# Pullout Behavior of a Polymeric Strap in Compacted Dry Granular Material
## 3 Results and Discussion
### The Calculated Values of the Friction Interaction Coefficient  
The pullout tests with one strap were performed to determine the influence of normal stress and grain size distribution on the friction interaction coefficient. From the performed pullout tests, it is found that the friction interaction coefficient at higher normal stress (150 kPa) is 1.5 to 2.0 times lower than at lower normal stress (50 kPa). In the case of the influence of the grain size distribution on the friction interaction coefficient at the same normal stress of 50 kPa, the friction interaction coefficient for backfill material C is 3.2 times higher than the friction interaction coefficient for backfill material A. At a higher normal stress (150 kPa), the difference is smaller. The friction interaction coefficient for backfill material C is 2.8 times higher than the friction interaction coefficient for backfill material B. The comparison of the friction interaction coefficient when pulling out one strap, two separated straps (CS), and two closely spaced straps (CC) shows that the maximum friction interaction coefficient is obtained for two separated straps, followed by for one strap, and, finally, for two closely spaced straps. The values of the friction interaction coefficient from all tests are shown in Figure 13.  
From the experimental results, it can be concluded that the grain size distribution has a significant effect on the friction interaction coefficient and that the relationship recommended by AASHTO [38] for the friction interaction coefficient calculation, where the friction interaction coefficient can be determined by Expression 3 (with a maximum friction angle of 36 degree), is suitable for crushed stone with a Cu < 4 as backfill material. For backfill material B, which meets the Cu > 4 criterion for greater depths in walls, a friction interaction coefficient of 0.79 is obtained experimentally, which is more than the coefficient that would be determined using Expression 3. For the backfill material C, the friction interaction coefficient would be significantly underestimated if using Expression 3. Therefore, it can be concluded that, when the maximum recommended friction angle of 36 degrees is taken in the calculation according to Expression 3, the interaction coefficients for crushed stone with Cu > 4 are significantly underestimated. Considering the fact that the straps are installed in the wall in such a way that two straps are usually located at a small distance from each other, the difference between the recommended values for the interaction coefficient and the measured values is even greater.  
Figure 13: Friction interaction coefficients for different backfill materials [32].  
Figure 13 also shows the values from the literature for the friction interaction coefficients for geosynthetic straps, where it can be seen that the friction interaction coefficients for backfill material A are below the line, while the friction interaction coefficients for backfill material B and C are well above the lines. A comparison with the friction interaction coefficients determined in other studies is shown in Figure 13.