## 3 Results
### Monotonic Loading Conditions
By adopting the Mohr-Coulomb failure criteria, the interface strength parameters, specifically the friction angle (\(\delta\)) and apparent cohesion (\(\mathrm{c_{a}}\)), were determined. Comparing the parameters derived from both types of equipment, it can be observed that the friction angle increases (from 13\({}^{\circ}\) to 19\({}^{\circ}\)), while the apparent cohesion decreases (from 38.6 to 30.2 kPa) when the small pullout box is used. Additionally, it is noted that the maximum pullout shear stress values are relatively similar, particularly under 50 kPa confining stress, indicating a different trend from that observed for the pullout resistance results. Figure 8: Monotonic pullout force versus frontal displacement of geogrid in small (SB) and large (LB) pullout boxes. The geogrid stiffness is an important parameter for several reinforcement applications, including reinforced soil structures and pavement design. The design methods of reinforced pavement base, such as that developed by [PERSON] and [PERSON] [49], for example, use the unconfined stiffness (J\({}_{\text{N}}\)) for designing the base layer. As the reinforcement is under confined conditions in the field, the confined tensile stiffness (J\({}_{\text{C}}\)) that can better represent the actual geogrid stiffness under field conditions was obtained in this study from the pullout test results. While the pullout test may not be the optimal method for this purpose, the criterion for assessing the confined stiffness was established as the ratio of the pullout force to the deformation of the geosynthetic between two designated measurement points, as defined in Equation (5):  
\[J_{C}=\frac{P_{R}}{\left(\frac{X_{i}-X_{i+1}}{d}\right)} \tag{5}\]  
where \(d\) represents the original distance between two adjacent measurement points, and \(X_{i}\) denotes the displacement of the \(i_{\text{th}}\) geogrid measurement point starting from the geogrid front end. For the purpose of this study, as shown in Table 3, analyses of confined tensile stiffness were conducted at 2% strain, considering the intermediate tell-tales (tell-tales T2-T3) for both types of equipment. Additionally, Figure 10 depicts the evolution of confined stiffness with increasing geogrid strain values. The same method and test apparatus were previously used by [PERSON] and [PERSON] [38] to determine the confined stiffness. Similarly, refs. [14; 16] adopted a comparable approach. Despite variations in equipment and materials, the stiffness curves presented in this study exhibited the same order of magnitude as those obtained by [PERSON] et al. [16]. Figure 10 indicates that, under lower normal stresses, the smaller box resulted in higher confined stiffness values, particularly for small strains (<2%). However, at a 2% strain, both types of equipment showed similar results. Specifically, for confining stresses of 25 and 50 kPa, the confined tensile stiffness decreased by 12.8% and 11.9%, respectively, when the larger box was utilized. However, a different behavior was observed under higher confining stress (100 kPa), where the influence of equipment scale was less significant, and both types of equipment presented relatively similar confined tensile stiffness values. Additionally, the confined tensile stiffness tended to increase with the confining stress, as also shown in Table 3.  
Figure 9: Evolution of maximum pullout shear stress with applied normal stress.