# 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
The plot of the difference of pullout resistance subjected to various boundary circumstances was presented in Figure 7.  
From Figure 7, under top rigid boundary condition, the difference of pullout resistance between rigid and flexible positive boundary was almost kept constant about 5 kN with the increase of the normal loading. However, under the positive rigid boundary condition, the difference of pullout resistance between rigid and flexible of the top boundary was gradually decreased and tended to be constant at 0.9 kN. From the above analysis, in the reinforced soil structures with rigid facing panel, the effect of top boundary conditions upon the pullout resistance was gradually decreased and tended to be constant. Moreover, the vertical stress can be regarded as the gravity of infill materials; thus, the height of infill materials above the reinforcements should be taken into account while designing reinforced soil structures. In practical engineering, the reinforced earth structures were mainly subjected to the coupling effect of the top and positive boundary condition. In addition to a single boundary condition, the influence of the coupling effect of the boundary conditions on the mechanical response of the geogrid-soil interface was also analyzed in the current study. From Figure 8, with the rise in the normal stress, a common characteristic was that  
Figure 7: Difference of boundary conditions with the increase of normal loading. the difference tended to be constant. The effect of the top rigid boundary condition on the difference between P(R) and P(F) was larger than that of the top flexible boundary condition. The top flexible boundary condition can ensure that the vertical loading on the soil sample was more uniform than rigid. However, according to the above conclusion, the effect of top boundary conditions on the mechanical behavior gradually decreased and tended to be constant with the increase of normal loading. Hence, the difference of T(R)&(P(R)-P(F)) and T(F)&(P(R)-P(F)) tend to be constant with increasing of the vertical loading. In the same way, the effect of positive rigid boundary condition on the difference between T(R) and T(F) was larger than that of the positive flexible boundary condition. Hence, the difference of (T(R)-T(F))&P(R) and (T(R)-T(F))&P(F) tend to be constant with increasing of the vertical loading. At present, the boundary condition of the pullout apparatus was generally rigid at domestic and abroad. However, the flexible boundary condition can imitate and duplicate the mechanical properties of the reinforced interface in the real reinforced structure. As a result, the link between rigid and flexible boundary conditions had to be established in order to offer a reference for the construction of the reinforced soil structure. In the current study, the boundary reduction coefficient (\(BRC\)) was introduced to discuss the effect and variation laws of top or positive boundary condition on the mechanical behavior of the geogrid-soil interface. \[BRC^{*}=\frac{P_{\text{T}(^{*})\&P(^{*})}}{P_{\text{T}(R)\&P(R)}} \tag{1}\]  
where the \(BRC\)-\({}^{*}\) is the reduction coefficient of the corresponding boundary, \(BRC\)-T is the boundary reduction coefficient when the top is the flexible boundary; \(BRC\)-P is the boundary reduction coefficient when the positive is the flexible boundary; \(BRC\)-T&P is the boundary reduction coefficient when the top and positive are both flexible boundary; \(P_{\text{T}(^{*})\&P(^{*})}\) is the pullout resistance under top flexible or positive flexible boundary condition; \(P_{\text{T}(R)\&P(R)}\) is the pullout resistance under top rigid and positive rigid boundary condition. Figure 9 showed the variation laws of the boundary reduction coefficient (\(BRC\)) of the top and positive boundary condition. From Figure 9, both the top and positive boundary affected the pullout resistance, at the same time the \(BRC\)-T was larger than that of the \(BRC\)-P. With regard to the top boundary condition, the \(BRC\)-T was 0.745 under 80 kPa.