Comparison of various testing methods for soil-geogrid friction parameters estimation to stabilise a thin soil layer on slopes

This study is based on previous results of Garcin et al. (2011) and Auray et al. (2012) which presented some sand/geogrid interface friction angles obtained following two testing methods: tests using a direct shear box following the standard EN ISO 12957-1 (AFNOR, 2005) and in-situ tests. In the present work, the same soil/geogrid interfaces were tested by an inclined plane device following the standard EN ISO 12957-2 (AFNOR, 2005). One major objective of this study is to assess the sand/geogrid interface friction angle when the geogrid is in tension, which is its role on site. So, the standard EN ISO 12957-2 was adapted to reach this goal. To better understand the interface shear behaviour, some wire sensors, not required in the standard EN ISO 12957-2, were used. A clear comparison of the various testing methods is made and the benefits/drawbacks are highlighted. It is shown that, if the two standards EN ISO 12957-1 and EN ISO 12957-2 are strictly followed, the soil/geogrid friction angles may not be assessed, principally due to the type of tested geogrid, which is not initially in tension. However, by adapting the testing procedure, the frictions angles obtained from the two standards are close even if each testing procedure includes some uncertainties.