**3. Seepage control techniques**

The two main goals in designing seepage control facilities include controlling the hydraulic gradient within overburden layers to ensure the seepage stability of foundation materials and reducing the seepage loss of reservoir water. For overburden layers where excavation and removal are feasible, deposits right beneath the impervious system of the dam body (e.g., clay or asphalt core and toe plinth) can be removed so that the seepage barrier can sit on a firm rock foundation. In cases where deep excavation is impossible, a horizontal, vertical or combined seepage control measure must be employed to meet the above goals. In the design specification for rolled earth and rockfill dams, a vertical seepage barrier that cuts through the overburden layers is recommended over an upstream horizontal measure. This can be evidenced from the cases listed in **Table 1**, in which all dams use at least one cutoff wall. In this section, seepage control techniques used in some of these example dams are reviewed.
