This European Standard specifies a method for measuring the steady-state liquid flow through a geosynthetic barrier, used to contain liquids in long-term applications. The test method and described apparatus allow the measurement of flows accurately down to 10-6 m3/m2/day. In particular circumstances where testing indicates that values obtained for a geosynthetic barrier lie below the threshold of sensitivity of this test method, then the value of liquid flow is declared as being less than10-6 m3/m2/day. Due to its long duration this test method is not suitable for production control testing. Clay geosynthetic barriers cannot be tested with this apparatus.