Mass failure and surface erosion of homogenous soil slopes under varying steady unsaturated flow

Soil-borne pollutant is a major environmental concern in river systems. Mass failure and surface erosion of stream banks represent major mechanisms by which contaminated topsoil and subsurface layers are transported into rivers. Recent studies show that initially engineered linear (planar) slopes, when subjected to hydromechanical loadings at the boundaries, change to curvilinear (convex/concave) profiles. This study investigates and provides a new way to quantify a steady state bound on the potential soil volume lost from riverbank as a consequence of hydromechanical loading-induced changes to slope geometry via a rotational failure mechanism and surface erosion. A two-dimensional (2D) formulation of Richards’ (1931) unsaturated flow equation is embedded within a Mohr-Coulomb limiting equilibrium analysis framework and numerically solved for representative riverbank configurations. Results indicate that for a bank height of 4 m, substantial volumes of soil (around 1.5-2.5 m3 per metre run) are potentially erodible in typical sandy and silty clay banks of sufficiently high permeability (to allow steady state condition to be established in the bank soils) due to moisture movements between steady state infiltration and evaporation regimes. Variations in matric head are most pronounced at slope crests, especially in narrower banks.