads was conducted to study the effect of different overlying loads on the rigid pile and soil of the composite foundation during excavation:when the additional load over the composite foundation increases,the pile The additional stress of the inter-soil body is more likely to be disturbed by excavation,the load is transferred to the rigid pile,and the axial force of the pile is increased,which is accompanied by a greater bending moment of the pile body;this makes the possibility of damage to the rigid pile affected by the excavation increased Big2.Based on the centrifuge test,a numerical model was established through the ABAQUS finite element software to study the law of overlying load diffusion and the effect on the deformation of the supporting structure.(1)Within the composite foundation,the additional vertical stress of the soil is distributed in an S shape with depth;the transmission of vertical stress is mainly concentrated in two areas;one is the shallow soil layer area,and the load is transferred to the pile through the cushion The soil spreads to the surroundings;the other is to spread the stress to the lower soil layer through the lateral resistance and end resistance of the pile to spread out.(2)When the overlying load is low,the deformation mode of the supporting structure is inward convex shape;when the load is large and the excavation is shallow,a deformation mode similar to that of a cantilever appears;after further excavation,the deformation of the supporting structure The mode is compound.3.Combining the centrifugal test and ABAQUS finite element analysis,the concrete damage structure model is introduced to study the damage mechanism of the composite foundation under different deformation modes of the supporting structure caused by the size of the overburden and the excavation depth.As the excavation near the foundation pit progresses,the rigid pile close to the foundation pit first appears damaged,bends and breaks,and then expands farther away.The soil between the piles is likely to slip after the pile body is damaged,and the whole is unstable.damage.When the cantilever of the supporting structure changes,the soil between the piles slides horizontally,and the damage to the pile is relatively light;when the supporting structure is convex or compound,the pile damage near the foundation pit is serious,and the pile body breaks,and the soil between the piles is The arc surface is unstable.4.Through the principle of minimum potential energy,determine the relationship between the rigid pile of the composite foundation and the deformation of the adjacent foundation pit,use the crack control of the rigid pile as the safe deformation limit of the composite foundation,and launch the foundation based on the safe deformation of the composite foundation during the excavation of the foundation pit Pit deformation control standard.The greater the overlying load,the stricter the safety deformation limit of rigid piles,and the smaller the allowable value of foundation pit deformation.In addition,the allowable value of the deformation of the foundation pit is also related to the depth of the excavation.With the progress of the excavation,the gap between the safety deformation control values of the adjacent foundation pit under different overlying loads is increasing.