Anisotropic Behavior Of Soft Clay And Its Effect On The Deformation Of Deep Excavations

In order to protect the environments, not only the safety and stability of excavations should be considered, but also its deformation should be controlled within certain scale, especially in the condition of soft foundation. Because it could cause excessive deformation and influence the well-balanced use of buildings, public establishments etc, al., thus, the preferably exact forecast of deformation is an important job of excavation engineering.Generally, in the numerical simulation, the constitutive model of soil doesn't include its anisotropic characteristic or its complexity doesn't make the engineers use it. However, the anisotropy has an important influence on the safety and deformation of excavations(Yang, 1997). In the construction, the relation of stress and strain will undergo changing and the direction of primary stress will also change. To great degree, the undrained shear strength and deformation depend on the direction of primary stress. If the unchanged mechanical parameters are still used in order to simulate the changed parameters of soil, it might induce differences between simulation and field data. The above factors must be included in the constitutive model for the purpose of exact forecast. This dissertation discusses the anisotropic behavior of soft clay and its effect on the deformation of excavations based on the study of other researchers.The main achievements of this thesis are as follows:1 Firstly, an anisotropic bounding surface constitutive model(Wei, 2005) is recommended, secondly, the deformation of triaxial compression tests and triaxial extension tests under condition of isotropic consolidation is analyzed qualitatively.In order to simply use the anisotropic constitutive model, the formulation of anisotropic undrained shear strength is deduced according to the undrained stress path. The formulation can express the effect of stress history and consolidated state on shear strength. At the same time, the ratio between extension and compression shear strength is confirmed about Shanghai soft clay.A formulation which reflects the initial anisotropy and induced anisotropy is deduced based on the formulation proposed by Casagrande and Carillo(1944). It could also contain the relation between anisotropic undrained shear strength and von Mises strength. Therefore, the classical von Mises strength criterion is modified. The modified von Mises criterion can reflect the anisotropic strength characteristic.2 The new anisotropic perfect elastic-plastic model is coupled into ANSYS9.0. On this basis, two examples in the study of Poh et al. (1997) are confirmed, then the deformation of excavation is exactly researched, the following are the conclusions: When we carry out study of anisotropy of soil, the influence of anisotropy on the deformation is becoming evident with the increasing degree of anisotropy on the condition of same excavation width, while the influence of anisotropy on the level deformation is greater than the vertical deformation within the certain excavation zone.The influence of anisotropy on the deformation changes with the different excavation width. With the increasing of excavation width, the influence of anisotropy on deformation changes from the level to vertical and become the dominant influence object within the certain excavation zone.The influence degree of initial anisotropy and induced anisotropy on the deformation of excavation is different from each other, and on the condition of soft clay, the influence of anisotropy on deformation is mainly induced anisotropy. For the excavation depth of 8 meters and the width changing from 10 meters to 40 meters, the influence of induced anisotropy on deformation occupied 70% of the whole anisotropy influence.The influence degree on the deformation is greater on the condition of reverse construction method than on the condition of normal construction method. The stronger the anisotropy, the greater the influence.3. Some excavation is analyzed with the coupled model and Drucker-Prager model, and the analysis results are compared with field data. The result shows the simulation result is more closed to the field data when anisotropy is to be considered.Finally, further studies on the research are discussed briefly.