## 4 Results and Discussion
### Permeability Quality Evaluation Test Results  
The permeability coefficient of three permeable asphalt mixtures with different cycles is shown in Figure 8.  
Figure 8: Permeability coefficient of three permeable asphalt mixtures with different cycles in spring and summer. (**a**) Spring clogging procedure; (**b**) Summer clogging procedure. Figure 7: The synthetic gradations of clogging materials in spring. As shown in Figure 8, it can be determined that the permeability coefficients of OGFC, SBS-OGFC, and COF-OGFC were 0.19 cm/s, 0.28 cm/s and 0.24 cm/s at the start of the clogging procedure. It is generally believed that, for permeable asphalt pavement, the smaller the asphalt content, the greater the number of connected pores and the better the permeability. The asphalt contents of OGFC, SBS-OGFC and COF-OGFC were 4.8%, 4.2%, and 5.18%, respectively. SBS-OGFC has the smallest asphalt content, so it has the largest permeability coefficient of the three asphalt mixtures. Interestingly, the asphalt content of COF-OGFC is higher than that of OGFC, but the permeability coefficient is larger than that of OGFC. This is because the oil shale waste has a large specific surface area, dense pore structure and various mesoporous shapes, which means a larger adsorption area and lower probability of redundant asphalt blocking the effective pore. The permeability coefficient of the three mixtures decreased rapidly with the increase of the spring clogging cycles. After eight spring clogging cycles, the permeability coefficients of the OGFC, SBS-OGFC, and COF-OGFC mixtures were 0.11 cm/s, 0.128 cm/s, and 0.122 cm/s, respectively. According to the technical requirements and practical experience of permeable asphalt mixture, it is recommended that the permeability coefficient of the surface layer should be greater than or equal to 0.12 cm/s. Thus, the permeable function of the three permeable asphalts no longer meets the requirements after approximately eight spring clogging cycles. It can be seen from Figure 8b that the permeability coefficient of the three permeable asphalt mixtures decreases with the increase of summer clogging cycles, and the declining trend seems to taper. After 50 summer clogging cycles, the permeability coefficients of the OGFC, SBS-OGFC, and COF-OGFC mixtures were 0.062 cm/s, 0.057 cm/s, and 0.069 cm/s, respectively, and after 20 spring clogging cycles, the permeability coefficients of the OGFC, SBS-OGFC, and COF-OGFC mixtures were 0.064 cm/s, 0.057 cm/s, and 0.072 cm/s, respectively. This also showed that there is a significant difference between the spring clogging behavior and summer clogging behavior of permeable pavement in seasonally frozen regions. The spring clogging behavior belongs to a rapid clogging mechanism, while the summer clogging behavior belongs to a slow clogging mechanism. This may be related to the coverage density of clogging materials, the particle size distribution of the clogging materials, and the porosity change caused by a F-T environment. The porosity of the three permeable asphalt mixtures with different cycles is shown in Figure 9.  
It can be readily observed from Figure 9a that the porosities of OGFC, SBS-OGFC, and COF-OGFC were 19.8%, 22.8%, and 21.3%, respectively, before the physical clogging procedure, which met the requirements of the Chinese standard (i.e., porosity of permeable asphalt pavement \(\geq\) 18%). The sequence of the porosity of the three asphalt mixtures before the physical clogging procedure was SBS-OGFC \(>\) COF-OGFC \(>\) OGFC, which is consistent with the results of the permeability coefficient. The porosity of the three permeable asphalt mixtures decreased rapidly with the increase of cycles. After 20 spring clogging cycles, the porosities of OGFC, SBS-OGFC, and COF-OGFC were 10.34%, 8.27%, and 9.97%, respectively. The decrease of the porosity of the three mixtures was mainly concentrated in  
Figure 9: Porosity of the three permeable asphalt mixtures with different cycles. (**a**) Spring clogging procedure; (**b**) Summer clogging procedure.