## 3 Results and Discussion
### Compaction Mechanism of OGFC-5 UIPFC with Different Thickness  
#### 3.2.1 Displacement and Stress of Loading Wall  
Figure 6 shows the displacement curve of the loading wall with respect to the loading time. Remarkably, there was an initial displacement at the time of formal loading because the wall needed to be preloaded before loading. Figure 6 shows the time it took for the SGC model displacement curves of different compaction thicknesses to reach the stable stage ranked 63.5 mm, 40, and 20 mm in descending order.  
The stress curve of the loading wall with respect to time is shown in Figure 7. The wall continued to move downward at a stress below 600 kPa, indicating that the model was still not fully compacted. The compaction tends to stabilize when the stress is maintained at approximately 600 kPa [38]. As shown in Figure 7, the stress of the 20 mm model was stable and fluctuated around 600 kPa at the earliest. Subsequently, the 40 mm and 63.5 mm models reached a stable stage.  
Figure 6: Displacement–time curves of loading wall.  
Figure 7: Stress–time curves of loading wall.  
A conclusion drawn from Figures 6 and 7 is that the 20 mm UPFC model was the first to reach a steady state. On the one hand, this may be because the 20 mm model is the easiest to compact fully, and it enters the stable state first. However, this may be because the 20 mm model could not be fully compacted, leading to a stable state in advance. Therefore, further analysis of the displacement of the loading wall is required to determine its specific cause. Aggregate crushing can affect the degree of interlocking between aggregate particles and the filling effect of asphalt. However, it should be noted that the DEM model does not take into account the effects of aggregate fragmentation and morphology, which may lead to misjudgment of the compaction process, thereby affecting the stability and durability evaluation of the UPFCs. Crushed aggregates require different compaction energies to achieve optimal compactness. Meanwhile, aggregate crushing affects the volume change and compaction curve of UPFCs. Although the DEM model does not consider the aggregate fragmentation and morphology, which may lead to a certain deviation between the time-displacement curve and the actual compaction, the simulation results can basically explain the above-mentioned laboratory test results, that is, when the UPFC target compaction thickness is too small, no matter what compaction parameters are used, the UPFC sample cannot meet the compaction requirements.  
The final displacements of all models with different heights were obtained, representing the compacted thickness of the model. The definition index \(\mathrm{D_{H}}\) is the compacted thickness per unit height of the model, which reflects the difficulty of compaction at different heights. The \(\mathrm{D_{H}}\) value was equal to the displacement of the loaded wall divided by the height of the model. The calculated \(\mathrm{D_{H}}\) values are listed in Table 8. The DH values ranked as D20, D40, and D63.5 in ascending order. This indicates that as the molding height decreased, the model became more difficult to compact. This result explains why a model displacement of 20 mm reached a stable state the earliest, which is consistent with the compaction data in Section 3.1.