Rain impact soil crust. IV. Packing of sand and silt fractions by raindrops

Abstract

Comparison of resistance to fast and slow deformation by rain-packed and artificially prepared sand and silt fractions showed that, whereas sands are left in loosely packed states after accrual under rain impact, silts are left more densely packed, i.e. below critical void ratio. This contrast is attributed mainly to the declining importance of hydraulic penetration with pore size. Rain-packed silt is dilatant and cannot deform without expansion which, in turn, requires water entry at a rate compatible with the required deformation. In the short duration of rain-impact events, hydrodynamic time lagging occurs because water can move only slowly through the small silt pores. Consequently, densely packed silt remains essentially rigid. However, drop-outflow sheets are able to entrain silt and distribute it over the surface as densely packed bed-load deposits, thus maintaining its dilatancy. Sand, its larger pores invaded by hydraulic penetration jets which maintain loose packing, cannot behave in this manner. Relative stability of the silt layer on soil surfaces, and its suppression of hydraulic penetration, allow transmission of drop-generated stress waves which cause the immediately underlying soil to become compacted into the main structural element of the rain-impact soil crust.