The influence of particle orientation on the loading condition of pebbles in fluvial gravel

The loading conditions of pebbles in fluvial gravel deposits were studied with different degrees of preferred particle orientation. Sediments that are comprised of non-spherical particles often show a preferred particle orientation, due to dynamic sedimentation. Here, the impact of this effect on the loading conditions of the particles and its implication on particle breakage was investigated by using discrete element simulations in three dimensions. The numerical models are based on the size and shape distribution of pebbles from a natural gravel sample. In addition, the particle size in some of the models was chosen to be uniform, to study the influence of the particle size distribution on the loading condition. Fluvial pebbles, whose shapes can be at best approximated by ellipsoids, were efficiently simulated in the discrete element models by the use of clumps. The results show that a preferred orientation of approximate ellipsoidal sedimentary particles has only a minor effect on the number and the position of particle contacts but leads to a significant load transfer from the rim to the centre of the oblate sides of the ellipsoidal particles, in comparison to an assembly of arbitrarily oriented particles. The comparison of the different particle size models indicates that the influence of the particle size distribution on the loading condition is relatively low. The results have significant implications for the breakage rate of non-spherical particles in sediments under load.