Based on the engineering background of No. 2 Xi'an Line metro shield tunneling, with a tunnel of 6 m diameter and 15m depth as the research object. The stress ratio curve of surface horizontal displacement and supporting is obtained by numerical simulation. Limit supporting stress ratio of the excavation face is between 0.1–0.15. Meanwhile, the research studies the plastic zones changing process at the excavation face from stability to destruction. With the decrease of excavation face supporting stress, the front soil displacement of the excavation face is increasing and the plastic zones in the front of the excavation face is expanding from the top to the earth's surface. When the supporting stress approaches the limit supporting stress (supporting stress ratio is 0.15), the diffusion area of plastic zones and the plastic zone expanded to the earth's surface, and the excavation face become unstable. By setting the horizontal and longitudinal monitoring lines, the research studies the changing of surface subsidence under different supporting stress ratio. For the transverse deformation, the smaller the supporting stress ratio is, the more obvious the settling tank is. While the supporting stress ratio is greater than 1.0, the surface uplifts. The greater the supporting stress ratio is, the more obvious the uplift is. For the tunnel longitudinal deformation, while the supporting stress ratio is greater than 1.0, all the ground uplifts, while the supporting stress is less than 1.0, the surface in the front of the tunnel subsides, as well, the surface in the back of the tunnel uplifts. When the supporting stress ratio is less than 0.2, all the ground subsides. While the supporting stress ratio is equal or less than 0.2, it can be clearly observed that about 5 m in the front of the tunnel is the maximum subsidence, by which a settling tank is formed.
With Chinese rapid development of social economy, the urbanization process is increasingly sharpened, the use of various ground resources tends to saturation, and the development and utilization of city underground space resources has become an important direction of social sustainable development. Improving for construction speed and having small influence on the environment, shield construction, as a special machine of tunnel construction, is widely used in the city underground tunnel construction, such as the subway and underground pipeline construction in Shanghai, Beijing, Nanjing, Guangzhou, Shenzhen, Tianjin, Chongqing and so on. But it is also a tough question to make a rational selection of the excavation face support pressure in engineering application. Xi'an Metro Construction applies shield tunnel construction method for the first time in loess area.
