# Local Stiffness Assessment of Geogrid-Stabilized Unbound Aggregates in a Large-Scale Testbed
## 3 Experimental Setup
### Large-Scale Testbed Equipped with BE Field Sensors  
The aggregate material, stabilized with a geogrid, was evaluated in a laboratory large-scale testbed through shear wave velocity measurements using the BE field sensors. The BE field sensor consists of a pair of BEs, a BE sensor-protection module, and a metal frame. The metal frame helps to fix BE sensor locations, which will maintain the fixed distance and orientation of both source and receiver sensors. Meanwhile, they also protect the cables [30].  
The same measurement system described above was used for the BE field sensor but with the addition of an amplifier, which can amplify the input signal transmitted from the signal generator into the source sensor by up to six times. The amplification is necessary for improving the signal's quality due to the longer propagation distance and higher energy loss. A schematic drawing of the BE field sensor used in this study (dimensions included), along with the measurement system, is shown in Figure 3.  
A large-scale testbed was utilized for evaluating the geogrid performance in mechanical stabilization. The testbed was built as a box with 182.9 cm (6-ft) length by 182.9 cm (6-ft) width by 61.0 cm (2-ft) height dimensions to provide sufficient space for constructing an aggregate layer. Oven-dried materials were used to maintain the material's conditions consistent throughout this study. The achieved density of the material was around 18.61 \(\mathrm{kN/m^{3}}\) (118.5 pcf), which is much lower than the MDD value owing to the dry condition. Previous studies have demonstrated that expanded polystyrene (EPS) geofoams can be used as a uniform subgrade material with relatively low strength [42; 43]. Thus, a 15.2 cm (6-in.) thick EPS geofoam was placed at the bottom of the testbed to function as a soft subgrade. The equivalent California bearing ratio (CBR) for the geofoam is determined as 5%, which is similar to a low-strength fine-graded soft subgrade soil [44]. The aggregate layer was constructed above the geofoam in four lifts, and each lift is approximately 115 mm (4.5-in.) with adjustments to accommodate the BE field-sensor location. The compaction  
Figure 3: Schematic of a bender element field sensor embedded in the base layer, along with the shear wave measurement system.  
was conducted using a vibratory plate compact with a consistent number of passes on top of each lift for uniform compaction. Further, the coarse aggregates near the BE sensor module were compacted through hand-tamping to ensure proper contact between the sensor and the aggregate material. The finished base course layer was 45.8 cm (18-in.) thick.  
In this study, the text matrix consisted of seven test scenarios with different geogrid placement and different BE field-sensor locations, as shown in Figure 4 and Table 3. Geogrids (GG1 or GG2) were placed 10 cm (4-in.) above the geofoam. When GG1 was placed, the BE field sensor was only installed at the bottom location, which is 2.5 cm (1-in.) above the geogrid. When GG2 was placed, the BE field sensor was placed at the bottom, middle [15.2 cm (6-in.) above the geogrid], and top [25.4 cm (10-in.) above the geogrid] locations. Geogrids were placed in the testbed without fixing them to the side of the box, which replicates the typical roadway construction practices. For the control tests without a geogrid, the BE field sensor was also placed at the bottom, middle, and top locations for comparison. A schematic drawing of the testbed is shown in Figure 4a, and the BE field sensor placement at the bottom location over a geogrid is shown in Figure 4b. The seven test scenarios with BE field-sensor locations and geogrid placements are summarized in Table 3 with the designated test number (Test no.).