On the Initial Stages of the Densification and Lithification of Sediments

Abstract

This paper presents a model that can simulate early rock-forming processes, including the influence of the initial packing of the grains on the subsequent rearrangement that occurs as a consequence of pressure-induced grain damage. The paper is concerned with the behaviour of assemblies of loose grains and the mechanics of early lithification. Consider the concept of shear-induced negative dilatancy, where any shear deformation has a tendency to produce densification even at very low pressures. As shear deformation progresses, positive dilatancy starts to contribute and at the critical state the two effects balance. This concept is encapsulated within the mathematics of the model. The model building scheme is first outlined and demonstrated using a hard particle model. Then, the concept of ‘self cancelling shear deformations’ that contribute to the shear–volume coupling but not to the macroscopic shear deformation is explained. The structure of the hard particle model is modified to include low levels of damage at the grain contacts. A parameter that describes bonding between the grains and possible damage to those bonds is incorporated into a term that, depending on its magnitude, also accounts for frictional resistance between unbonded grains. This parameter has the potential to develop with time, increasing compressive stress, or in response to evolving chemical concentrations. Together these modifications allow densification in the short term, and the formation of sedimentary rocks in the long term, by pressure alone, to be simulated. Finally, simulations using the model are compared with experimental results on soils.