Erosion in Relation to Filter Design Criteria in Earth Dams

The current design practice for filters in earth and rock fill dams is an outgrowth of a well known concept proposed by Terzaghi. The main effect of amplifications that investigators have added to the Terzaghi criteria is to permit the use of widely graded sand-gravel mixtures as filter materials. The use of these criteria without consideration of the erodibility characteristics of the core material and crack potential of the filter has caused near failure and total failure of several dams. A new method is presented for quantifying the erodibility of soils based on the use of critical shear stress, TC, developed due to hydraulic flow in the cracked zone of a dam. The limitation of the classification proposed for evaluating erosion from a knowledge of Atterberg limits of soils is considered. A direct correlation between the critical shear stress required to cause erosion and the piping failure of several dams whose filters were designed under the Terzaghi criteria, is presented. Core fines with TC values less than 4dynes/cm2 have been shown to be one of the contributing factors causing piping failure of dams. The need to correctly quantify erodibility and to carry out filtration tests using the eroded fines and proposed filter material is recommended for the safe design of protective filters. Research is necessary to determine the percent of fines less than #200 sieve size needed in a filter in order that it will retain fines which have been eroded and also so that it will not sustain a transverse crack.