On the distinct phenomena of suffusion and suffosion

Seepage-induced internal instability is a phenomenon whereby fine particles are transported from a non-plastic soil. A distinction can readily be made between a washed-out soil structure that remains intact and one in which some form of destruction or collapse of the structure accompanies the migration of fine particles. The three variables of a measured value of mass loss, a measured value of volume change and a value of change in hydraulic conductivity, deduced from measurements of hydraulic gradient and flow rate, are sufficient to quantify, and hence distinguish between, seepage-induced internal instability phenomena. The term ‘suffusion’ is advocated to describe the non-destructive response, which may be quantified by a mass loss, no change in volume and an increase in hydraulic conductivity. The term ‘suffosion’ is recommended to describe the instability phenomenon whereby the transport of fine particles by seepage flow is accompanied by a collapse of the soil structure. Accordingly, this distinct internal instability phenomenon may be quantified by a mass loss, a volumetric contraction and a change in hydraulic conductivity.