Study On The Influence Of Disturbance Caused By Foundation Excavation On Pile Foundations Of Existing Buildings

During the construction of the foundation pit,disturbances are caused to the soil,which causes great changes in the stress path,stress-strain relationship and strength deformation of the soil.In order to analyze the influence of soil disturbance caused by excavation of the foundation pit on the adjacent existing building pile foundation,this paper Based on the analysis of the disturbed area of excavated soil in the foundation pit and the simulation of the soil disturbance in the indoor vibration test,the consolidated undrained test,the unconsolidated undrained test and the unconfined compressive test were conducted,and the strength and disturbance degree of the soil were studied.The relationship between the conventional shear test and the torsional shear test under different stress and moisture content was analyzed;the shear behavior of pile-soil interface under different conditions of disturbance was analyzed;In order to analyze the strength loss of the soil and the impact of high stacking on the adjacent piles during excavation of foundation pits,this article uses the collapsed Building 7 of the Lotus Garden in Shanghai as an example to pass through the disturbed area of excavation.For the soil strength reduction in different disturbance degree areas,the 3D foundation excavation model was established using ABAQUS finite element analysis software to study axial force,bending moment and displacement change of the foundation pit adjacent to the existing building foundation.Impact,reached the following conclusions:(1)Through the study of the existing disturbed soil and the analysis of the laboratory test of the disturbed soil in this paper,the feasibility of the small-scale shaking table test to simulate the disturbance of the soil in different disturbance areas was determined,and the foundation excavation soil was preliminarily determined.The strength reduction coefficients in the bulk disturbance areas(strong disturbance,middle disturbance,and weak disturbance)are 0.45~0.57,0.62~0.8,and 0.85~0.9,respectively.(2)Under the same normal stress and moisture content,the shear stress-shear displacements of the two shear modes of torsional shear and direct shear are quite different,and the change is more significant with the increase of normal stress.Thetorsional shear-pile-soil interface shear strength is 1.5~2.1 times the shear strength of direct shear.(3)After the disturbance of soil,the original structure of the soil is seriously damaged,and the shear strength at the interface of the pile and soil decreases with the loss of the strength of the soil;the strength loss after the disturbance of the soil and the strength between the pile and soil interface Loss relationship,fitting empirical formulas.The relationship between the strength loss of disturbed soil and the loss of shear strength between piles and soils is established.The loss of soil strength caused by excavation of foundation pits and the loss of frictional strength at the interface of piles and soils are evaluated,which can provide reference for construction.(4)After the reduction of soil strength in the disturbed area of foundation pit excavation,the uplift displacement of the soil at the bottom of the pit increased,and the horizontal displacement of the pile significantly increased.The displacement from the ground to the foundation pit of the building before and after the reduction was 57.2mm and 74.8mm,respectively.The displacement from the top of the building to the side of the foundation pit is 47.7mm and 87.2mm,respectively.The displacement of the pile before reduction is increasing with the distance from the pit,while the displacement of the pile after reduction is linearly decreasing with the distance from the pit.The maximum bending moment of the pile without reduction was 50.152kN·m,and the maximum bending moment of the pile after reduction was 32.952kN·m.The bending moment of the pile was not up to 52kN·m.Without maximizing the axial force of the pile 2721.0 kN,the axial force of the rear pile was reduced by 2267.9 kN,which exceeds the design value of the pile vertical load of 2000 kN.(5)The displacement of the bottom of excavation of the foundation pit under loading is more obvious.The horizontal displacement of the pile before and after the loading is changed in the opposite direction.The displacements of the building ground to the foundation pit before and after the loading are respectively 21.5mm and 74.8mm.The displacement to the excavation side is 58.9mm and 87.2mm respectively.The displacement of the pile top under the condition of no-load is increasing with the distance from the pit,and the displacement of the top of the pile piled up with thedistance from the pit is basically increased.Linearly decreasing,and the pile top displacement is significantly greater than the pile without pile top displacement.The pile load significantly increases the bending moment of the pile,the maximum moment of no-load is 20.4kN·m,and the maximum value of the load moment under load is33.0kN·m.The maximum axial force of the pile without piles was 1770.6 kN.Due to the high pile load,the axial force of the pile significantly increased.The maximum pile axial force was 2267.9 kN,which exceeded the design value of the pile bearing capacity by 2000 kN,resulting in the final pile being piled.Press off.