Strain Accumulation Mechanisms in Unconsolidated Sediments during Compression

Pride and Berryman (2009) proposed a model to predict pressure dependence of effective elastic bulk modulus for unconsolidated sediments, by progressively allowing the creation of new contacts during compression. They assume that the gaps around rattlers are distributed according to a power law with distance, in addition, the model allows two different strain-accumulation mechanisms: linear or quadratic, the latter being associated with grain rotation (Goddard, 1990). We have observed that the model of Pride and Berryman can be simplified without losing its generality, assuming a flat distribution of gaps around rattlers, given appropriate values for the maximum gap. We have used this simplified model to study how the strain-accumulation mechanism affects the coordination number during isotropic compression. We tested our model on sand data from Zimmer (2003). We observed that the majority of the experimental trends lay between the pure linear and the pure quadratic accumulation trends. We conclude that the strain accumulation in unconsolidated sediments can be well described as a combination of the two mechanisms. We noted also that rotation affects larger grains (diameter approx. 500 micron) more than smaller grains (diameter approx. 100 micron).