Based on a large deep foundation pit project,the deformation characteristics of the pit support enclosure system,force characteristics,and changing trend of surrounding ground surface settlement during the excavation process of the narrow and long deep foundation pit with steel braces were studied through automated monitoring and numerical simulation.Midas GTS was used to establish the pit model,and the effects of the horizontal spacing of the steel brace,the distance of the third steel brace from the bottom of the pit,the preloaded axial force of the steel brace,and the change of the pile diameter of the pit enclosure on the deformation and force of the retaining pile structure and the surrounding ground surface settlement were investigated for the sections with and without steel brace,respectively.The results show that:① The horizontal spacing of different steel braces has different control effects on the deformation of the retaining structure and the surrounding ground surface settlement;smaller horizontal spacing of steel braces indicates smaller values of pile displacement,pile bending moment,and ground surface settlement of the foundation pit;② the position of the third steel brace has little effect on the stability of the foundation pit;③ the deformation of the retaining pile in the foundation pit and the surroundi ng ground surface settlement can be effectively controlled by applying the preloaded force of steel braces;④ with the increase in the diameter of the retaining pile,the deformation of the retaining structure and ground surface settlement values are reduced.Once it exceeds a certain pile diameter,the sensitivity of the pit deformation and the surrounding surface deformation to the diameter of the retaining pile will be weakened;⑤ for the foundation pit project with the completion of the base slab pouring,the steel brace does not have an obvious effect in controlling the deformation of the pit structure,the force,and the surrounding ground surface settlement.Through the comprehensive consideration of engineering conditions,the steel brace can be removed.