A theory of design of blankets for dams on pervious foundations is presented. Rules for optimal design of rectangular and triangular blankets are formulated. Analysis is given for trapezoidal blankets also. Only the blankets on the upstream side of the dams are considered because the downstream blankets are not effective in reducing the uplift force. They will need to be made much stronger than the upstream blankets in order to withstand the unbalanced uplift force. Usually on the downstream side, drainage holes are provided to relieve the uplift pressure.