| Summary: | 博士 === 國立臺灣科技大學 === 營建工程系 === 87 === This research presents an effort to develop a generalized effective stress constitutive law for Taipei clay, which can be applied to deep excavation analysis in Taipei. The approach of this study follows the general considerations of material research. First, it is necessary to understand thoroughly the mechanical behavior of Taipei clay, based on advanced laboratory test results. Then the effective stress-strain relationship is formulated according to the existing mechanical theories and characteristics of soil behavior. Finally, predicted results are compared with reliable experimental data to evaluate the accuracy and limitations of the models. Test results indicated that Taipei clay is not a highly structured material and that Taipei clay has normalized behavior. All significant Taipei clay characteristics have been taken into account during model derivation, including anisotropic behavior, strength anisotropy, strain softening, coupled volumetric and deviatoric behaviors, and rate-dependent behavior. In addition, laboratory tests were conducted to confirm the properties of consolidation and secondary compression of Taipei clay. Yielding behavior was also thoroughly investigated.
In this study, rate-independent and rate-dependent soil constitutive models were derived, considering isotropic and anisotropic loading surfaces respectively. Among the rate-independent relations, Modified Cam Clay Model gives a fundamental effective stress constitutive relationship for idealized soil, which was derived from the theory of plasticity. However this model could not characterize the anisotropic properties. The proposed anisotropic model, adopting a distorted ellipsoid yield surface, is derived based on plastic theory and non-associated flow rule. It can rationally simulate the anisotropic behavior and strength anisotropy for normally consolidated and slightly overconsolidated Taipei clay. In addition, strain softening can also be described in the proposed model. Under the framework of the above rate-independent constitutive relationships, the rate-dependent effective stress soil models is established, considering the concept of visco-plastic, existing state boundary surface introduced by critical state soil mechanics, Taylor''s secondary compression theory, and Bjerrum''s delay hypothesis. Compared with rate-independent model, the rate-dependent only two additional soil parameters, the secondary compression index and the void ratio at unit mean effective stress , are used in. The rate-dependent soil behavior can then be predicted using these models, including shear responses varies with strain rate, undrained and drained creep, etc. In addition, the shape of loading surfaces, assumed in these models, was modified to rationally predict the time effect for Taipei clay. It should also be noted that the tertiary creep behavior of Taipei clay can be simulated using the proposed anisotropic rate-dependent soil model.
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