Study Of Land Subsidence Induced By Construction Of Urban Metro Tunnels For Mutil-Tunnel System

In underground engineering of city, the constructions of the tunnel will inevitably produce the land subsidence,the building asymmetry decline and other negative influence to the enviroment,this kind of phenomenon is especially outstanding while constructing mutative and large transect multi-tunnel system of urban metro. Because buildings and various underground pipelines of the circumjacent metro are numerous and there usually has strict and restrictive request of land subsidence for the tunnel construction,it is seems to be very important to make more accurate forecast for the land subsidence and adopt corresponding measures to control the land subsidence. In this paper there is a research topic:the ground subsidence due to constructing the mutative and large transect multi-tunnels between the Hepingxiqiao station ~ Beituchengdonglu station section of No.5 Beijing metro,and by means of the large and current ANSYS software carrying out the finite element numerical simulation,giving emphasis to study the law of the ground subsidence while constructing the multi-tunnels,and the scheme optimization of the multi-tunnels construction,and the influence factors of the ground subsidence,and so on.The reason that the ground produce settlement is that the stratum takes place loss. When anticipating the ground settlement caused by tunnels construction,it is crucial to reasonably simulate advance timbering process of the tunnels excavation and the mechanical effect of diversified timbering measures. This paper establishes the target function of the land subsidence,combined with the theory of the dynamic programming and by means of the FEM numerical simulation computation,and gains the typical land subsidence graph of single tunnel and the land subsidence distributing plot of two tunnels. For two parallel tunnels,the construction of one tunnel will affect the land subsidence of the other tunnel at different degree.In the process of the tunnel excavation,the epaulement and the podium of the tunnel bear biggish stress,and there will obviously appear the phenomenon that the stress concentrate in the corner of the tunnel. In the process of the construction it is must reinforce the surveillance and measure to these parts which are disadvantageous to the structure stabilization of the tunnel. Under the condition of necessity,it is must increase the rigidity and intensity of the reinforced region and enhance and improve the designed parameters of the tunnel structure properly.The pivotal technique for the metamorphic-section tunnel construction is the transition between different mutative and large transect tunnels,and in the process of construction it is proper to dig the tunnels from the small cross section expanding to the big cross section with a method of trumpet. In order to guarantee that the transition from the small cross section to the big cross section is safe,it should adopt the reliable construction method which run before to protect the tunnel's crest slow and steady and consolidate the tunnel's wreath step by step.The double-arch tunnel is the transition from the large-section tunnel of turnoff to the main tunnel and link tunnel.In the progress of construction,it can change three-hole construction method to two-hole. This paper carries on the calculation and comparison to these two kinds of constructions method from the view of construction progress,tunnel safety and so on,and finally confirm that the construction method of the double-arch tunnel is two-hole. For the switch between the double-arch tunnel and the standard-section tunnel,based on the calculated results from 3-D numerical simulation, analysing and comparing the land subsidence and the tunnel structure's stress of the two construction methods,and finally confirm that the switch is from the double-arch tunnel to the standard-section tunnel.Setting the inverted arch has important function to the whole stability of the tunnel structure,and it not only enhanced the safety of the tunnel construction,but also extend the wear of the tunnel's use over a long period of time. In the progress of the tunnel's construction,it must construct the inverted arch in time,and close the shoring wreath as early as possible.The main factors which influence the rock stability contain geology circumstance, dimension of tunnel section,tunnel span-ratio ,depth of the tunnel and so on.