Particle-scale simulation of clay using molecular dynamics

The overall aim of this PhD research is to enhance the framework of the particle-scale simulations of clay to understand the engineering behaviour of clay. This research used coarse-grained molecular dynamics (CGMD) in which the elementary particles are clay particles that are modelled as rigid, flat ellipsoids. The interaction between two ellipsoids is represented using a potential function that describes potential energy as a function of their separation distance and relative orientation. This research proposed several methods to improve the conventional CGMD framework for particle-scale simulations of clay; a new potential function that can accurately capture kaolinite non-monotonic interactions was developed, a new approach to model the anisotropy of surface charge on kaolinite particles was introduced, and a methodology of introducing inter-particle friction was suggested. Using the improved CGMD framework, this research conducted various particle-scale simulations of kaolinite; To explore the differences in the compression behaviour of normally- and over-consolidated clays, isotropic compression simulations were performed using two potential functions, the first monotonic and the second non-monotonic. The results of the simulations revealed that the non-monotonicity of the relationship between the potential energy and the separation distance may explain the differences between the responses of normally- and over-consolidated clays. Also, assuming two pore water conditions, the first acidic (pH=4) and the second alkaline (pH=8), isotropic compression simulations were performed to show that the pore water pH dependency of the particle interaction significantly influenced the microfabric formations and mechanical responses. Additionally, results of one-dimensional (1D) consolidation simulations indicated that implementation of interparticle friction was essential to capture an anisotropic stress tensor of clay and an irreversible unloading-reloading loop. Finally, shear simulations were performed to show that the inter-particle friction significantly influenced shear behaviour of clay.