Erosion mechanics of soils with an impermeable subsurface layer

More than 50% of annual soil loss in a number of temperate regions of the world occurs when frozen soils are thawing. Such losses occur as a consequence of relatively high surface soil water contents due to the presence of a subsurface impermeable layer. This laboratory study addressed the soil mechanical principles governing erosion of soils with an impermeable subsurface layer (frozen or compacted, etc.). We examined (i) the effect of a freeze-thaw cycle on soil cohesion and (ii) the effect of a subsurface impermeable layer on soil detachment during raindrop impact for two soils: a Galva loess (silty clay loam, mixed, mesic Typic Hapludoll) and a Nicollet glacial till (loam mixed, superactive mesic Aquic Hapludoll). A Mohr diagram was constructed, based on a series of triaxial tests at three matric potentials. Soil cohesion for each treatment was determined from the Mohr diagrams. Soil water content treatments of 0.15 and 0.25 g g(-1) were imposed before freezing and thawing. The freeze treatment was 90 min at -12 degrees C, followed by a 30-min thaw period. Immediately thereafter, shear strength and detachment were measured. Soil cohesion values were < 50 kg m(-2) and were not affected by freezing or by water content-at-freezing. The single-water-drop detachment values for soils with an impermeable layer vs. without an impermeable layer increased from 0.016 +/- 0.0065 g to 0.054 +/- 0.0265 g for loess and 0.036 +/- 0.0071 g to 0.145 +/- 0.0635 g for till. With an impermeable subsurface layer, soil matric potential is driven towards 0 Pa, resulting in extremely weakened soils prone to high soil-detachment values and quite likely high soil-erosion losses.