The Reasonable Buried Depth Of Beijing Subway Tunnels By Shallow Tunnelling Method

With the rapid development of economy and the acceleration of urbanization, urban problems, such as traffic jam and slow driving speed, become more and more serious. Urban rail transportation comes out as an effective measure to the problem of urban construction. As we all know, the city subway tunnel construction causes the inevitable disturbance to the ground, stress redistribution and the movement of the ground. The city subway tunnels have many engineering characteristics which include shallow depth, complex construction environment and numerous underground facilities, so the deformation of strata can lead to many different degrees of environmental impact, for example cracking of neighboring structures and buildings and the leaning underground pipeline destruction. Therefore, how to design the depth of city subway reasonable, to maintain the stability of surrounding rocks and to meet the environmental requirements has become an important issue of the city subway construction, which should be solved immediately.This thesis gives the recommended depth of Beijing subway tunnel, basing on the subway construction in Beijing by the methods of investigation of field monitoring data, model test and numerical simulation from two aspects of the structural stability of surrounding rock and environmental requirements. Obtained the following key finds:(1) Investigate field monitoring data:the reasonable control standards of the ground settlement and the related regulation of the max value of surface settlement are given by probability statistic, the definition of reasonable buried depth is given base on this proposed.(2) Laboratory study on tunnel excavation:Progressive failure characteristics of the surrounding rocks of tunnels and different types of collapse under different buried depth are summarized by a large number of laboratory tests which deal with the influences of the buried depth of tunnels on the failure mechanism of tunnels.(3) Using numerical simulation method and building area models to simulate the standard running tunnel cross-section with regard to the ground conditions of Beijing: On the basis of elastic-plastic theory and large amount of computational analysis, the rule of displacement in the ground and the feature of surface subsidence are given. It is concluded that the arch structure only forms in the area about 1.2 times of the equivalent diameter D up of the tunnel and the settlement curve will produce inflection point about 1.6 times D up of the tunnel. (4) The definition of pressure arch is given combining with the computing results. Based on both the formation conditions and the control requirement of surface ground settlement, the proposal value 2.0D～2.5D (D is the equivalent diameter of tunnel) of burying depth for running tunnel and 0.5～0.7 times of thickness span ratio for metro station in Beijing are put forward.