THE NUMBER OF CONSTRICTIONS CONCEPT AS A MEANS TO PREDICT THE FILTRATION BEHAVIOUR OF NONWOVEN GEOTEXTILE FILTERS

Most of the filtration criteria developed for nonwoven geotextiles are based on the comparison between their filtration opening size (FOS) and an indicative diameter of the soil to be filtered. In recent published experimental data, there is evidence that the FOS might not be the sole parameter controlling the filtration behavior of soil-geotextile systems. Significant behavioural differences have been observed between systems composed with nonwoven geotextiles of similar FOS but with different fibrous structure, ranging from equilibrium to severe piping. It is clear that the nonwoven geotextile structure has a great influence on the filtration of a soil-geotextile system. The number of constrictions concept has been recently proposed as a simple means to describe the structure on nonwoven geotextile filters. Filtration tests have been performed in order to verify the existence of a correlation between this parameter and the filtration behaviour of soil-geotextile systems. Gradient ratio test results showed that high numbers of constrictions may result in the development of a severe piping, and a number of constrictions of 45 represents an acceptable upper limit. Even if theoretical analysis suggests that low number of constrictions could lead to blinding mechanism, test results revealed no clear evidence. For the covering abstract see ITRD E117244.