Optimum design of front-braced grouting steel pipe system in the foundation pit engineering in Shanghai

Through comparative analysis of the code-calculated and measured deformation characteristics of the front-braced grouting steel pipe system under different geological conditions in Shanghai, optimized design methods and measures for improving bearing capacity and controlling the deformation of retaining piles are proposed and verified by numerical simulation. The results show that: (1) For foundation pits with poor soil conditions at both the pit bottom and the front-braced pile bottom, the measured deformation of the retaining system is significantly larger than the code-calculated value. The optimal horizontal angle of the front-braced piles is 45°. Zoned excavation and the addition of precast cushions during construction can reduce the total deformation of retaining piles by approximately 10%. Further deformation control can be achieved by reducing the single excavation area. (2) For foundation pits with poor soil at the pit bottom but good soil at the front-braced pile bottom, the measured deformation of retaining piles is slightly larger than the code-calculated value. The horizontal angle of front-braced piles can be increased to 50°~55°, which shortens the pile length while maintaining the same horizontal bearing capacity, thus improving economic efficiency. Deformation can be controlled via zoned excavation and cushion installation, and attention should be paid to the shear design between retaining piles and wales. (3) For foundation pits with good soil at both the pit bottom and the front-braced pile bottom, the measured deformation of retaining piles is close to the code-calculated value. The horizontal angle of front-braced piles can be reduced to 30°~35°, which shortens the pile length under constant horizontal bearing capacity, controls the deformation of the pile top, and also improves economic efficiency.