Field Test On Deformation Characteristics Of A Curved Foundation Pit Excavation In Nanjing Metro

With the development and utilization of urban underground space,the foundation pit engineering is larger,deeper and has more complex shape,which also puts forward new requirements for the theoretical system and design method of foundation pit engineering.At present,the research on curved foundation pit is not enough.In the design and construction of foundation pit engineering,the influence of the shape feature is often neglected,which may easily cause waste of construction materials or pose a safety hazard.Based on the curved foundation pit of the entrance and exit line of north extension Nanjing Metro Line 1,uses theoretical analysis,field monitoring and numerical simulation to study the excavation deformation of curved foundation pit.The main research contents and results are as following:(1)The field measurement of curved foundation pit and the statistical analysis is carried out.Field measured data show that the ground settlement around the foundation pit is in the shape of a “spoon”,and the maximum ground settlement is about 0.19% H.The deformation of the envelope structure is in the shape of a “parabola” with two large ends in the middle,and the maximum horizontal displacement of the envelope structure is about 0.22% H.Besides that,the deformation on the positive side of the foundation pit is greater than the deformation on the negative side of the foundation pit.This is because the retaining structure on the negative side of the foundation pit protrudes into the soil,which is beneficial to control the deformation of foundation pit,and the situation on the external and internal sides is opposite.After regression fitting of field measured data,the results shows that the surface settlement and the distance from the measurement point to the side wall of the foundation pit showed a cubic relationship.The horizontal displacement of the deep foundation pit retaining structure on the left side of the mountain is in a cubic relationship with the depth,and the horizontal displacement of the deep foundation pit on the right side of the mountain is in a quartic relationship.(2)A three-dimensional numerical analysis model is established.The calculated results are in good agreement with the measured results,which verifies the rationality of the model.On this basis,the effects of curve radius,support spacing,steel support diameter(steel support wall thickness 16mm),depth of supporting pile embedding,and full weathered rock thickness of the foundation pit on the deformation of the foundation pit are analyzed.The results show that as the radius of the curve increases,the deformation on the male side decreases gradually,while the deformation on the female side increases.As the support distance increases,the deformation of the foundation pit increases,and the diameter of the steel support also has a greater effect on the deformation of the foundation pit.As the depth of the supporting pile increases,the deformation of the foundation pit gradually decreases.As the embedded depth of the supporting pile increases,the deformation of the foundation pit gradually decreases;as the thickness of the fully weathered rock decreases,the foundation pit deformation gradually increases,but the increase is small.(3)Sensitivity analysis of the influencing factors controlling the deformation of the foundation pit is carried out,it is concluded that the sensitivity relationship of the factors affecting the deformation of the foundation pit in this project is: lateral distance of support>embedded depth of supporting pile> steel support diameter> curve radius> longitudinal distance of support> full weathered rock thickness.(4)According to the characteristics of the project and the sensitivity analysis results of influencing factors of foundation pit deformation,the optimization scheme of enclosure structure is put forward and the numerical simulation is carried out.The results show that the ground settlement and the horizontal displacement of the retaining structure of the foundation pit are reduced,and the differential deformation of the left side and right side of the mountain and the external and internal sides of the foundation pit are also reduced.The optimization scheme is conducive to improving the safety of the foundation pit project.