## 4 Results and Discussion
### Evaluation of pavement Foundation Drainage Performance  
The drainage performance of the pavement foundation layer significantly affects the performance and durability of the pavement surface system. At the CRCP-12 site, Photos taken during the field investigation process are shown in Figure 10a. Field observations revealed severe random and corner cracking in the pavement and the water accumulated close to the curb. The water accumulation can cause insufficient bearing capacity of the foundation. Reduction in the bearing capacity of the subgrade materials may not only lead to a non-uniform support condition but also cause subgrade erosion. Additionally, the test section of the site CRCP-12 is close to a manhole and sewer pipelines (i.e., green paint marks the pipeline directions in Figure 10), where the subgrade material is likely less well-compacted, which makes it more susceptible to scoured and loose material. The GPR scan results show that a 5 cm thick layer was observed between the concrete and the bottom foundation layer as shown in Figure 10b. This was confirmed by the observation at the bottom of the core hole shown in Figure 10c. This further suggests that the pressure-injected layer was applied to address the erosion and loosening of less compacted materials. To quantitatively assess the drainage performance of pavement foundation materials, the CHP tests were performed in core holes at each site. The CHP test results are summarized in Table 3. Tests performed on surface crack locations were marked as cracks, tests on regular surface locations were labeled as regular, and tests performed at joints were marked as joints. The calculated hydraulic conductivity values (K\({}_{\text{CHP}}\)) for the core holes without cracks are denoted as regular as shown in Table 3 and are representative of each testing site. The foundation layers provide better drainage at sites CRCP-23-B and CRCP-12, which have fewer fine particles than the other sites as shown in Figure 7. It seems to indicate that more coarse particles and fewer fines are beneficial for better permeability. Figure 6 shows that the CRCP-23-A had more joint deterioration and D-cracking. It indicated that the permeability of the foundation material with relatively fewer fine particles yieldsgood drainage performance, which may lead to a longer service life of the PCC pavement surface structure. The foundation layers provide very poor drainage at the other sites, at site JPCP-4-A, the CHP test result for the core hole at a joint is very low. It indicates that the joints at this site were well-sealed, effectively preventing water infiltration. The K\({}_{\text{CHP}}\) at site CRCP-4 is low but still higher than that of sites JPCP-4-A and JPCP-4-B, suggesting that the foundation of site CRCP-4 has greater permeability. Figure 6 indicates that despite having the same service life, site CRCP-4 exhibited more significant premature failure than the other two sites. This could potentially be related to water entering the foundation, leading to the washout of foundation materials and consequently resulting in insufficient bearing capacity of the subgrade. Additionally, the CHP test results indicate that most core holes with cracks show higher permeability compared to those without cracks. This suggests that the cracks allow water to more easily seep into the foundation, resulting in higher flow rates beneath the cracked slabs. Figure 11 shows the observed cracking pattern and corresponding GPR scan results at site CRCP-23-A. Figure 11a shows a photo taken during the field investigation, also depicting water accumulation near the curb and severe random cracks and corner cracks on the pavement. The cracks allow water to seep into the foundation, which may accelerate  
Figure 10: (**a**) Fautilted concrete slab, (**b**) GPR 2D scan result, and (**c**) injected material at Site CRCP-12.