Cyclic Constitutive Modeling Of Soft Clays And Long-term Predictions Of Subway Traffic-Load-Induced Settlements

With a rapid development of subway construction, long-term settlements of subsoils induced by traffic loading have received much attention recently. Predicting, controlling and reducing settlements have been a great interest to geotechnical engineers. However, little effort has been made to behaviors of saturated soft clay subjected to long-term cyclic loads. Most of constitutive models in the literature are not capable of capturing both anisotropic and cyclic behaviors of soft clay simultaneously. There are no practical methods available for predicting subway train-load-induced long-term settlements under anisotropically consolidated conditions. Anisotropic and cyclic behavior of Shanghai saturated soft clays is investigated by conducting a series of cyclic triaxial tests on anisotropically consolidated samples. Based on critical state theory and bounding surface theory, two constitutive models are herein developed. One is an anisotropic bounding surface model and the other is a bounding surface model incorporating kinematic hardening, which account for anisotropic and cyclic properties of soft clays. Finally, a practical method is proposed for predicting the subway train-load-induced long-term settlements. Present research is of importance both for theoretical analysis and practical engineering.The main achievements of this thesis are as follows:1. A series of triaxial compression tests and cyclic triaxial compression tests on anisotropically consolidated soft clay are conducted on GDS triaxial apparatus. Response of deformation and pore pressure are compared based on tests performed with different consolidated stress, cyclic stress amplitudes and drained conditions. Results show that accumulative plastic strain increases with increasing effective mean normal stress and cyclic stress level. Moreover, accumulative deformation and pore pressure increase rapidly at early stage and attain a stable state with large cyclic numbers. In partially drained condition, accumulative axial strain and pore pressure are certainly greater than those in undrained condition.2. Based on bounding surface theory, an anisotropic bounding surface constitutive model is developed for predicting anisotropic and cyclic behaviors of soft clay. The proposed model can take into account the initial anisotropy by introducing anisotropic tensor and shape parameter based on yield surface of MCC. Simultaneously, for anisotropic tensor, rotational hardening law is introduced to predict stress anisotropy. Unlike other simple bounding surface models, the proposed model involves a kinematic mapping centre described by the following working rules. For virgin loading, the mapping centre is located at the origin of stress space, whereas...