Study On The Stress And Deformation Characteristics Of The Pit In The Pit Retaining Structure Of The Open Cut Tunnel Built By The Public Railway

In recent years,China’s urbanization construction has also got rapid development,and major cities in China have begun to attach importance to the rational and orderly development and utilization of underground space,so China’s foundation pit engineering has also got a rapid development stage.In the actual field engineering,in order to meet some special use functions,the second excavation or even multiple excavation processes are carried out at the bottom of the excavated large foundation pit,and the internal new pit formed is the inner pit or the pit in the pit.The excavation of pit in the pit will cause the change of soil stress path,change the original stress path,and form a new complex stress path,which has a negative impact on the stability and safety of the entire pit.This paper is based on the model experimental study of the public-rail tunnel project in Aixi Lake,Nanchang,and the study of the force-deformation characteristics of the enclosure structure in the pit of the public-rail open-cut tunnel.The finite element model was established by PLAXIS and compared with the test,and the effect of excavation in the pit on the force deformation of the enclosure structure under different working conditions was studied.The specific research contents are as follows:(1)Based on the model similarity theory,a more reasonable model test scheme is designed,and the model test device,test materials and equipment,test items and arrangement,and test steps are introduced in detail.Through the model test,the following conclusions are obtained: during the process of excavation from 10 cm to 40 cm of the outer pit,the maximum ground settlement increased from 0.05 mm to 0.32 mm,with an increase of 540%;In the process of excavation from 10 cm to 40 cm,the maximum ground settlement increased from0.60 mm to 0.65 mm,with an increase of only 8.33%.With the increase of the area ratio,the surface settlement of the pit outside the foundation pit increases gradually.When the area ratio is 0.4 and 0.6,the maximum surface settlement of the pit outside the foundation pit is0.64 mm and 1.07 mm,with an increase of 67.18%.The maximum surface settlement of the pit outside the foundation pit changes significantly.(2)Based on the Aixihu highway and rail tunnel project in Nanchang City,the excavation and enclosure model of pit in pit is established,and the finite element analysis is carried out;The model calculation results are compared with the field monitoring data to verify the feasibility of the model,that is,the rationality of selecting parameters;According to the results of finite element calculation,the horizontal displacement law of the diaphragm wall of the outer pit in each stage of pit excavation is analyzed.(3)Using the PLAXIS finite element software,the finite element model of pit enclosure structure is established,and the influence of different stacking loads and different area ratios on pit enclosure structure is studied through the control variable method.The following conclusions are obtained: the horizontal displacement of the inner and outer enclosure structure increases as the area ratio increases,and there is a critical area ratio.When the area ratio exceeds this,The earth pressure between the inner and outer pit retaining structures changes from infinite earth pressure to finite earth pressure.The positive and negative bending moments of the diaphragm wall in the outer pit are large,and increase with the increase of the area ratio,and the negative bending moment increases more significantly;The cast-in-place pile in the inner pit mainly bears positive bending moment,and the positive bending moment increases significantly with the increase of the area ratio.In the case of unbalanced loading in the outer pit,the lateral displacement of the left diaphragm wall increases with the increase of the left overload P1,and the lateral displacement of the right diaphragm wall decreases with the increase of the left overload P1.When the left overload is30 k Pa and the right overload is 10 k Pa,the right diaphragm wall begins to reverse displacement,that is,horizontal displacement toward the outside of the pit.