|M.Sc Student||Kopito Dor|
|Subject||An Anisotropic Hypoplastic Constitutive Model for Soft|
Soils and its Use in Tunneling Problems
|Department||Department of Mechanical Engineering||Supervisor||Professor Assaf Klar|
|Full Thesis text|
Correct evaluation of tunneling induced ground displacement is a central problem in geotechnical engineering. When constructing a tunnel to be part of sewage system, metro or a service tunnel, one must consider the influence of the excavation process on existing structures. Recent studies have shown that common models, used in numerical simulations for prediction of soil displacements due to tunneling, predict wider and shallower surface settlement troughs than those measured in the field. Further research has shown that in order to improve the prediction, the constitutive laws must consider the following three main characteristics:  non-linear behavior,  anisotropy, and  plasticity. In this thesis, an anisotropic hypoplastic constitutive law was developed with the hope it can answer the discussed problem. The developed model is based on the hypoplastic modified cam-clay model of Mašin (2012), and is extended to include anisotropic behavior through the use of a failure dependent non-linear anisotropy tensor. The characteristics of the new model were evaluated using the response envelope concept. Numerical analyses of tunnel excavations in undrained soft clay were considered for demonstrating the effect of the model on the developed surface settlement trough. It was found that by using the proposed anisotropic hypoplastic model, a narrower and deeper surface settlement trough can be obtained in comparison to the one obtained using the isotropic model. A wide parametric study, conglomerating stress relaxation, volume loss, anisotropy parameters, was performed to depict the effect tradeoff between the parameters. It was found that not only the final settlement trough varies due the choice of the model parameters (which can be resulted by the total soil stiffness) but also the incremental evolution of the settlement trough shape is influenced by the anisotropy parameters. In conclusion, the results indicated that the use of the proposed model can help overcome the deficiencies associated with currently used continuum based models for prediction of tunneling induced ground displacements.