Criteria for soil retention and permeability are important in the proper selection of a geotextile in filtration design. Gradient-ratio tests have been performed to assess the compatibility of nonwoven geotextiles with soils exhibiting a range of gradations. Observations of any soil piping through a geotextile were used to evaluate soil-retention criteria. The American Society for Testing and Materials test apparatus includes additional ports that were used to define a modified gradient ratio, which is a better indicator of flow condition. Measurements of the distribution of water head through the soil sample and geotextile specimen, and its variation with imposed hydraulic gradient and time, are used to establish the permeability of the soil and soil–geotextile composite. The composite layer is relatively thin. A unified interpretation of the results is presented that shows a gradient ratio greater than one means the permeability of the composite is less than that of the protected soil: the condition is acceptable if it can be demonstrated that no impediment of seepage flow will result. Key words : filtration, geotextile, soil retention, gradient ratio, permeability.
A dynamic gradient ratio test apparatus
