Experimental and Numerical Investigation of Bottom Outlet Leakage in Earth-Fill Dams

Leakage dissolution and the induced internal erosion failures in earth-fill dams and foundations significantly threaten the long-term operational safety of geotechnical structures. This phenomenon can be more dangerous in conjunction with the damage of internal water-supply bottom outlets due to the leakage-induced pore water pressure, which reduces soil effective stresses and therefore affects dam stability. However, the internal instability induced by bottom outlet leakage has not been thoroughly investigated. Therefore, in this paper, a bottom outlet leakage model testing system is designed to simulate and investigate bottom outlet leakage in earth-fill dams. This leads to a detailed investigation on seepage behavior in earth-fill dams, by considering the different positions of bottom outlet leakage, i.e., at the upstream side, the middle dam section, and the downstream side. Furthermore, numerical analyses are carried out to study the leakage-induced slope instability. Based on the experimental and numerical results, the following phenomena are observed: (1) when bottom outlet leakage occurs, the whole phreatic line is elevated and hydraulic head increases significantly at the leaking position. The equipotential line bends to the point of leakage and the seepage field at the leakage point is drastically affected; (2) in the direction perpendicular to the bottom outlet, the hydraulic head decreases nonlinearly as the distance from the bottom outlet increases; (3) seepage discharge increases as hydraulic head increases, in an approximately nonlinear relation. The seepage discharge due to bottom outlet leakage at the upstream side of the dam is much larger than that at the middle dam section and the downstream side; (4) the factor of safety of slope stability decreases as leakage-induced hydraulic head increases. The potential failure surface of the dam slope is circular when leakage occurs at the middle dam section and the downstream side. For the failure at the upstream side, seepage-induced tensile stress reaches the tensile strength, which leads to slope damage; and (5) based on the experimental and numerical investigations, suggestions are given for designing the monitoring scheme for bottom outlet leakage problems in earth-fill dams.