Clogging of toe drain drastically affects phreatic seepage in earth dams

In aged levees, toe (blanket) drains get clogged with time due to seepage-induced suffusion and translocation of fine soil fractions from the upstream to the downstream part of the embankment. These particles deposit on the top of the drain (usually, Terzhagi's graded gravel) as a cake. Also, high hydraulic gradients in the vicinity of the drain move the fine particles into the body of the coarse filter material such that “internal colmation” takes place. In this paper 2-D seepage to a clogged drain is studied experimentally, analytically and numerically. In a sandbox, we illustrate the difference in the position of a phreatic surface and the seepage flow rate between an equipotential toe drain and a clogged one. In the analytical solution, a potential flow model is used and the Neumann (Kirkham-Brock) boundary condition on the clogged drain surface (horizontal segment) is imposed. A circular triangle is mapped conformally onto a reference half-plane, where Hilbert's boundary value problem for a holomorphic function is solved. For a given size of the levee, clogging causes a significant rise of the phreatic surface, although the seepage flow rate drops. In HYDRUS2-D simulations, a FEM-meshed Richards’ equation for a saturated-unsaturated 2-D flow is used for solving in a composite polygon, which mimics a vertical cross-section of a rectangular levee and a clogged-colmated blanket drain. Numerical results are in rough agreement with the analytical model.