Research On Foundation Pit’s Deformation Under Combined Actions Of Double-row Supporting Piles And Composite Pile Group Foundation

The combined action system composed of double-row supporting piles and composite pile group foundation can control the deformation of foundation pit. Also, it is used in the supporting design of foundation pit widely. However, at present, the design method and theory analysis of the combined action system are not perfect enough. The monitoring data are not much enough, too. The mechanical mechanism of the combined action system is not clearly recognized. Also, the numerical calculation accuracy is still needed to be improved. Therefore, it is very necessary to do more detailed and deeper research on the problem.Based on the foundation pit of culture and art center theatre stage located in the capital of Shandong province, finite difference software FLAC3D is used to do numerical simulation of the double-row supporting piles and composite pile group foundation. Elastic-plastic entity unit and linear pile unit are applied and interaction between soil and pile are considered in the numerical simulation.3D simulation analysis of foundation pit has been done during the whole process of excavation, supporting and construction. Mechanical parameters of soil mass, design parameters of double-row supporting piles, CFG piles and bored piles are analyzed entirely. And the more sensitive parameters are found out. The back analysis is done through orthogonal test method. With the target of on-site monitoring data, different parameters are used in calculation and the results are analyzed. As a result, the more excellent mechanical parameters and design parameters are got. At the same time,"entity unit pile" and "structural unit pile" are considered when the foundation pit is excavated to different depths. And the results are compared to each other. Finally, regard soil stiffness as the only variable, pilebolck’s horizontal displacement or deep layer’s horizontal displacement as the equivalent target, quantitative analysis of composite pile group foundation is researched to see its ability of improving soil properties.The calculation results show that, soil mass’elastic modulus, soil mass’friction, soil mass’cohesion, soil mass’lateral pressure coefficient, stiffness of bored piles, cohesion of soil mass and bored pils and other parameters have greater impact on foundation pit’s deformation. The more perfect scheme can be got from back analysis using orthogonal test method. And the results using the more perfect parameters are well coincided to on-site monitoring data. Through the comparison of "entity unit pile" and "structural unit pile", double-row supporting piles’and composite pile group foundation’s mechanical behaviors are clearly recognized. The foundation of CFG piles mainly reduces foundation pit’s upper horizontal displacement while the foundation of bored piles mainly cuts down deep layer’s horizontal displacement of soil mass. If soil stiffness is analyzed as the only variable, soil stiffness will be improved to be1.5times of its value by CFG piles foundation and6times of its value by bored piles foundation. The results can provide great reference to the choosing of the parameters of combined action system.