Peat is a highly organic material that poses significant environmental and geotechnical engineering challenges due to its hydrological relevance and atypical mechanical behaviour. Understanding its unsaturated response is essential for infrastructure built over organic soils, particularly under increasing seasonal variability associated with increased climate stresses. Modelling the water retention behaviour of peat remains complex due to its high compressibility and the fabric rearrangements induced by drying and wetting cycles. This study presents an experimental characterisation of the shrinkage and water retention behaviour of natural and reconstituted fibrous peat from the Netherlands. A combination of high-resolution laser scanning and suction measurements was employed to monitor volume change and water retention throughout drying. The results are interpreted through a framework that distinguishes between inter-and intra-ped porosities, allowing for the separation of their respective contributions to shrinkage and retention. Complementary mercury intrusion porosimetry (MIP) analyses provided insight into the evolution of pore size distribution during drying, supporting the interpretation of a sequential engagement of pore sizes. The findings underscore the importance of accounting for differential multiscale porosity evolution and fabric structure when evaluating the hydro-mechanical response of peat.

A relevant part of the geotechnical infrastructure in the north of Europe and overseas is built on soft organic soils, including peat. Peat is extremely vulnerable to climate-related hazards as increased temperature accelerates drying, shrinkage and decomposition of the organic matter. Peat exhibits dramatic changes in volume with changes in water content. As the material deforms, the pore space evolves and changes the water retention response. The evolution of the pore space leads to a hysteretic relationship between suction, water content, and void size distribution. In this work, data from free shrinkage-swelling and suction measurements on natural fibrous peat subjected to drying and wetting cycles are presented and discussed. The water retention and shrinkage behaviour of the samples are modelled by accounting for capillarity and considering the evolution of the pore size distribution. X-Ray computer tomography was used to explore the change in the pore space upon shrinkage and drying. The experimental evidence shows that peat experiences distinct shrinkage zones including one where accelerated contraction occurs. Such behaviour is explained as a consequence of the interactions of an aggregated fabric. This is supported by the conceptual modelling approach that highlights the pivotal role of the evolving pore space.