Upper Bound Limit Analysis For Base Stability Of Deep Excavation In Soft Clay

In deep excavation in soft clay, a major concern is to analysis base stability against base heave failure. The traditional methods for base stability analysis, such as the the bearing capacity method and the slip failure method, can not take into consideration all the influencing factors such as soil undrained strength anisotropy, depth of the excavation, embedded depth of retaining wall, etc. Based on the typical field monitoring data of deformation induced by excavation in soft clay, the deformation mechanisms based on the distorted Gauss incremental displacement induced by excavation are proposed for both wide and narrow excavation in soft clay, including the kinematic field in the pit. Based on the proposed deformation mechanism, two new methods for stability analysis are proposed considering strength anistropy of soil within the framework of upper bound theorem for both wide and narrow excavation respectively in soft clay. The propsoed methods are verified by case studies. The main contributions of this thesis are generalized as follows:1. Based on the the typical field monitoring data of deformation induced by excavation in soft clay, the deformation mechanism based on the distorted Gauss curve incremental displacement profile is proposed. Based on the modified deformation mechanism, a method for base stability analysis is proposed considering the strength anisotropy and rotation of principal stress axis under plane strain conditions within the framework of upper bound theorem for wide excavation in soft clay. The method is verified by two case studies and is proved to be more proper for base stability analysis as compared with traditional methods:the bearing capacity method and the slip failure method.2. For narrow and deep excavation in soft clay, the deformation mechanism based on the cosine fuction lateral incremental displacement profile is modified to the deformation mechanism based on the distorted Gauss curve lateral incremental displacement profile, with conjecture of kinematic field in pit.. A method for base stability analysis of narrow excavation in soft clay is proposed considering the strength anisotropy under plane strain conditions within the framework of upper bound theorem. The method is verified by a case study and is proved to be more proper for base stability analysis as compared with the slip failure method and Terzaghi-Peck method.3. The basal heave failure accident of North2excavation at Xianghu Station of Hangzhou Metro Line1is analyzed by the method proposed in this thesis. The results of the analysis show that the primary reason for the accident is the over excavation of certain construction sections, which is the same as the conclusions made by the official accident investigation group. Parametric study of basal heave control measures is conducted, and some beneficial conclusions which can serve as a reference for similar engineering are gained.