Results of Geosynthetic Interface Shear Round Robin Tests

Friction is determined experimentally in modified laboratory direct shear (interface shear) tests. Results of these tests vary greatly due to several factors including test equipment, configuration, and setup. Recently, two round-robin test series were performed by the German Geotechnical Society (DGGT). Both series show significant variations in the characteristics of friction stress vs. displacement curves and the peak friction stress values. For the second test series the boundary conditions of the tests were specified more precisely. However, the results showed nearly the same variation. Additionally, in 2013 a round-robin test series was performed with ASTM D5321 after a significant revision of this ASTM test standard was published. One interface was tested. This round-robin test program was set-up to evaluate the effectiveness of the current test method. However, similar variation was found in this test series as the two series initiated by the DGGT. This paper will present results of the round-robin test programs and identify ways in which to improve the current interface shear test standards. Particular attention will be given to the test equipment and setup including boundary conditions, clamping, gripping, etc.