The term peat is commonly used to describe organic soils, composed of fragmented plant remains and fibers. Despite the extensive research on the behaviour of peat during the last 50 years, the vast majority of the attention was devoted to the volumetric behaviour. Only few attempts are reported to model the deviatoric behavior of peat, which is fundamental in many geotechnical applications as dykes founded on peat layers. This Thesis represents an attempt to understand the current knowledge on the deviatoric behaviour of peat with the aid of an hierarchical simple elasto-plastic model.

The constitutive model was conceptually developed aiming for the use of a limited number of parameters with an easy way of determination. This choice aligns with the needs of engineering practice, which demands the use of models with few and comprehensive parameters. The constitutive ingredients of the model have been derived from direct experimental evidences. The model requires 10 parameters, 5 out of them are the classical parameters of the well-known Modified Cam-clay model.\\
\\
The high compressibility of the peat along with its high friction angle intensify the end restraints occurring due to the platens of triaxial apparatus. In order to account for the effects of the boundaries, the constitutive model has been implemented in the Finite Element Programme LAGAMINE and the tests have been simulated as Boundary Value Problems. The model is in qualitative agreement with the experimental data and, in particular, with quantitative agreement until 20\% of strain for the case of consolidated drained triaxial tests. To be able to reproduce properly the case of consolidated undrained triaxial tests which impose an isochoric response, one parameter associated with the flow rule of the model should be changed. This change of a parameter is related to the fact that the fibers in the peat specimen are not considered explicitly in the constitutive equations, although they can alter the deformational response of peat under some kinematic constraints and influence the mechanical response in a predominant way at high strain levels.

Overall, the constitutive model developed in this Thesis gives good quantitative results until 20\% of axial strain. Further research on the influence of fibers on the response of peat under different strain paths and on the consequent matrix-fiber interaction should be done in order to have a model with predictive capabilities until high strain levels.