Optimization Design On Composite Support Of A Certain Pit With Soil-Nailing And Pile-Anchor Structure In Beijing Region

With the deepening of urbanization, more and more big cities develop to deep underground. A super-deep pit in Beijing (excavation depth 28m) was studied in this paper. The design, monitoring and optimization of the complex support system with soil nail-pile anchor were carried out. According to the specifications, the scheme of dewatering, soil-nail wall and the pile-anchor were designed. And the pre-laid inclinometer tube, Load Cell, settlement observation points in the scene were used to monitor and analysis the rationality of the design; At the same time, the parameters of foundation pit were optimized using Plaxis finite element methods including the upper soil-nail wall panel stiffness, soil-nail diameter, pile stiffness, pile length, spacing of piles and so on. Through this study, some significant results are gained as followed:1. In the current design, the anchor tension is within the allowable range. The maximum value of anchor is in the fourth layer. Anchor tension of the positive pit edge corner is the same with the tension value of its central pit, so full attention should be paid in angle bracing design.2. Horizontal displacement in pit center was significantly greater than that of the edge. The horizontal displacement of the center and edge are 13.8mm and 6.4mm, respectively. During the winter, the horizontal displacement of the center pit is effected by the winter frost. Then in the coming spring, its horizontal displacement will be reduced, but this phenomenon does not appear in the edge of the pit.3. When regarding the horizontal displacement of the pit as the control standards, the proposed optimization of the final pit design parameters are: using C20 concrete for panel, using 120mm diameter for upper soil nail wall, the length of the breaker row with C30 concrete is 32m, its spacing is 2.0m.