The gradient ratio test is used to assess soil-geotextile compatibility in filtration applications. Experience gained in performing the test is summarized for ten soils and four non-woven geotextiles, and the potential for application of the test method is demonstrated. Details of a new, modified test device are then presented which address a perceived limitation of the existing configuration of the test device. Results of a preliminary series of tests, performed to commission the modified gradient ratio test device, suggest the filtration behaviour of the geotextile is essentially independent of normal stress.