This paper discusses the most common forms of drain clogging and its influence on seepage under concrete dams built on permeable rock foundations. The increments of the uplift pressure caused by reduction in drain length or diameter, reduction of its effective flow section, reduction of flow to the drain and the reduction of the permeability of the rock mass around the drains is discussed. The parametric studies were based on a nonlinear numerical model developed for three dimensional seepage analyses of concrete dams with permeable continuous or fissured rock foundations. The main conclusions are that all causes abovementioned could induce clogging but some are more critical than others. Reduction of the drain’s length, filling of the drains with granular or fine materials and joint clogging around the drains tend to be the more critical situations. Dams sitting on high permeability materials are more susceptible to clogging than those with low permeability foundation since the flow captured by the drains is higher in the first case. The importance of proper drain maintenance to maintain the dam's stability is stressed.
The upstream and downstream reservoirs, and seepage through the foundations, generate the main external forces acting upon a concrete dam. Fig. 1 indicates schematically these forces, plus the dam’s weight, for a typical concrete gravity dam, without drains.