Experimental Investigation And Finite Element Analysis Of Consolidation-Creep Coupling Effect Of Soft Soil

Soft soils show low rate of consolidation and significant rheological characteristics, which brings about some problems such as insufficient bearing capacity and too large settlement of soft soil foundation. Therefore, the conventional calculation methods based on classical elastic-plastic soil mechanics have obvious shortcomings for soft soil. In order to have a quantitative control over deformation properties and potential failure in soft soil engineering and provide reliable prediction and appropriate countermeasures, a deeper understanding of intrinsic deformation mechanism due to the coupling effect of consolidation and creep in soft soil is needed. A calculating model that reflects such effect is needed, too.By means of theoretical analysis, laboratory tests and numerical simulation, the time-dependent deformation characteristics of soft soil, which result from consolidation-creep coupling effect, are investigated. The main contents and conclusions presented in this thesis can be summarized as follow:1. Based on the present research results, the theories of consolidation and creep of soft soil is systematically discussed. It is an effective approach to combine Biot consolidation theory, constitutive relation of soil and rheological model for the description of consolidation-creep coupling effect of soft soil.2. Meanwhile, a series of laboratory tests were done for representative soft soil samples from Pearl River Delta, including conventional triaxial shear tests, one-dimensional consolidation tests, direct shear creep tests and triaxial shear creep tests, in order to study the deformation mechanism of consolidation-creep coupling effect and compare the deformation characteristics under different loading methodsand drainage condition. The test results show that in one-dimensional consolidation tests, coefficient of secondary consolidation, which represents volumetric creep, has a certain law of variation. It has a close relationship with the present consolidation condition and is correlative with compression index. The test curves of direct shear creep tests can be fitted and described by exponential decay function or generalized Kelvin model. In triaxial creep condition, deformation characteristics change with drainage condition, which indicates that the consolidation-creep coupling effect is affected significantly by drainage condition. Based on test results and Singh-Mitchell creep equation, an empirical creep model considering drainage condition is proposed. Moreover, a nonlinear elastic-viscous calculating model called modified generalized Kelvin model is proposed, which conforms to the deformation regulations of soil mass. The method to determine the model parameters is presented, too.3.Combining the proposed model, stress-strain relationship and Biot consolidation theory, a modified generalized Kelvin consolidation model is set up. Based on it, a finite element program is compiled, which can account for the consolidation-creep coupling effect of soft soil. In application to a certain project of preloading technique with prefabricated vertical drains, the rationality of the model and the validity of the program were verified. The influences of some factors on the time-dependent deformation are studied and several proposals are provided as follow: To get a more accurate prediction of deformation properties in soft soil foundation, the consolidation-creep coupling effect should be taken into account, especially if the drainage condition is bad; To weaken creep effect and prevent from creep rupture, the drainage condition of the soft soil foundation should be improved and load rate should be controlled carefully.