On the Initial Stages of the Densification and Lithification of Sediments
- PDF / 1,605,363 Bytes
- 23 Pages / 439.37 x 666.142 pts Page_size
- 2 Downloads / 208 Views
On the Initial Stages of the Densification and Lithification of Sediments C. M. Sands1 · H. W. Chandler1
Received: 12 March 2015 / Accepted: 1 September 2015 © International Association for Mathematical Geosciences 2015
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. Keywords Constitutive modelling · Critical state · Yield surface · Rock forming · Consolidation · Lithification
B 1
C. M. Sands [email protected] University of Aberdeen, Aberdeen AB24 3UE, UK
123
Math Geosci
1 Introduction When rock surfaces grind together during earthquakes or fault movement (Mair and Abe 2008; Wibberly et al. 2008), fine particles become detached from the rock surface. The particles are subsequently ground to finer material and, depending on the mineral content of the parent rock, sands, silts and clays are formed. Non-mechanical processes, such as freeze–thaw and repeated heating and cooling, also contribute to the production of granular assemblies. Slow consolidation increases the density of clay sediments such that slates and shales eventually form (Baker et al. 1993; Ellis and Darby 2005). The rearrangement of un-bonded granular materials (Paterson and Tobisch 1993) subjected to shear and moderate compression is important in many geomechanical situations. Shear, particularly cyclic shear, facilitates the rearrangement of loose sand grains into a denser packing, particularly when supported by moderate pressure or liquefaction during earthqua
Data Loading...
