# Effect of Boundary Conditions on the Mechanical Behavior of the Geogrid-Soil Interface
## 3 Results and Discussion
### Effect of Boundary Conditions on the Mechanical Response of the Geogrid-Soil Interface  
Figure 6 depicted a plot of pullout displacement vs. pullout resistance derived from pullout testing of HDPE uniaxial geogrid-reinforced coarse sand under various boundary conditions. The curve of pullout displacement-pullout resistance with varied boundary conditions practically coincided in the beginning stage of the pullout test under the same normal loading, as shown in Figure 6. This demonstrated that when the pullout displacement was minor, the stiff and flexible boundary conditions of the top and positive end had an insignificant effect on the friction characteristics of the reinforced interface. The reason was that due to the small relative displacement, the boundary conditions had not yet worked, the potential energy produced by the interaction of the geogrid-soil was insignificant. However, with the gradual rise in the pullout displacement, the curve of the pullout test with different boundary conditions began to present different effects, and the effect  
Figure 6: The plot of pullout displacement vs. pullout resistance derived from pullout testing of HDPE uniaxial geogrid-reinforced coarse sand under various boundary conditions: (**a**) 80 kPa; (**b**) 100 kPa; (**c**) 120 kPa; (**d**) 200 kPa. became more and more significant. For example, under the vertical loading of 100 kPa, the pullout resistance under the combination of top rigid boundary and positive rigid boundary condition (T(R)&P(R)) was the largest among the four types of boundary conditions, and then the pullout results under the combination of top flexible boundary and positive rigid boundary condition (T(F)&P(R)). Last was the pullout results under the combination of top rigid boundary and positive flexible boundary condition (T(R)&P(F)), the combination of top flexible boundary and positive flexible boundary condition (T(F)&P(F)). The experimental results of lab pullout test showed that the combination of top and positive boundary conditions had a noticeable influence on the mechanical response of reinforced interface. Thus, the boundary conditions cannot be ignored while designing and analyzing reinforced soil structures. In addition, the difference in pullout resistance between T(R)&P(F) and T(F)&P(F) was insignificant. Meanwhile, the pullout test curve presented a trend from hardening to softening with the change of boundary conditions from rigid to flexible. The reason was that under rigid boundary condition, the reinforcement was pulled out at a zero displacement and generally subjected to additional stress during the test. However, the additional stress was gradually decreased when the boundary condition changed from rigid to flexible. Considering the complexity of the combination of the boundary conditions, based on the principle of the single variable, the influence of top and positive boundary conditions on the friction properties of interface was analyzed, respectively. Under the normal loading of 100 kPa, both the positive boundary condition was rigid, namely P(R). When the top boundary condition ranging from rigid to flexible, the pullout resistance changed from 9.20 to 8.199 kN, with a decrease of 10.88%. In the same way, both the top boundary condition was rigid, namely T(R). When the positive boundary condition ranging from rigid to flexible, the pullout resistance changed from 9.20 to 4.123 kN, with a decrease of 49.71%. Stress concentration was the responsible that the pullout resistance of geogrid under rigid positive boundary condition was greater than that of flexible positive boundary condition. In addition, the flexible boundary condition can redistribute the interaction force between the infill materials and the inner wall of test box or loading plate, then reducing the potential energy. On the contrary, under the normal stress of 100 kPa, both the positive boundary condition was flexible, namely P(F).