# Geosynthetic Interface Friction at Low Normal Stress: Two Approaches with Increasing Shear Loading
## 2 The Direct Shear and the Inclined Plane Tests: Pros and Cons  
The direct shear is probably the most known and widespread methodology of test for studying the interface friction. It is standardized by the ASTM D 5321-8 [10] and by the EN ISO 12957-1 [11] and it is carried out on specimens having minimum size of 300 mm \(\times\) 300 mm, arranged horizontally and fixed to two supports, one of which remains still during the test while the other can slide with respect to the first. Once a prefixed normal stress is applied, half the device is pushed at a constant speed of 1 \(\pm\) 0.2 mm/min while the contrast force necessary to keep firm the other half of the device is measured, until a maximum displacement of 50 mm is reached. In order to identify the failure envelope, the tests are repeated for at least three different values of normal pressure (i.e., for the European standard 50 kPa, 100 kPa and 150 kPa), and subsequently the envelope friction parameters (c\({}^{\prime}\), \(\varphi\)) are evaluated by linear interpolation of the results. On the other hand, the inclined plane test is standardized only by the European EN ISO 12957-2 [12] and it should be noted that, strictly speaking, this rule refers only to interfaces between soil and geosynthetics, even if its indications can be easily extended also to the case of contact between geosynthetic and geosynthetic. The equipment used for this type of test consists of a tiltable plane, above which a steel block is placed. This element is able to slide along the plane and its motion can be directed by lateral guides or by vertical carriages, acting outside the contact area of the geosynthetics. The first geosynthetic, of the examined interface, is fixed on the inclined plane while the second is bound to the bottom of the upper block; in analogy with the direct shear test, the specimens should have a minimum size of 300 mm \(\times\) 300 mm. The test starts with the plane in the horizontal position and, gradually, the inclination of the plane is increased, at a constant speed of 3 \(\pm\) 0.5\({}^{\circ}\)/min. During the test, the relative displacement of the upper block respect to the plane is monitored and the inclination angle of the plane, for which the sliding of the block occurs, is researched. According to the EN ISO 12957-2, the interface friction angle \(\varphi\) is equal to the plane inclination angle \(\beta\) reached when the upper block showed a reference displacement of 50 mm:  
\[tan\ \varphi=tan\beta_{\rm 50 mm} \tag{1}\]  
Referring to this assumption, it is important to note that the displacement value of 50 mm, taken in analogy with the direct shear test, is actually an arbitrary reference; to remember that this friction angle is evaluated according to the standard EN ISO 12957-2, in the following, it will be denoted as \"standard friction angle\" (\(\varphi_{\rm stand}\)). Moreover, we should point out that Equation (1) is correct only in the hypothesis that the block is in a static equilibrium, while its condition is more properly kinematic, given that it is in motion with speed and acceleration that are not always negligible [13]. For these reasons, some attempts to improve the inclined plane procedure are still in progress [14], as well as to research alternative procedures [15; 16]. On the differences between the two methodologies, it should first be noted that the direct shear test is carried out at three different values of normal stress, in order to delineate the failure envelope, while the inclined plane test is conducted at a single value of normal stress and consequently it is interpreted in terms of secant friction angle, rather than the tangential one. This is not the only difference: for example, the range of normal stress, usually investigated, is higher for the direct shear test. The common laboratory shear devices allow perform tests under normal stress varying between extreme values of about 25 kPa and 500 kPa.