Research On Numerical Analysis Of Large Open Caissons For Deep Foundation Pits In Soft Soils

Open caissons are widely used in foundation pit engineerings by virtue of theirmany advantages such as high stiffness, large load-carrying capacity, smallconstruction site, small effect on surrounding soils, and so on. The application ismore common in areas where the soil condition is poor. At present, the design methodof open caisson supporting structures are mostly based on the traditional Rankine’searth pressure theory and Coulomb’s earth pressure theory. Calculation methods oflaterally loaded pile, such as elastic subgrade reaction method, ultimate subgradereaction method and p ycurve method, are usually used in the calculation of opencaisson’s displacement and deformation. These analytical methods have theadvantages of clear concept, easy to understand, simple formular and convinent forapplication. However, they cannot reflect the interaction between soil and shaft wall;cannot reflect the change of the stress in the soils and open caissons and theirdeformation during the whole process of excavation. The problems can be solved wellby the finite element numerical analysis. Nevertheless, finite element numericalanalysis for open caisson supporting structures currently is usually focused on theconditions which the section of open caisson is regular and the surrounding conditionof the foundation pits is not complex, so the results have many diffences with actualengineering because of many simplifications are used, although it can make updeficiencies of analytical methods to some extent. So It is necessary tocomprehensively consider actual project, then establish proper numerical analysismodels, and comprehensivly reveals the mechanical and deformation characteristicsof caisson supporting structure and surrounding soils, so that provide valuablereference structural design for open caissons.Large general finite element simulation analysis software, i.e., ADINA is used tomake a3D numerical analysis model for a large open caisson of a foundation pit insoft soils. The model fully reflects the complex site conditions surrounding thefoundation pit and the irregular section of the structure of open caisson. It uses theelement birth and death function to simulate soils excavation and the open caissonsinking.In order to better simulate the soils and embody their nonlinear stress-strain relationship, a calculation programme of Duncan-Chang model is written based onADINA secondary development environment and tested by simulating triaxialcompression test. The results indicate that the program is reliable.The Morh-coulomb model and the Duncan-chang model are adopted to calculatethe numerical model. The mechanical and deformation chatacters of the shaft wall andsurrounding soils are systematically analyzed according the calculation results. Toverify the accuracy of the numerical model, calculating displacement values arecompared with measured displacement values.In addition, in view of one of the shaft walls came into big deformation pointinginside the open caisson and the concrete at one of the corners of the shaft wall hadmany cracks, the model to simulate the interaction between the open caisson and soilsis set up. Concrete material in ADINA is adopted to simulate the actual concrete, andthe Rebar element which is peculiar element in ADINA is selected to simulate steel,and the concrete cracks are successfully simulated.