# 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
Conversely, it is difficult to manage very low normal stress, because to the way in which the normal load is applied, that is through a hydraulic press. On the other hand, the inclined plane test is carried out only under very low normal stresses, generally equal to 5 kPa, which can be eventually increased to slightly higher values, up to about 10-15 kPa. This difference of the investigated range of normal stress makes difficult the comparison of the results obtainable with the two types of tests, the direct shear and the inclined plane, and consequently the evaluation of the equivalence of the two approaches. However, the results reported in some studies [17; 18] seem to indicate that the extrapolation of the results of direct shear tests to the field of low normal stresses may overestimate the interface shear strength with respect to the strength parameters provided by inclined plane tests. In this regard, it should be pointed out that the failure envelope of the interfaces between geosynthetics can be curvilinear and, therefore, the choice of the type of laboratory tests should be related to the actual stress condition to which the interface is subjected on site. For example, using the results of direct shear tests, performed as said at medium-high stresses, for the design of the capping of a landfill, condition characterized by low vertical stress, may be very little precautionary. Similarly, the opposite case of the extension of the inclined plane test results to the design of lateral liners, where the geosynthetics are subjected to high normal stresses, could be unsafe. In addition to the different stress range, another possible reason for differences between the results provided by the two types of test may be the different way of application of the shear stress. Indeed, in the case of direct shear, the interface experiences an imposed displacement at constant speed of 1 mm/min, while the response is evaluated in terms of shear force. On the contrary, in the inclined plane the interface is subjected to a gradually increasing shear stress, given by the component of the weight parallel to the plane, which increases as the plane inclination increases, and the interface response is detected in terms of block displacement. In summary, in the first case a displacement is imposed and the shear force is detected while in the second case a shear force is imposed and the displacement is detected. Although few studies on this subject are currently available [19], it can be stated that the approach followed in the inclined plane test is much more similar to the real kinematics involved by the sliding of an interface placed on a slope. Moreover, the findings of recent research seem to suggest that the direct shear approach could overestimate the friction when the interface can manifest very slow sliding phenomena, at speeds lower than that usually imposed in the direct shear test [14]. Continuing the discussion, another aspect to consider is the maximum displacement, allowed by the two devices, that is generally noticeably different. In the case of the direct shear, the standard requires a displacement of 50 mm and, in general, the maximum stroke of the devices is slightly higher than this value. This level of displacement may be insufficient to detect the residual shear resistance [8], but the only way to study the shear strength at large displacements with these devices is to perform many cycles of displacement, with motion reversal. However, this approach does not exactly correspond to the physics of the real phenomenon, since it involves a reordering of the geosynthetic fibers, or a different action on the microscopic asperities, at each inversion of the motion direction. This problem does not arise in the case of inclined plane tests, in which the maximum interface displacement can be several tens of centimeters, depending only on the length of the plane over which the block slides.