Application Of Small Shaft Construction Method In Foundation Pit Excavation Above Metro Tunnel

After more than 30 years of large scale urban construction,the land resources in economically developed urban areas are in short supply.Therefore,the urban underground space resources need to be effectively utilized.At present,a large number of underground garages,underground subway stations and tunnels,underground shopping malls,basement and other underground projects in urban construction has become widespread.Deep foundation pit project in the construction of these underground structures will inevitably occur in large numbers,thus bringing many deep foundation pit engineering problems.There are more construction projects for earth excavation at the top of the tunnel.Now,the theory of excavation and unloading is still at an immature stage.It is necessary to further investigate the deformation of the tunnel due to unloading of the foundation pit so as to provide a reliable scientific basis and guidance for the design and construction of the foundation pit of the relevant project.In this paper,through theoretical analysis and numerical simulation,this paper systematically studies the small shaft method of floating upward in the controlled tunnel in Qianhai,Shenzhen.The main contents include:1.This paper introduces in detail a method of excavation of the foundation pit above the tunnel-small shaft method,this method can effectively control the existing horizontal tunnel under the float,is now a wide range of engineering practice in Shenzhen has been repeatedly verified.After that,the process technology is maturing.This paper introduces and analyzes in detail the construction sequence of the small shaft method,and explains the reason why it can control the floating of the tunnel from the mechanics mechanism.2.In this paper,the municipal project of Shuangjiehe Road in Shenzhen Qianhai is taken as an example.At first,the project adopts the sub-block excavation scheme and part of the earthwork unloading,which causes the longitudinal deformation of the existing under-lying tunnel to exceed the maximum allowable value.After the use of small shaft method for earthexcavation.In this paper,the finite element software was used to compare the original excavation of the sub-block and the present excavation scheme of the small shaft method to observe their effect on controlling the floating of the tunnel.Through the actual monitoring data and concrete analysis,it shows that the small shaft method has a great advantage in controlling the floating of the tunnel.3.In this paper,the numerical simulation method is used to study the influence of uplift piles,excavation sequence,excavation step depth,ground reinforcement,and small shaft parameters on the control of tunnel floatation in the small shaft method.The Hardening Soil constitutive model was used in numerical simulation to establish a three-dimensional finite element model for the interaction of small shafts,tunnels and soils.This article systematically studies the small shaft construction method and draws constructive opinions,providing theoretical basis and guidance for the actual construction design of the small shaft construction method.