The role of finite strain kinematics in the natural fibre reinforcement of peat and other soft organic soils

Peat is a highly organic and fibrous soil that often presents significant challenges in geotechnical engineering due to its unconventional high compressibility, shearing resistance and anisotropy. While there is empirical evidence about the role of fibres, a mechanistic model that systematically explains their contribution to the response of the material is lacking. This study presents an experimental and numerical methodology to investigate the reinforcing role of fibres on the mechanical response of peat. An experimental campaign characterised the geometric and mechanical properties of individual peat fibres, highlighting size-dependent variability in tensile strength and stiffness that was modelled with a stochastic approach developed for fracture mechanics. Dynamic image analysis provided a detailed understanding of fibre size distributions, and a novel function was proposed to flexibly model fibre orientations in three dimensions. These findings informed the development of a numerical framework which incorporates large-strain kinematics to examine the effects of fibre reorientation and volumetric changes during material deformation. The results highlight the importance of fibre kinematics in shaping the stress-strain behaviour of peat and offer a framework for further exploration of the role of fibres in soft organic soils. The numerical results compared with laboratory data highlight that fibre reinforcement during shearing depends strongly on the previous strain history and the alignment between fibre orientation and the loading direction.