Application Of Different Support Forms In Segmentation Of Long And Narrow Deep Excavation

The construction of urban rail transit is rapid.Restricted by the narrow urban space,long and narrow subway pits are excavated in cities with heavy traffic and dense buildings.Reverse and semi-reverse methods are often used to ensure the rapid recovery of traffic in the excavation area.Few theoretical studies have been taken on deformation prediction of deep excavation with reverse construction method,and most of them consider 2D deformation of supporting structure of deep excavation with internal support.There is little coordination of deformation characteristics and longitudinal deformation of deep excavation with multiple support.In order to study the application of different supporting forms in the long and narrow deep excavation,this paper relies on the deep excavation engineering of Wuxi Metro to carry out the research.The main contents and conclusions are as follows:(1)The comprehensive stiffness is quoted and revised to quantitatively analyze the flexural capacity of the subsection support structure;referring to the latest MSD(Mobilized Strength Design)method for two-dimensional plane prediction of deep excavation in soft soil,the concept of spatial effect in three-dimensional analysis of deep excavation is introduced,and the deformation coordination law of subsection support structure in three-dimensional situation is put forward.The comprehensive stiffness of the supporting structure in the reverse section of the relying project is much greater than that of the diaphragm wall section and the cast-in-place pile section.Based on the deformation prediction of deep excavation by MSD method and the deformation coordination results of different support modes,the closer to the corner position of deep excavation,the higher the flexural safety factor of deep excavation support.(2)Statistical field monitoring data,using the same section data mirror layout method,showing the deformation trend of deep excavation support structure.For the long and narrow deep excavation with bracing,the action point is on both sides of the section.The actual deformation is not completely symmetrical.Because of the isolation of excavation space,the asymmetry of the deformation on both sides of the deep excavation is transmitted and coordinated through the bracing.The deformation curve of diaphragm wall of reverse foundation pit shows that the horizontal displacement above the first steel support develops slowly,and the following deformation characteristics increase rapidly.The horizontaldisplacement of cast-in-place pile mainly occurs in the upper part of the enclosure structure,and that of diaphragm wall tends to be larger in the middle and smaller at both ends.The occlusive cast-in-place pile has the characteristics of the bottom of the retaining structure moving into the pit with the deformation.(3)The simplified numerical model of long and narrow subway deep excavation is established,and the modified Cambridge constitutive model is introduced.The deformation mode of diaphragm wall and the deformation coordination of diaphragm wall at the junction of open excavation section and reverse excavation section are emphatically analyzed.The results show that the influence range of spatial effects of homogeneous model is 10m-15 m,and that of theoretical calculation is about 8m.The calculation theory is conservative.