Study On Mechanisim Of Deep Pneumatic Caisson Techniques

Because of having many advantages, more and more pneumatic caisson method is used in the deep foundation construction of the high-rise skyscrapers, the huge trans-sea bridge, the deep underground station, the large underground oil depository, the huge hydropower station, and so on. However, the equipment used in this method is very expensive and the high technology is necessary. The techniques of this method are mastered only by few developed countries in the world. So far, based on the central wind well construction between the Nanpu station and Yaohua station in line 7, we study this technique from three aspects: experimental testing, theory study and in-situ measurement analyzing. The mechanism of pneumatic caisson sinking is systematically studied. The main contents are following below:1. The physical and mechanical characteristics of the representative silty clay in the in-situ spot of pneumatic caisson construction are experimentally tested. By using the advanced triaxial system, the silty clay in the in-situ spot of pneumatic caisson construction is used to be experimentally tested. The experimental test includes three kinds of types, namely triaxial testing under isotropic pressure condition, triaxial testing under K0 consolidation condition and triaxial testing under air pressure condition. The different physical and mechanical characteristics can be studied by using the different experimental conditions. The conclusion about the shear strength, the relationship between the stress and strain, the effective stress path and the peak value of strength are drawn, which can provide the physical and mechanical parameter and warrant for the dynamic numerical simulating of pneumatic caisson.2. The problem of ultimate bearing capacity of ring footings is the pivotal problem on the foundation design and construction of the pneumatic caisson. Only when the ultimate bearing capacity of ring footings is exactly determined, the reasonable structure and width of the ring footing can be properly selected and the prosperous sinking is guaranteed and the pneumatic caisson construction is achieved. In this study, by using the slip line method and the finite differencing method, the ultimate vertical bearing capacity of ring footing is systematically studied, including the influence of the ratio of the inner radius to the outer radius of the ring footing and the inner frictional angle on the ultimate bearing capacity of ring footing. The ultimate bearing capacity of ring footing under different working state and its distribution form are obtained, which provides the theoretical foundation for the structure design and construction of pneumatic caisson.3. The course of pneumatic caisson sinking is numerical simulated. The course of pneumatic caisson construction and sinking is numerical simulated with physical and mechanical parameters of soil obtained in experiments by using finite element method. The soil pressure near around the pneumatic caisson and the effect on the surroundings are analyzed for several kinds of construction conditons and the numerical simulating results are compared with the measured results in-situ, which can be referred in the same and similar coming projects.4. Systematical monitoring on the structural inner stress and deformation of pneumatic caisson and the effect of pneumatic caisson sinking on the surrounding environment are proceeded. The datum about the footing soil pressure, the soil pressure and the lateral frictional resistance on the lateral wall and the lateral displacement and settlement are cleared up and systematically analyzed. Based on this, the deformation characteristics of pneumatic caisson are opened out and the influence of pneumatic caisson sinking on the surrounding environment is estimated. Accordingly, the structure safe is warranted and the deformation of surrounding environment can be controlled in the permission and the pneumatic caisson construction is instructed. In addition, it provides the reference for design and construction of the following this kind of project and accumulates precious original datum for subject development.