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Vertical particle segregation in structural crusts: experimental observations and the role of shear strain

1995; Elsevier BV; Volume: 67; Issue: 3-4 Linguagem: Inglês

10.1016/0016-7061(95)00012-d

1872-6259

Charles Bielders, Philippe C. Baveye,

Groundwater flow and contamination studies

There is ample evidence that coarse-textured soils can be highly sensitive to crust formation under rainfall. One of the most controversial aspects of crust formation on such soils is the frequent occurrence of a thin clay band buried below a washed-out layer. In order to better understand the mechanisms that may lead to this vertical sorting in structural crusts, mixtures of 92.5% sand and 7.5% clay were exposed to simulated rainfall for 1 h at 63 mm h−1. The sand fraction of the samples consisted of a binary mixture of contrasting particle-size classes between 106 and 1000 μm, mixed in varying proportions. Micromorphological changes in surface structure were observed on thin sections made from undisturbed samples. Before rainfall, the clay was present as coatings around the sand grains. In all samples subjected to rainfall, a washed-out layer 2 to 2.5 mm thick formed at the soil surface. Except for the samples containing 50% or more 106–150 μm sand, the lower boundary of the washed-out layer was marked by an accumulation of clay material, generally in the form of a band. This clay material was present as microaggregates that were probably derived from the initial grain coatings. Within the washed-out layer a vertical sorting of the sand grains was sometimes observed, which resulted in a relative concentration of the coarsest particles at the surface. The sorting was more pronounced as the ratio between the diameter of the large and small sand grains increased, and as the relative percentage of finer sand decreased. By analogy with a physical model derived for particle segregation in granular media, it is proposed that the vertical sorting of particles according to their size was caused by the shear strain created in the first few millimeters of the samples by drop impact. The strain is believed to have induced temporary changes in pore size which allowed the preferential downward movement of finer grains that would otherwise riot percolate freely. The present observations suggest that strain-induced segregation may be one of the main mechanisms leading to clay band formation in structural crusts formed on coarse-textured soils. The implications of the model are discussed in terms of the range of conditions under which segregation is expected to occur. The experimental results also point to the distinctive role of the particle-size distribution of the sand fraction on the morphology of structural crusts.