Optimization Study On Retaining And Protecting System For Deep Foundation Excavation In Metro Projects

The deep foundation pit is common used in Beijing metro designs, but the retaining and protecting for foundation excavation is very single and the bored pile with steel-tube bracing system is used mostly, and so is the excavation in the Olympic centre station and the Olympic centre– panda crossroad interval in the Beijing metro Olympic branch line. Based on the monitoring information of excavation, it's obviously that over-excavation, placing support out time and disturbing construction are weaknesses for such bracing system.Technical and economic comparison is made between bored pile with steel-tube bracing system and bored pile with prestressed anchor cable bracing system, so as to study on feasibility and superiority for anchor cable to be used in Beijing metro excavation projects. When numerical simulated by FLAC3D, the soil's mechanical parameter is not easily to fix, and back analysis of displacement has offered an effective way to fix these parameters.According to the calculate results of FLAC3D, training sample of BP neural network is established between pile displacement and unknown mechanical parameters. The network input is pile displacement, and the network output is unknown parameters. After the training of BP neural network is successful, those needed parameters are received through the network simulation of actual measured displacement of pile. However, it has the problem of generalization of neural network for functional approximation. Uniform design method is used to get training sample in order to overcome this problem and to reduce sample count. The back analysis is designed by two steps. Firstly, the first and second layers of soil's mechanical parameter are gained according to the work condition that the second steel shotcrete has been placed with excavating to its height mark; and secondly, those of the other layers are gained according to the work condition with excavating to the bottom.During the design of anchore bracing system, deformation and force are obtained by FLAC3D simulation. After comparison of several programs: such as bolted every pile or bolted every two piles, the superior design program is determined and that is: bolted every pile with three rows in every fracture plane. Finally technical and economic comparison is made between bored pile with steel-tube bracing system and bored pile with prestressed anchor bracing system, and we can see that in aspects of cost, progress...