This document describes an index test method that covers laboratory measurement of water flux through saturated clay geosynthetic barrier (GBR-C) specimens using a flexible wall permeameter at constant head. This test method is applicable to GBR-C products with no additional sealing layers attached (e.g. polymeric or bituminous). This test method provides a measurement of flux under a prescribed set of conditions that can be used for manufacturing quality control. The test method can also be used to check conformance. The flux value determined using this test method is not considered to be representative of the in-service flux of a GBR-C.