On the Initial Stages of the Densification and Lithification of Sediments

This paper presents a model that can simulate early rock forming pro- 7 cesses, including the influence of the initial packing of the grains on the subsequent 8 rearrangement that occurs as a consequence of pressure induced grain damage. The 9 paper is concerned with the behaviour of assemblies of loose grains and the mechan- 10 ics of early lithification. Consider the concept of shear induced negative dilatancy, 11 where any shear deformation has a tendency to produce densification even at very low 12 pressures. As shear deformation progresses, positive dilatancy starts to contribute and 13 at the critical state the two effects balance. This concept is encapsulated within the 14 mathematics of the model. The model building scheme is first outlined and demon- 15 strated using a hard particle model. Then the concept of ‘self cancelling shear de- 16 formations’, that contribute to the shear-volume coupling but not to the macroscopic 17 shear deformation, is explained. The structure of the hard particle model is modified 18 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 20 into a term that, depending on its magnitude, also accounts for frictional resistance 21 between unbonded grains. This parameter has the potential to develop with time, 22 increasing compressive stress, or in response to evolving chemical concentrations. 23 Together these modifications allow densification in the short term, and the formation 24 of sedimentary rocks in the long term, by pressure alone to be simulated. Finally 25 simulations using the model are compared with experimental results on soils.