# Local Stiffness Assessment of Geogrid-Stabilized Unbound Aggregates in a Large-Scale Testbed
## 5 Results and Discussion
### Vertical Stiffness Profile  
The BE modulus (\(E_{BE}\)) of the unbound aggregate base material was measured at five stages of static loading. Figure 10 presents the modulus depth profiles of the specimens quantified via the BE field sensor measurements. The BE moduli with geogrid stabilization using GG2 are represented by solid lines, and BE moduli without any geogrid stabilization  
Figure 8: Shear wave velocity graphed with bulk stress as determined at the 15 stress states. Figure 9: Relationship between Poisson’s ratio and applied bulk stress. are shown with dashed lines. The BE moduli with geogrid stabilization using GG1 are shown as symbols only. Note that the tests with GG1 were performed with the BE sensor placed at the bottom location only. The modulus of the unbound aggregate material is primarily influenced by the application of confining pressure [47; 54]. Hence, the BE moduli for each test gradually increased with the higher magnitude of applied vertical stress, although the changes in elastic moduli with vertical stress were small, owing to the low magnitudes of the applied vertical stresses. Similarly, the BE modulus gradually increased with depth due to the higher confinement from the overburden, i.e., the weight of the upper layer. A comparison between the BE moduli with and without geogrid stabilization clearly shows the geogrid-stiffening effect. The BE moduli measured from the BE sensors at the middle and the bottom locations with GG2 (i.e., Tests 1, 2, and 4) were higher than the moduli from the BE sensors with no geogrid (i.e., Tests 3 and 5), whereas the BE moduli of stabilized and unstabilized sections were in a similar range at the top location (i.e., Tests 6 and 7). At Load Stage 5, geogrid stabilization with GG2 resulted in 16.2% and 12.9% higher BE moduli at the bottom and middle locations, respectively. A large increase in BE modulus from the top to the middle location indicates the extent of the stiffened zone. On the other hand, there was no geogrid-stiffening effect at the top location. Thus, it can be concluded that the stiffened zone generated by geogrid stabilization extends more than 15.2 cm (6 in.) above the geogrid location, but the geogrid stabilization effect completely diminishes at a height of 25.4 cm (10 in.) above the geogrid. In terms of the geogrid aperture, the extent of the geogrid stiffened zone was between 8 and 13.3 times the geogrid's aperture size. The BE moduli measured from the BE sensors at the bottom location with GG1 and GG2 (i.e., tests 1 and 2) indicate that GG2 provides a higher stiffness enhancement. At Load Stage 5, the shear wave velocity measured at the bottom location increased by 16.2% with GG2 stabilization compared to the unstabilized section, whereas the velocity only increased  
Figure 10: Vertical stiffness profiles of samples with and without geogrid stabilization. by 8.3% with GG1 stabilization. In a previous study, [PERSON] et al. estimated local stiffness enhancements with GG1-and GG2-stabilized aggregates from repeated load triaxial testing and concluded that GG2 stabilization led to a better geogrid-aggregate interlock, which is in accordance with the current test results [29]. Note that the percentage increase in modulus was significantly lower than the percentages achieved in previous studies, which reported modulus increases of up to 250% [28,29]. The difference in the stiffening effect can be attributed to three factors. First, the applied confining pressures and the compaction densities achieved in this study were remarkably lower (-84% of the MDD was achieved in the constructed aggregate lifts in the testbed). Further, an appropriate size ratio between the geogrid aperture and strike-through aggregate particles facilitates interlocking, thus enhancing the effectiveness of the geogrid stabilization [37,55,56,57,58,59]. The geogrid aperture size to D\({}_{50}\) ratio (S/D\({}_{50}\)) ranging from 2 to 3 was reported to achieve the most effective modulus enhancement through interlocking [56,59].