The paper presents the scheme of modifed apparatus to test the local hydraulic gradient in soil-geotextile system according to ASTM D 5101. Based on the test results, gradient ratio in soil-nonwoven geotextile and gradient ratio in soil for internal unstable slightly clayey sands and nonwoven geotextile K-500 during fltration process under total hydraulic gradients i = 1–10 was carried out. The test results were analyzed for evaluation of using the nonwoven geotextile as a fltration layer according to clogging criteria, presented by Haliburton and Wood (1982).The analysis of tests ^’ results showed, that nonwoven geotextile could be flter for protected internal unstable soils with no more than 27% fnes contents.

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