Viscous compression of clay and peat

Viscous compression, the delayed slow compression of soils after loading, has emerged as a challenging process contributing to land subsidence in soft soil areas. Despite previous research on clay soils, there is still limited understanding of the processes and mechanisms of viscous compression of organic soils. As peat is more susceptible to viscous compression than clay, and the subsurface of subsiding deltas can contain substantial bodies of peat, understanding of processes, mechanisms and drivers is needed to predict the potential for and amount of viscous compression to occur and assess the effect of mitigation measures to delta subsidence. This study integrates findings from prior research on viscous compression behaviour of clay for a comprehensive comparison of the structural, geomechanical, chemical, and biological characteristics of clay and peat, to evaluate to what extent compression mechanisms in clay operate in a similar way in peat. The study focuses on mechanisms of viscous clay compression, which are: expulsion of micropore water, changes in the adsorbed water layer, and particle interactions. Our review establishes that these mechanisms also manifest in peat, albeit with varying contributions to the reorientation of peat fibres. Notably, the distinct pore structure and larger average pore diameters of peat result in water expulsion behaviour that is different from clay. Additionally, the negative electrical charge on clay mineral surfaces is stronger than that of peat fibre surfaces, influencing attraction or repulsion forces among particles and the adsorbed water. This study introduces decomposition of organic matter as an additional long-term control of subsidence. Decomposition weakens the peat structure and facilitates particle reorientation, which enhances the susceptibility to compression. On the other hand, when organic material is already decomposed, it shows lower compressibility compared to fibrous organic material.