# 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
When the top boundary condition ranging from rigid to flexible, the pullout resistance changed from 4.123 to 4.421 kN, with an increase of 7.23%. In the same way, both the top boundary condition was flexible, namely T(F). When the positive boundary condition ranging from rigid to flexible, the pullout resistance changed from 8.199 to 4.421 kN, with a decrease of 46.10%. From the above analysis for the lab pullout tests under various boundary circumstances, it was observed that the effect of the positive boundary condition on the mechanical properties of the interface was significantly greater than that of the top boundary condition, which was consistent with the results obtained from [PERSON] [18] and [PERSON] [29], also indicating the effectiveness of flexible rubber adapted to simulate flexible boundary in the current study. Next, the difference of the top boundary condition ranging from rigid to flexible under rigid positive boundary was slightly greater than that of the flexible positive boundary (10.88% > 7.23%). In the same way, the difference of the positive boundary condition ranging from rigid to flexible under rigid top boundary was less than that of the top flexible boundary (49.71% > 46.10%). It was proven again that the influence of the positive boundary condition was greater than that of the top boundary condition. The reason was that the particles of the geogrid-soil interface would move along with the geogrid during the test, such that the particles would interact with the positive wall of the test box. However, if the inner positive wall of test box was a rigid boundary, its stiffness was far greater than that of the infill materials, and the positive wall of test box would not deform when the particles were interacting with the positive wall. Therefore, a large number of particles would accumulate at the positive wall when the geogrid moved along the pullout direction. With the increase of the density of particles close to the positive wall, the pullout resistance monitored by the data acquisition system increased. On the contrary, if the inner positive wall of the test box was a flexible boundary, the force offered by the geogrid would make the particle move and rotate when the particles were interacted with the positive wall, redistributing stress concentration closed to the positive boundary, and then reducing the potential energy. From the above analysis, it was determined that the pullout resistance under positive rigid boundary measured by the sensor could not truly reflect the mechanical response of the reinforced interface. Therefore, the increment of pullout resistance caused by the stress concentration at the positive wall of test box should be subtracted to be regarded as the real pullout resistance of the reinforced soil interface. The flexible positive boundary can effectively reduce the effect of stress concentration. Based on the above analysis, the positive flexible boundary condition can simulate and reproduce the mechanical characteristics of the geogrid-soil interface in the actual reinforced structure. Hence, the pullout test results under the positive flexible boundary condition should be taken as the basis when designing reinforced structures. In addition, the effect of the positive boundary was greater than that of the top boundary. The reason was that the positive boundary had dissipated most of the internal force of the particles, the potential energy of the particles was low when the particles move to the top boundary. Additionally, from Figure 6, with the rise in the normal loading, the variation in pullout resistance between T(R)&P(R) and T(F)&P(R) was gradually decreased. However, the difference in pullout resistance between T(R)&P(R) and T(R)&P(F) was not obvious. To quantitatively analyze the impact of top and positive boundary conditions on the difference in pullout resistance with the rise in the normal loading, the results of tests were analyzed.