Abstract: To improve the disaster prevention and mitigation of metro shield tunnel, model test of similar material and numerical calculation is studied in this paper to simulate the construction of building adjacent to foundation pit in Beijing. Under unloading-loading conditions of the foundation pit above, the mechanical characteristics of shield tunnel are studied, and the influence of spacing between the bottom of the foundation pit and the shield tunnel top and the loading strength of foundation pit are also analyzed. As shown by calculation, in unloading stage, the vertical displacement of shield tunnel gradually increases. When the foundation pit is excavated to the bottom, the vertical displacement reaches the maximum value. Under loading condition, the vertical displacement of shield tunnel can be restored. The greatest impact on shield tunnel always occurs at the end of the foundation excavation. At this point, the work of foundation slab construction should be completed as soon as possible. In the unloading-loading process of foundation pit , earth pressure of shield tunnel presents calabash type symmetrical distribution. At the top and bottom of the shield tunnel the earth pressure is larger than the soil pressure in the middle of the shield tunnel. In unloading stage, the earth pressure of shield tunnel vertical axis was significantly reduced. In loading stage, the earth pressure of shield tunnel can be restored. With the spacing between the bottom of foundation pit and shield tunnel top increases, the displacement of shield tunnel, the displacement difference between arch bottom and arch and the horizontal convergence are incrementally reduced. When the spacing greater than 3 times of the depth of deep excavation, the additional deformation of shield tunnel caused by foundation pit unloading-loading effect is slightly. When the loading strength of foundation pit 2 times than the unloading strength, the vertical displacement of shield tunnel can be restored to its original state.

Key words: shield tunnel, soil unloading, the foundation loading, model test of similar material, earth pressure