# Dynamic Responses of Asphalt Concrete Waterproofing Layer in Ballastless Track
## 1 Introduction  
The waterproofing layer, as an important part in railway substructure, is usually set on the surface of subgrade to prevent the infiltration of water from the surface into the body of the subgrade. The infiltration of water will lead to a decrease of soil strength and cause various engineering issues under the dynamic load of the train, such as mud pumping and subgrade settlement [1]. The application of the waterproofing layer for the seasonally frozen regions is also essential for controlling the frost heave of subgrade [2]. The traditional materials used for the waterproofing layer on the high-speed railway in China is cement concrete. The related field investigations show that the early constructed concrete waterproofing layer in north China is prone to cracking due to the brittleness of concrete materials and because maintenance is relatively difficult [3]. In this case, the asphalt concrete, as a viscoelastic material with both mechanical strength and flexibility, has been applied to the waterproofing layer for the railway subgrade. In recent years, the asphalt concrete was used to replace the traditional granular materials in the subballast layer of ballast railway in the USA, Japan, Italy, Spain and France [4; 5; 6]. Many test sections of asphalt concrete subballast layer were constructed on the railway in these countries and the long-term monitoring results show that asphalt concrete has a good performance of water resistance and crack resistance and can reduce the subgrade deformation; the latter is beneficial for reducing the life cycle cost of railway engineering [7; 8]. In addition, the asphalt concrete was also applied as the track bed for theballastless track and the full-scale test results indicated that the tensile strains at the bottom of the asphalt layer with the thickness of 30 cm are less than 100 \(\mu\)\(\epsilon\), which guarantees a long service life [9]. The application of asphalt concrete for the subgrade in high-speed railway was also carried out in China, but the results of early attempts were not satisfactory, due to the insufficient compaction in the field [10]. Then the mastic asphalt concrete was developed to deal with the field construction problem in cold regions and paved in the test section on the Harbin-Qiqihar high-speed railway in 2014 [11; 12]. The later monitoring results proved the good durability and stability of asphalt concrete waterproofing layer (ACWL) compared to the traditional concrete waterproofing layer [13]. The aforementioned ACWL was paved in the middle and shoulder area of the roadbed, which is not convenient for rapid construction of large machinery and does not bear the load from the superstructure. The full cross-section ACWL was also proposed in China to further improve the function of the waterproofing layer. So, it is necessary to investigate the mechanical behaviors of full cross-section ACWL under the train load considering the load bearing function. The dynamic behavior of asphalt subballast layer has been analyzed by both field tests and numerical simulation. The related research results show that the asphalt subballast layer is helpful to reduce the vibration of the track structure and extend the service life of the whole structure [14; 15]. Moreover, the dynamic response of asphalt concrete for the subballast layer was tested by complex modulus test and the 2S2P1D (2 springs, 2 parabolic creep elements and 1 dashpot) model was used to describe the linear viscoelastic behavior over a wide range of frequencies and temperatures [16]. The excellent waterproofing function of ACWL has been confirmed by many studies, but the research on dynamic responses of full cross-section ACWL is still limited, especially for the ballastless track in China. It is expected that the application range of ACWL will continue to expand with the large-scale construction of high-speed railways and the CRTS III ballastless slab track structure is most promising in China.