Objective: to investigate the mechanisms governing the coefficient of earth pressure at rest, K0, at the level of soil elementary structure, i.e., its microstructure. The fundamental components of soil microstructure are solid-phase particles. The study focused on a discrete medium represented by clean medium-sized quartz sand. Methods: this paper considers aa idealized monodisperse medium with periodic grain packing, i.e., a discrete medium composed of particles identical in size and shape and arranged in a regular pattern. Apparently, this type of medium is a major simplification, as natural soils consist of grains of varied sizes, which may differ by several orders of magnitude. Modelling that accounts for the actual particle-size distribution significantly complicates the research process. At the same time, only monodisperse granular assemblies allow the limiting states of packing densities to be identified without ambiguity and make it possible to derive analytical expressions for the coefficient of lateral earth pressure. For this reason, this study reviews and summarizes available theoretical approaches developed for evaluating this coefficient in media of that type. In the numerical simulations, the granular media was described within the framework of discrete media mechanics. Under this approach, an elementary soil volume is treated as a mechanical system composed of a finite number of interacting grains, governed by the fundamental laws of dynamics. Numerical modelling of the medium during the formation of K0 was simulated using the discrete element method. Results: the results indicate that the available theoretical relationships remain valid only within a limited range of porosity and packing density. When porosity approaches its limiting value, the coefficient of lateral earth pressure rises sharply relative to the values of the theoretical predictions. A comparison with laboratory measurements of K0 reveals a substantial discrepancy between the simplified model and experimental evidence. In particular, the model suggests that loose sand should exhibit a greater K0 than densely packed sand, whereas laboratory experiments demonstrate the reverse relationship. Practical significance: the results of this study have theoretical significance for soil mechanics. By interpreting the development of the lateral earth pressure coefficient through the principles of discrete media mechanics the study makes it possible to examine, in detail, how each characteristic of the granular system contributes to the emergence of K0.