Research On Stability Of Dry-Dock And Soil Retaining System In Caisson Tunnel Engineering

Immersed tube method is one of the most advanced construction methods for building underwater tunnels. A number of geotechnical engineering problems are involved in such kind of projects. The soil properties are changeable during different periods of construction, and many complicated factors must be considered in analyzing the stability of the foundation system. Although several Caisson Tunnels have been built, no code for guiding design and construction was developed. Several geotechnical problems in Caisson Tunnel engineering still need to be further studied.The main achievements of this thesis are as follows:(1) The dry-dock is the field of constructing the tunnel pieces. It usually has large dimensions and will be exposed for several years. During construction, the dry-dock will be excavated, refilled with water and redried. In such periods, the soil will be unloaded and disturbed, the ground water level and the pore water pressure will change, and seepage will occur. All of these may have great influence on the behavior of soil. The slope stability of dry-dock under different conditions was analyzed considering the environment influence and soil properties at every different construction state.(2) The soil mass in foundation of dry-dock and underwater trench will be unloaded after the excavation works. The strength parameters of soil unloaded will change seriously from the initial condition. Based on laboratory tests, the proper parameters of unloaded soil were determined and the unloading ratio and the depth affected were derived with the FE analysis. Using these results, the bearing capacity and the deformation of the foundation were calculated.(3) The lattice style soil retaining system is a kind of complicated structure. Different segments have distinct thickness and length. And the environmental forces acting on it are changeable. Analyzing the stability and security of the diaphragm wall becomes very challenging. Code method related to soil retaining wall and FEM were both used in such kind of analysis。(4) In order to simulate the behavior of soil more precisely, the Duncan-Chang model was introduced and linked to the famous FEM software ABAQUS by developing a section of program. The influence of Caisson Tunnel engineering on the surrounding circumstance was predicted by the developed software. The simulation results agree well with measured data.(5)The results achieved in the thesis has been applied for the design and construction of the Haihe Caisson Tunnel project in Tianjin, and also can be referred by the similar tunnel project in Northern areas of China.