A laboratory test program was conducted on partially clogged nonwoven geotextiles under pressure. The results of test showed that the presence of soil particles inside the geotextile reduced its compressibility and permittivity

a reduction factor (RF) of equivalent opening size was proposed during filtration. RF of 1.5－2.5 was suggested for the drainage of bank slope. The expression presented by Giroud (1996) demonstrated a satisfactory forecast of coefficient of permeability of non-clogged geotextiles under pressure. Comparison of measured and predicted permeability values for partially clogged geotextile specimens indicated that an expression of equivalent porosity has a better prediction of soil impregnated specimens under pressure.