Abstract: To ensure the construction safety and determine the safety redundancy of the envelope structure, a threedimensional refined numerical model based on the modified Cambridge model is established based on a metro station foundation pit engineering constructed by covering topdown method in Beijing, China. On the basis of verifying the rationality of the numerical model by the data measured in the field, the thickness of the underground diaphragm wall and the floor slab and the diameter of the column are changed. The influence of the change of the parameters of the envelope structure on the deformation of the foundation pit and the change of the redundancy against continuous damage are analyzed. The results show the following: (1) The coupling effect of the space effect and the pit angle effect of the foundation pit leads to the distribution rule of the maximum horizontal displacement value of the underground diaphragm wall: the horizontal displacement value of the middle underground diaphragm wall of the long side of the foundation pit is the largest, the horizontal displacement value of other positions is relatively small, and the horizontal displacement value of the underground diaphragm wall of the short side of the foundation pit is the smallest. (2) As the thickness of the underground diaphragm wall and floor becomes smaller, the horizontal displacement value of the deep layer of the underground diaphragm wall gradually increases, and the change of the maximum horizontal displacement value of the underground diaphragm wall increases exponentially; the surface subsidence value increases gradually, surface settlement mainly occurs in the range of 2 times the depth of the foundation pit, and the surface settlement in the range of 2~4 times the depth of the foundation pit is no longer significant. (3) The stiffness of the floor in the covering topdown method is greater than that of the inner support in the opencut method, and the resistance to deformation is stronger, but the horizontal deformation of the envelope structure is still affected by the geological conditions and structural materials to a certain extent. (4) When the thickness of the underground diaphragm wall and the floor reaches 1.0 m, the redundancy of the vertical envelope structure tends to be stable, and the continuous increase of the thickness of the underground diaphragm wall and the floor will not increase the ability of the structure to resist continuous damage.

Key words: deep foundation pit, redundancy, covering topdown method, numerical simulation, excavation unloading