# Geosynthetic Interface Friction at Low Normal Stress: Two Approaches with Increasing Shear Loading
## 5 Test Results  
Referring to the first interface, GMB-GCD\({}_{1}\), Figure 3, shows a typical result of an inclined plane test, on dry virgin specimens, in terms of displacement of the block as a function of the plane inclination. Remembering that, in this type of test, the interpretation is performed according to Equation (1), by assuming always a static condition for the block, the angle of plane inclination is also equal to the mobilized friction angle. Conversely, Figure 4 shows an example of a direct shear SSI test, on the same interface and on dry virgin specimens; in detail, Figure 4a shows the increasing horizontal force and the corresponding block displacement versus the time, while Figure 4b shows the mobilized friction angle and the block speed as a function of the block displacement. In analogy with \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Interface**} & **Tested** & **Number of Inclined** & **Number of Direct** \\  & **Conditions** & **Plane Test** & **Shear SSI Test** \\ \hline GMB-GCD\({}_{1}\) & dry/wet & 54 & 34 \\ GTXw-GTXw & dry & 22 & 16 \\ GCD\({}_{2}\)-GGR & dry/wet & 35 & 41 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the number of tests carried out for each interface. Figure 3: Results of an inclined plane test on the GMB-GCD\({}_{1}\) interface. what is usually done in the case of the inclined plane test, also in this case the interpretation of the results was made assuming a static equilibrium of the block (Equation (2)), even if the block speed and acceleration were not actually negligible. It is interesting to observe that, virtually flipping the axes of Figure 3, the evolution of the mobilized friction respect to the block displacement is similar to the curve of Figure 4b. Following an approach already presented in other research [22; 23], after the first evaluation, carried out on virgin specimens, the tests were repeated several times, repositioning the block, after each sliding, in the initial position and proceeding in the same way for both the inclined plane and the direct shear SSI tests. Numerous values of the friction angle were thus obtained, related to the amount of the cumulated displacement experienced by the interface, which was pushed up to approximately 1.6 m, thus allowing us to investigate the wear effect [24]. Three pairs of specimens were studied, for each device, in the dry condition; moreover, the tests were repeated in the wet condition on three different pairs of specimens, for the inclined plane, and three others for the direct shear, for a total of 12 pairs of specimens and about 72 tests achieved. A comparison of the results, for the two conditions and for the two types of tests, is showed in Figure 5a, for the dry condition, and in Figure 5b, for the wet condition, in terms of standard friction angle versus cumulated displacement. The data show a good agreement between the two types of tests and a similar trend of the interface friction with the cumulated displacement is highlighted by both the methodologies, corresponding to a moderate reduction of the available friction with the increase of the wear. Albeit theoretically the nonuniformity of the normal stress could change the measured mobilized friction, giving rise to phenomena of progressive failure, in the literature there are not many experimental data on this topic. Consequently, the tests in dry condition were repeated, on another set of three pairs of specimens, with a different mass distribution of the sliding block, to analyze the possible influence of the normal load eccentricity. A uniform distribution of mass, inside the block, was adopted, instead of the configuration with the center of mass positioned backward and previously described.