Development and Evaluation of an Effective Stress Based Model for Soft Clays
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The term, natural soft clay, is commonly used to describe a normal or slightly overconsolidated clay. This soil type may be encountered in e.g. Scandinavia, Eastern Canada and South East Asia. The mechanical response of natural soft clay may be relatively complex and more research is still required to understand the entire process with respect to short and long time aspects. The observed mechanical behaviour in the field and in the laboratory has been investigated through a literature study to outline the main characteristic features which dominate the mechanical response. This study has shown that the viscous and ageing effects are the dominating features in natural soft clay. The observed features have made a foundation of a new constitutive model based on effective stresses. The model utilizes the theory behind the Modified Cam Clay model and invariants as framework. The model requires 17 input parameters to access all its implemented features. Three additional input parameters are available to switch between different methods of simulating the stress history, evolution of anisotropy, and failure criteria in the general stress space. The model assumes associated flow and utilizes three hardening laws which introduce eight state (hardening) parameters in total. The model is implemented in the commercial software PLAXIS as a user defined soil model through the interface between PLAXIS and FORTRAN. The FORTRAN code is supported by additional MATLAB coding where the differential equations controlling the mechanical response are assembled. The initializing of the state parameters is performed in the FORTRAN code. The model has been verified against laboratory tests such as constant rate of strain and undraind tri-axial tests and shows acceptable performance with respect to the implemented features. The performance of the suggested model has been investigated during a full scale boundary value problem for the test embankment located at Onsøy, Norway. The model shows fairly good predictions of the soil response and the dissipation of excess pore water with respect to field measurements. These predictions have further been compared to other known soil models. Keywords: Natural soft clay, creep, anisotropy, destructuration, recovery, Onsøy.