Exploring the linkage between mechanical behaviour and particle-scale interaction of kaolinite

The response of clay to mechanical compression is highly dependent on its stress history. Generally, normally consolidated clays exhibit a relatively soft response, while overconsolidated clays exhibit a much stiffer response upon reloading. In the literature this difference has been qualitatively attributed to differences in clay microstructure. In this paper, coarse-grained molecular dynamics simulations are used to propose that an additional contribution comes from the non-linear, non-monotonic relationship between the inter-particle forces and the separation distances. At large separation distances, clay particles interact by way of repulsive non-contact forces when the particles initially become close enough to interact. The strength of the mutual repulsion increases with decreasing separation until a maximum repulsive interaction energy, termed an energy barrier, is reached. Once this energy barrier is overcome, the particle interactions become attractive so that the particles effectively become bonded to each other. A new approach to interaction models for particle-scale simulation is used in this paper to show that the compressive forces experienced by particles under engineering stress levels are sufficient to push particle pairs into this attractive force regime; and that, upon subsequent unloading, these particles remain bonded to each other. The difference in macro-scale compressibility between normally consolidated and overconsolidated clays can be explained, at least in part, by this attraction and by particles irreversibly bonding together.