The gradient ratio test is used to assess soil-geotextile compatibility in filtration/drainage applications. Modifications to the test permeameter and sample preparation technique are reported which provide for good repeatability of results. Data are reported for three soils and non-woven geotextiles. A unified interpretation of the water head distribution, gradient ratio and permeability is presented which contributes to a better understanding of those factors governing d gn criteria proposed for interpretation of the gradient ratio test.