Rock physics modelling of microstructural changes and hysteresis during loading and unloading of Paleogene smectitic clay

The deformation properties of Paleogene smectite-rich clays are of concern in relation to infrastructure in Denmark. The clays are known locally as “high plasticity clays”, but when consolidated naturally or in the laboratory, a significant part of the deformation is reversible. However, the oedometer modulus is not constant, as unloading-reloading loops contain significant hysteresis. By using methods developed for rock physics interpretations of borehole acoustic data, this paper demonstrates how the reversible deformation and hysteresis can be interpreted as the result of an interplay of the stress-controlled changing area of particle contact, repulsion between particles due to the layer of hydrated ions adsorbed on each particle surface, as well as the stress-induced merging of adsorbed layers of hydrated ions. This adsorbed-layer merging is reversible and hysteretic and can explain the hysteresis found in loading/unloading experiments. No supposed reversible slipping of solid particles is required. The energy associated with hysteresis would reflect the transport of ions between the merged adsorbed layers and the free pore water.