Study On Stress Mechanism And Optimal Design Of Internal Support Axial Force For Metro Deep Foundation Pit In Loess Area

With the rapid development of urban underground space,a large number of deep foundation pits were founded.Because of the complexity and uncertainty of the foundation pit engineering,there is a large gap between the mechanical calculation model and the actual situation on the design of the foundation pit,so the internal force of the enclosure is difficult to match the measured value.Campshed & inner support is a very common supporting form and its deformation and stress is more complex,so it is necessary to study the stress and deformation characteristics of the campshed & inner supporting structure and analyzing its influence factors and optimizing the design of support structure.This paper takes Xi ’an subway no.3 Qinglong Temple interchange station as the research object,aiming at the campshed + inner supporting form and combining the indoor model test with the numerical modeling to simulate the excavation of the foundation pit.In the process of foundation pit excavation,the lateral earth pressure,internal force of pile and the inner-support axle-force were monitored.The soil pressure distribution is obtained through the inversion of the pile bending moment,and based on inversion results,the axial force of support was adjusted,and then the internal force of pile was optimized.There are the main conclusions:(1)The strut axial forces’ growth was stable,after the support was set up and with the next operation condition the strut axial forces will increase dramatically and when the next axial force of support was set up the last axial force of support was minished.In the process of excavation and erection,the axial force of support tends to be stable.(2)In the process of foundation pit excavation,with the increase of the depth of foundation pit,the pile shaft bending moment increases gradually,and the maximum bending moment is gradually move down.The function of the inner supporting pre axial force can effectively reduce the internal force of the supporting pile and big differences on the pile shaft bending moment and these differences takes place in the every support moment,the inner supporting pre axial force has significant adjustment on the pile shaft bending moment.(3)According to the inversion of the bending moment of pile soil pressure of pile and the measured results are basically consistent which lives up to zero distribution assumption.The results shows that the earth pressure is closer to the actual situation when the distribution is zero.(4)During the process of the foundation pit excavation,the existence of the inner support assure the safety of the foundation pit and regulate the supporting structure of the internal force effectively.In the process of foundation pit excavation,by adjusting the axial force of support,turning passive support form into active support form the inner support,which makes the internal force distribution of the pile is more reasonable and optimized the pile shaft bending moment.