Safety Assessment of Shield Tunnel in Loess Stratum Crossing Lake in Long Distance

In order to evaluate the construction safety of shield tunnel passing through lake, relying on the section project from Qujiangchi West Road to Qujiangchi Hanyao of Xi ’an Metro Line 8, the tunnel excavation model of shield tunnel passing through Qujiangchi in loess stratum was established by using multi-physical field coupling software, and four typical working conditions were analyzed from three aspects: the stability of shield excavation surface, the ground settlement of shield construction and the structural safety of shield segments. The results are as follows. With the shield tunnel excavation, the plastic zone of surrounding rock gradually increases and presents a “horizontal ellipse” shape, and in four typical working conditions, there is no plastic zone that runs through the shield excavation face and the ground surface, and runs through the left line and the right line surrounding rock, that is, the excavation face is relatively stable during shield construction. The maximum settlement of the construction surface under four typical excavation conditions is 20.42 mm, which appears on the surface just above the tunnel line. The maximum principal stress and maximum shear stress of the segment structure are relatively small, both of which occur at the arch waist of the first half of the tunnel, so the shield segment is relatively safe. The research results have important engineering significance for ensuring the safety of shield tunneling through rivers and lakes, and can also provide reference for the construction and operation of similar projects.