# Intelligent Compaction Measurement Value in Variability Control of Subgrade Compaction Quality
## 4 Results and Discussion
### Variation Laws of Overall Compaction Quality of Subgrade
#### 4.1.1 Thickness of Filling Layer  
Experimental data of three conditions (3)4(5) in Table 3) are analyzed to develop the influence of thickness of the filling layer on the overall compaction quality of the subgrade in Figure 5. Figure 5a shows that the distribution curve of the final compaction value with different thickness of filling layer has an obvious stratification phenomenon at the same roller passes. When the thickness of the filling layer is increasing, the final compaction value \(E_{vib}\) decreases gradually, which indicates that the overall compaction quality of the subgrade decreases with the increasing thickness of the filling layer.  
It was found that the difference of \(E_{vib}\) between thicknesses of 40 cm and 50 cm is very small. However, the final compaction value in thickness of 60 cm is much lower than that in thicknesses of 40 cm and 50 cm. Two possible reasons can be explained for the phenomenon. One reason is that the subgrade with a thinner filling layer has reached a higher strength before the 8 th roller pass. Interestingly, it is not obvious to improve the strength of the filling layer. However, the strength of the subgrade with a slightly thicker filling layer increases relatively with more roller passes. Another reason is related to the effective compaction depth of the vibratory roller. It was provided that the compaction effect of a 20-ton vibratory roller on the filling thickness of 40 cm is the best, but the compaction effect of it on the filling thickness of 60 cm is poor [20]. Other research shows that the dynamic pressure produced by the vibratory roller decreases sharply at the depth of 40-60 cm [21]. When the filling layer is thicker, it is difficult to transmit the dynamic pressure produced by the vibratory roller to the lower part of the filling layer, which makes the lower part difficult to be compacted, and then, it affects the compaction effect of the upper filling layer.  
Figure 4: The 3D FE model established with the ABAQUS program: (**a**) mesh generation of elastic double-deck subgrade; (**b**) simulation results of deformation.  
Figure 5b,c show that the compaction quality of the subgrade decreases at any stage of rolling with increases in the thickness of the filling layer. The lines shown on these figures are presented to show that the difference in \(E_{vib}\) between 40 cm thickness of the filling layer and 50 cm thickness of the filling layer gradually narrows in the last three roller passes. The strength growth of the subgrade is slowed down because the overall strength of the subgrade has been already high in the case of the 40 cm filling layer. Comparatively, it is too thick for the 60 cm filling layer to be fully compacted, and then the subgrade strength is still increasing slowly after each roller. This variation law is consistent with the deduction of this phenomenon based on the depth of the compaction effect.  
As shown in Figure 5d, it can be referred that increasing the subgrade strength mostly concentrates on the first four roller passes, and then, it increases slightly on the last four roller passes. According to the suggestion in Technical Conditions of Intelligent Compaction Control System for Highway Subgrade Filling Engineering [19], it can be determined that the strength reaches a stable state, in which the compaction quality will not be changed significantly again as the increased rate of \(E_{vib}\) of two adjacent rollers does not exceed 1%.