Research On Supporting Performance Of A Deep Foundation Pit In Nanchang Area

Generally in the busy urban areas,city foundation pit excavation surrounding environment being more complex,with critical structures,underground facilities and pipelines surrounding it,has given more and more serious requirements for the deformation control,which means the demands for city excavation have changed from strength control to deformation control.For the excavation of the foundation pit in Nanchang area,there is little research on deformation or sufficient understanding deformation law,therefore to study the deformation law of foundation pit excavation in Nanchang area means great theoretical and practical significance.To focus on the issues of deformation caused by foundation pit escavation in the district of Nanchang,these works and achievement are as followed:(1)The analysis of the supporting forms and the deformation of the retaining structure of the 23 foundation pits in Nanchang city has been carried out,which is for the first time,leading to a fact that the deformation form of Nanchang area is mainly protruding abdomen form.And the ratio of the maximum horizontal displacement of retaining structures and excavation depth is between 0.04% ~ 0.2%,with a mean of 0.12,which is less than the general statistics of hard clay area.(2)Using finite element software MIDAS/GTS,a Nanchang subway station 2d plane model is established.The deformation and affecting factors of the foundation pit in Nanchang area were explored by changing the parameters of the soil layer and the supporting body.It was found that when the pile diameter of the surrounding pile is less than 1.0m,the deformation of the pile body and its surrounding surface can be well controlled by increasing the pile diameter.After the pile diameter exceeds 1.2m,the deformation is not controlled by increasing the pile diameter,and the pile diameter is not good.When the thickness is less than 0.8m,the pile deformation curve will show an anti-bending point at the first support installation.The pile embedded in the weathered mudstone 2.5~3.0m can guarantee that the pile end will not appear "kick leg" phenomenon.Under the premise of satisfying the structural requirements,the installation position should be as close to the gravel layer as possible to control the deformation.(3)Through the monitoring data of a deep excavation in Nanchang,it was found that the horizontal displacement and the displacement of the soil outside the pit in thesilty clay area of the site are relatively larger.In the design of the support structure,if the thickness of such soil is large,it is recommended to make a difference design.The horizontal displacement of the crown beam of the foundation pit is less deformed at the corners and larger at the middle of the long edge,showing a clear spatial effect.(4)A 3 d model based on foundation pit calculation results showed that,the ground settlement outside the curve shows "spoon" type.The main influence area is within 0.7(H is the excavation depth)H from the pit wall,the secondary impact area is within the range of 0.7H to 1.8H,the maximum settlement outside the pit at 4~5m.