| In recent years,with the continuous development of economy and the acceleration of urban construction,land resources are becoming increasingly tense.In order to make full use of the land,high-rise buildings are springing up in our country,which promotes the development and utilization of underground space.At present,the underground space of various uses has been widely developed all over the world.The development and construction of these underground spaces has produced a large number of deep foundation pit projects,and its scale and depth continue to increase,which also brings great challenges to engineering design and construction.Because of the difference of region,site and surrounding environment,the selection of foundation pit support type is also different.At present,the internal support system is widely used in deep foundation pit engineering.There are many kinds of layout ways of horizontal internal support structure in the internal support system.Different layout ways of horizontal internal support structure can be selected according to different surrounding environment and foundation pit site.Different layout ways of horizontal internal support structure bring different support effects.The plane shape of the foundation pit is different,and there will be many corners,and the size of the corner will have an impact on the stress and deformation of the support structure,and it will cause damage to the foundation pit if it is serious.Therefore,what kind of horizontal internal support structure layout is more conducive to the structural stress and deformation and foundation pit stability needs to be studied.At the same time,the foundation pit will encounter various adverse factors in the construction process,such as the impact of horizontal internal support components and columns,the partial pressure and asymmetric excavation may cause the horizontal displacement of the top of one side of the support pile to move to the outside of the foundation pit,which will affect the overall stability of the foundation pit.Therefore,it is of great significance to analyze the layout of the horizontal internal support structure,the angle change of the plane corner of the foundation pit and the influence of the unfavorable factors on the stability of the foundation pit.Based on the research background of a deep foundation pit supporting project in Nanchang,this paper uses the finite element software MIDAS/GTS to analyze the influence of the horizontal internal supporting structure layout,the angle change of the plane corner of the foundation pit,the uncontrollable factors in the construction,as well as the three-dimensional simulation analysis of a deep foundation pit project in Nanchang,and compares the results with the monitoring data.The main work and research results are as follows:(1)Through the analysis of the internal force and deformation influence of the three layout ways of the support structure,i.e.triangle layout,zigzag layout and no connecting rod layout,it is found that the surface settlement,pit bottom uplift,horizontal displacement of the support pile and internal force of the support structure calculated by the two layout ways of triangle layout and zigzag layout have little change,However,the horizontal displacement and the internal force of the supporting structure are obviously increased when there is no connecting rod between the supports.(2)Based on the analysis of the stress and deformation of the foundation pit support under the four conditions of the plane angle of the foundation pit:110°,120°,130°and 140°,it is found that the ground settlement around the foundation pit,the uplift of the pit bottom,the horizontal displacement of the support pile and the internal force of the support structure increase with the increase of the plane angle of the foundation pit.The plane angle of the foundation pit increases from 110°to 140°under the three kinds of horizontal internal supporting structures,i.e.triangular arrangement,zigzag arrangement and non connecting rod arrangement.The maximum shear force of the support increases by 70.5%,72.9%and 74.9%respectively.And under the three kinds of horizontal internal structure arrangement,the maximum shear force of the support is at the joint of the support beam and the crown beam which is farthest from the foundation pit angle.When the plane angle of the foundation pit is too large,the connection position between the support beam and the crown beam is prone to shear failure,which indicates that the connection between the angle support and the crown beam or the waist beam is very important,and if the design is improper,it is easy to shear failure.(3)Through the simulation analysis of asymmetric displacement of foundation pit,fracture of support beam and fracture of column,the results show that the moment of pile body increases with the displacement from one side of foundation pit to the outside of the pit,and the axial force of support beam increases with the displacement from 0 mm to 15 mm.The maximum axial force of support beam increases by about 22.5%,which indicates that when the asymmetric displacement of foundation pit occurs It has influence on the safety of supporting structure.The location of the fracture of the support beam and the column is different,which has different influence on the overall stability of the foundation pit.(4)Through the simulation analysis of the whole process of the excavation of the basement foundation pit of the headquarters building of China Construction Group Co.,Ltd.,it is found that the pit bottom heave reaches the maximum in the center of the foundation pit,and presents the deformation law of "large in the middle,small on both sides".With the increase of the distance from the edge of the foundation pit,the surface settlement increases,reaches a maximum value,then gradually decreases and finally tends to a stable state.The change trend of horizontal displacement of supporting pile body presents a "bow shape",and the maximum value of horizontal displacement of supporting pile is 7.5mm,which meets the specification requirements.By changing the parameters of the supporting structure,it is found that the bending moment and displacement of the supporting pile body decrease with the increase of the width of the internal support section,but the axial force and bending moment of the support increase,and the excessive axial force and bending moment of the support may damage the support.With the increase of the diameter of the supporting pile,the displacement of the supporting pile is gradually less,and the bending moment of the supporting pile increases with the increase of the diameter of the supporting pile,which is unfavorable to the strength of the supporting pile.In engineering design,the diameter of row piles should be selected reasonably according to the actual situation,which can not only make the deformation of supporting structure more reasonable and safer,but also reduce the engineering cost.When the pile diameter is fixed,the displacement of the supporting pile increases with the increase of the distance between the supporting piles,With the increase of the distance between the supporting piles,the bending moment of the supporting piles increases,and the excessive distance between the supporting piles has an impact on the stability of the supporting structure.(5)Through the comparison of monitoring data and numerical simulation results,the results show that the deformation laws of foundation pit and supporting structure are basically the same,but the numerical simulation results are different from the actual monitoring values,which is caused by the variability of.construction conditions.Through the combination of foundation pit monitoring and numerical simulation,it can provide effective help for our actual project construction. |
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