# Geosynthetic Interface Friction at Low Normal Stress: Two Approaches with Increasing Shear Loading
## 3 Devices  
In this work, two different devices were adopted. The first device was an inclined plane apparatus (Figure 1), characterized by a plane with a length of 1.10 m and width of 0.25 m and a steel block with a contact base of 0.21 m \(\times\) 0.42 m.  
The first geosynthetic specimen was fixed to the base of the block, while the second was fixed on the plane. In order to ensure a straight sliding, the block was constrained by two lateral guides and it was equipped with four lateral wheels, to avoid introducing significant additional friction forces. The inclination of the plane was measured by means of an accelerometer, while the block displacement was detected with a Linear Variable Displacement Transducer (LVDT). For the block, a rectangular shape was adopted, instead of the standard measures of 0.30 m \(\times\) 0.30 m, to minimize the effects of the normal load eccentricity, while maintaining the same contact area of 0.09 m\({}^{2}\). In this regard, in the device setup adopted in this research, the height of the center of gravity of the block, for a mean normal stress of 5 kPa, was of only 34 mm and the center of mass was backward of about 12 mm. This configuration allowed to be obtained a uniform distribution of the normal  
Figure 1: The inclined plane device: (**a**) side view and (**b**) sketch.  
stresses for an inclination angle of 21\({}^{\circ}\), reputed to be an acceptable compromise between the maximum and minimum values of interface friction observed during the tests.  
The second device was a not standard apparatus, similar to a direct shear except for the way the shear stress was applied. It was composed of a steel block, with dimensions of 0.30 m \(\times\) 0.30 m, resting on a horizontal plane. Also in this case, the first geosynthetic specimen was fixed to the base of the block, while the second was fixed on the plane. The block was hooked, with the interposition of a load cell, to a steel cable at the end of which, after a series of pulley, a counterweight was attached (Figure 2). The test was carried out by gradually varying the shear force over time by means of a simple mechanism based on a hopper, through which some sand drops smoothly into a bucket (the counterweight) and ended when the block displacement reached the values of 50 mm.  
During the test, the horizontal force applied to the block was detected by the load cell while the block displacement was measured by a UVDT. Starting from the hypothesis of a static condition, the measurement of the horizontal force applied (H), related to the weight of the block (W), makes possible the evaluation of the mobilized friction angle, time by time, by means of the following simple equation:  
\[tan\ \varphi=\frac{\mathrm{H}}{\mathrm{W}} \tag{2}\]  
Once the limit static equilibrium is reached, Equation (2) would no longer be valid, but, in analogy with what is normally done for the inclined plane test, it was extended to the whole test, even if the speed and acceleration of the block were not actually negligible.  
To remark the difference with the usual direct shear, in the following this type of test will be indicated as SSI test, where the acronym indicates that the experiment was carried out following a shear stress increase.