The effects of moisture content on filtration of mixed core soils in embankment dams

Mixed or intermediate soils are widely used in the construction of embankment dams. Unfavourable climate conditions and poor quality control may result in the use of mixed soils as under-compacted dam cores, with moisture content highly in excess of the optimum. It is thus essential to design safe filters to prevent internal erosion and piping from wet, under-compacted mixed-soil cores in embankment dams. In this study, 87 no-erosion filter tests were performed on seven mixed core soils having dissimilar geotechnical specifications with diverse moisture contents and compaction ratios. The experimental findings showed that the filter performance (success or failure at preventing erosion) depends on moisture in excess of optimum, the filter D₁₅ (particle size with 15% finer in filter gradation) and the fines content of the core soil. An increase in excess moisture necessitates a smaller D₁₅ to inhibit erosion, to the extent that filters respecting accepted criteria may fail to hinder erosion in the case of excessively wet and under-compacted cores. Overall, for group II, cores the criterion of D₁₅ < 0.7 mm was found to be applicable for soils with compaction ratios over 96%, provided that they are non-dispersive and have a fines content less than 80%. For a compaction ratio of 92–96%, the filter criterion of D₁₅ < 0.5 mm is adequate.