Depending on the micromechanical properties of its components and submitted to a vibrational excitation, a 2D set of spherical particles can exhibit the whole range of behaviour of common dry particulates such as sand : Spontaneous heaping, size segregation , convection, surface fluidisation etc. Modelling sand with a 2D set of particles has the crucial advantage of allowing a direct observation of the interplay of these processes, as well as allowing theoretical investigation and making up privileged benchmarks for computer simulations. Based on our most recent experiments, this paper is devoted to the recognition of the pertinent parameters which govern the physics of these materials. As an example, we discuss the processes governing vibrational size segregation and we set-up and illustrate the basic equations which rule the progressive decompaction mechanism responsible for bulk motions. In particular we show how the interplay of both the friction at the boundaries, the aspect ratio of the container and the ability of the granulate to convert vertical into horizontal stresses, combine into a single a-dimensional parameter χ, which governs the decompaction process.