# Application of Mastic Asphalt Waterproofing Layer in High-Speed Railway Track in Cold Regions
## 4 Field Validation
### Subgrade Water Content Monitoring Results  
The variation of subgrade water content during two years after construction was recorded through buried water content sensors. All the sensors were buried in the depth of 0.25 m from the top of the subgrade, and the layout of water sensors is shown in Figure 9. As the surface water relies on precipitation, the precipitation data acquired from the local weather station are shown in Figure 13 for  
Figure 11: Features of cracks measured in two times of field investigations: (**a**) November 2016; and (**b**) August 2017. Figure 12: Status of construction joint after two years of operation: (**a**) construction joint in good condition; and (**b**) local debonding of construction joint. reference. Figure 13 shows the subgrade water content variations in the line center, left and right side of slab track of three different cross sections, namely cross Sections 1-3. As the test section is located in a seasonally frozen area, the water content of subgrade is influenced by two factors, namely the freezing of water and penetration of surface water. The subgrade water content remains at a low level during winter, due to partially frozen subgrade water, and it increases to a relatively high level after thawing in spring, then the value is mainly affected by penetration of surface water. There is an obvious difference between subgrade water content variation of the test section and the normal section. From the field monitoring data, the subgrade water content in both cross sections of test section is much more stable than the subgrade water content of the cross section in the normal section, indicating better waterproofing performance of MAWL. Meanwhile, the data from line center are always more stable than the data from slab side, which is consistent with the results of field investigation, that is, there are fewer cracks in the middle area than in the shoulder. The waterproof performance of the concrete waterproofing layer obviously deteriorates over time. The increase and extension of cracks reduce its capability of preventing surface water penetration, thus it becomes easier for the surface water to penetrate into subgrade after raining, and it also takes longer time for the seepage water to get out of subgrade via evaporation. As shown in Figure 14, for concrete waterproofing layer in normal section, the amplitude of fluctuation of subgrade water content in 2016 became larger than that of 2015, indicating that subgrade water content became more sensitive to precipitation in the past two years. It is necessary to point out that the measured water content of right side of cross Section 3 is obviously higher than the other two lines, which is caused by the special position of the sensor. As shown in Figure 15, a water groove was built right over the water content sensor in the right side of cross Section 3, and, unfortunately, a cutting-through crack was found on the bottom of the groove. Therefore, the surface water will be collected into the water grove in rainy days, which facilitates the water penetration through the crack, resulting higher subgrade water content than the other two cross sections. The overall behavior of MAWL in the test section was much better than concrete waterproofing layer in the normal section. The subgrade water content in most monitoring points of the test section always remained at a stable level during the two years of the monitoring period. The mastic asphalt waterproofing layer showed good waterproofing performance in both winter and summer days. However, cases of deterioration were also found in the test section. On the right side of cross Section 1, as shown in Figure 14c, the subgrade water content increased slightly after heavy rains in 2015, while the fluctuation became larger in 2016. To find the causes, a closer examination was done around this area during the field investigation in August 2017, and construction joint failure was determined to be the main reason.