Evaluation of geosynthetic interface friction at low normal stress: comparison between two different test procedures

The interface friction between geosynthetics has previously been extensively studied with reference to medium-high contact stresses. The interface strength at low contact stress is a less investigated chapter whose importance, however, is evident for the stability of geosynthetic landfill barriers during the construction stage and of the covering systems. The inclined plane is the most used laboratory test for measuring the geosynthetic interface friction at normal stresses as low as 5 kPa. This type of test, however, is not free from criticalities, due to the non-uniformity of the contact stresses induced by the inclination of the plane. Alternatively, it is possible to extrapolate the results of the classic direct shear test, which, however, generally cannot be performed at such low contact stress values. The paper compares data results provided by inclined plane tests and an experimental apparatus capable of performing horizontal shear tests at a vertical stress of 5 kPa. The peculiarity of this device is to guarantee a constant growth rate of the tangential stress equivalent to what happens with the inclined plane test. The good agreement of the interface strength data obtained for two different geosynthetic interfaces leads the way for possible future innovations and insights on the topic.