A finite-strain hyperviscoplastic model and undrained triaxial tests of peat

This paper presents a finite-strain hyperviscoplastic constitutive model within a thermodynamically consistent framework for peat which was categorised as a material with both rate-dependent and thermodynamic equilibrium hysteresis based on the data reported in the literature. The model was implemented numerically using implicit time integration and verified against analytical solutions under simplified conditions. Experimental studies on the undrained relaxation and loading-unloading-reloading behaviour of an undisturbed fibrous peat were carried out to define the thermodynamic equilibrium state during deviatoric loading as a prerequisite for further modelling, to fit particularly those model parameters related to solid matrix properties, and to validate the proposed model under undrained conditions. This validation performed by comparison to experimental results showed that the hyperviscoplastic model could simulate undrained triaxial compression tests carried out at five different strain rates with loading/unloading relaxation steps.Comment: 30 pages, 16 figures, 4 tables. This is a pre-peer reviewed version of manuscript submitted to the International Journal of Numerical and Analytical Methods in Geomechanic

Undrained Triaxial Experimental Investigations and Hyperviscoplastic Modelling of Peat Materials

This PhD research includes two parts, viz. experimental investigation on the undrained mechanical properties of undisturbed fibrous peat and a finite strain constitutive model within a thermodynamically consistent framework based on the experimental results of the tested peat. From the laboratory investigation, nonlinear behaviour with large strain was observed from the loading-unloading tests on peat. The influence of cell pressure, strain rate, stress relaxation on the constitutive behaviour as well as strain recoveries from unloading were investigated. Relaxation tests were carried out for overstress quantification as well as to obtain the equilibrium state. The tested peat was categorised as a rate-dependent material with equilibrium hysteresis. Structural anisotropy was also investigated by testing the undisturbed vertical and horizontal specimens under the same conditions. Although in the proposed model, the structural anisotropy was not included, the experimental data provided the foundation for the future development of anisotropic constitutive models. The observed material behaviour motivated a rheological model comprising four parallel layers, each consisting of elastic, viscoelastic and elastoplastic elements. The proposed hyperviscoplastic model was derived from the entropy inequality. Each part of the model was verified against their analytical solutions. The model parameter fitting started with the rate-independent equilibrium tests, where the hyperelastoplastic model was fitted to the defined laboratory equilibrium test. Two compression-relaxation tests, carried out at different strain rates, were used for the parameter estimation of the rate-dependent hyperviscoplastic model. The hyperviscoplastic model was validated against five strain rate tests under various load cases as well as an undrained triaxial creep test. The finite strain constitutive model derived within a thermodynamically consistent framework showed its versatility in simulating peat behaviour in various load cases by a good agreement of the experimental results. Also, from the process of the deriving the hyperviscoplastic model, it is perceived that it would be easy to extend the current model with more features in a mathematically and thermomechanically consistent manner