## 4 Test Results and Analysis
### Influence of Gradation Index on Slope Stability  
Through the slope flow test, the relationship between the slope flow value of asphalt concrete with different gradation indexes and the test time was obtained, as shown in Figure 2.  
It can be seen from Figure 2 that under different gradations, the change law of slope flow value was similar to that of test time, which was mainly divided into three stages: the slope flow value was almost linear growth stage; slope flow value gradually stable stage; slope flow value stable stage. That is to say, the slope flow value increased greatly in the first 6 h (the completion rate of slope flow value reached 70-85%), the change of slope flow value tended to be stable in 6-12 h (the completion rate of slope flow value reached 85-100%), and the slope flow value was basically unchanged in 12-48 h. The soil structure  
Figure 1: Test devices and specimens. theory [21; 22] can be used to explain the change of slope flow value and the three stages of test time. (1) In the initial stage of the test, the temperature in the oven increased, the temperature of the asphalt concrete specimen itself increased from room temperature (16 \({}^{\circ}\)C) to 70 \({}^{\circ}\)C, the asphalt mortar softened, and the original structure of the asphalt concrete gradually deformed. The stability of the asphalt concrete was mainly the skeleton structure formed by the coarse aggregate and the cohesion of the asphalt. After the asphalt was softened, it played a lubricating role. Under a certain slope, the specimen was affected by its own gravity, the skeleton structure of the asphalt concrete changed, and the secondary structure began to form. Due to the change in the primary skeleton structure of asphalt concrete, the secondary skeleton structure did not yet form, and the growth rate of slope flow value was large. Therefore, the curve relationship between the slope flow value and the test time was linear at the beginning. (2) As the test continued, the original structure of asphalt concrete was gradually destroyed, and the secondary structure was gradually formed. The slope of the tangent line of the slope flow value and the test time curve decreased, showing a trend of growth weakening, because the secondary structure was gradually stable to resist slope flow, and the slope flow value was gradually stable. (3) From the middle of the test to the later stage, the original structure of the asphalt concrete specimen was completely destroyed, and the secondary structure inside the asphalt concrete was completely generated. The specimen had stability, and the completion rate of the slope flow value can reach 95-100% of the final flow value. (4) Different from the soil structure, asphalt concrete is a multi-level spatial network structure composed of aggregate, filler and asphalt. The properties of different materials play different roles in asphalt concrete, such as high bitumen aggregate ratio and high fluidity. With the increase in test time, as long as the aggregate in asphalt concrete can form a stable structure, even if the asphalt concrete may have a small displacement again, it will gradually stabilize, in general. Figure 3 illustrates the relationship between the gradation index of asphalt concrete and the slope flow value. It can be clearly seen from the figure that the slope flow value decreased with the increase in the gradation index, and the thermal stability of asphalt concrete was enhanced. This was because as the gradation index of asphalt concrete increased, the proportion of coarse aggregate gradually increased, while the proportion of fine aggregate decreased. As the skeleton structure of asphalt concrete, the increase in  
Figure 2: The relationship between the slope flow value of different gradation index and test time. coarse aggregate ratio can improve the slope stability of asphalt concrete. The selection of the gradation index was considered from two aspects of slope stability and asphalt concrete impermeability.