Filtration of Broadly Graded Cohesionless Soils

The filtration mechanism of broadly graded soils is different from that of uniform soils. The well-known filter criteria have been found unsatisfactory in selecting filters to retain the former soils. It is obvious that in such cases, the filter criteria must take into account the self-filtration process taking place at the base-filter interface, as well as the internal stability of the base soils. Cohesionless broadly graded soils were selected to quantify the potential migration of base particles, the self-filtration layer, and the quantity of particles carried away. Results indicated that the quantity of fines carried away and the thickness of the selffiltration layer could be related to the gradation curve profile and to the soil broadness coefficient. Compatibility tests were also performed to evaluate the representative grain size of the base promoting the self-filtration process. The representative grain size of linearly graded soils was found to be equal to d50. For gap-graded soils, the test results indicated that dSF corresponds to the lower size of the gap.