Geotextile filters for internally stable/unstable soils

A series of filtration tests were performed to examine geotextile filters and internally unstable soils (broad and gap-graded soils) under non-steady state hydraulic conditions. Eight different nonwoven geotextiles with four model soils and one natural soil were tested. It was observed that for broad and gap-graded soils the internal stability can be characterized by the coefficient of curvature, Cc, of the soil, and that these soils have a strong effect on the filtration performance of the soil/geotextile system. Soil piping failure was measured for some filtration systems for soils with a Cc greater than seven. The instantaneous application of a high hydraulic gradient only has a short-term influence on the filtration performance. Digital image analyses were performed on the soil/geotextile interfaces after the filtration tests. These digital images have shown that the vaults or arches formed by soil particles at the soil/geotextile interface occurred only for internally stable soil structures and were altered by sudden changes in hydraulic gradient. Based on this study, a new retention criteria is proposed, which accounts for the internal stability of the soils.