## 1 Introduction  
Asphalt concrete has been tentatively used for a full-cross-section waterproofing layer in the substructure of ballastless track in China [1, 2]. Asphalt concrete has also been employed as an isolation layer of the high-speed railway foundation, sub-ballast layer of ballast railway, or reinforcement layer of the railway subgrade in Germany, Japan, France, Italy, etc. [3, 4, 5, 6, 7]. Compared with cement concrete, which is brittle and prone to crack in cold regions, asphalt concrete has the advantages of waterproofing, anti-cracking ability, vibration reduction, and deformation coordination with the subgrade [8, 9]. The accumulation of water in the subgrade can lead to subgrade strength decrease and subgrade settlement. In seasonally frozen regions, the frost heave of the accumulated water in subgrade will cause uneven deformation. Thus, asphalt concrete with good anti-cracking performance is conducive to the integrity and stability of ballastless track [10, 11, 12]. The high-speed railway development offers a broad application market for asphalt concrete. The designer of the high-speed railway usually treats the under-rail structure as a completely continuous state. In other words, slippage or debonding is not allowed between adjacent layers [13; 14]. As a layered structure composed of track-slab, cement concrete base plate (CCBP), asphalt concrete waterproofing layer (ACWL), and cement concrete base or supporting layer, the adjacent layers in the ballastless track subgrade are bonded by the interfacial adhesion force which is much smaller than the strength of the concrete layers [15; 16]. In this regard, the interface is the weakest part of the whole structure. The stiffness difference between asphalt concrete and cement concrete will lead to the inconsistent deformation trend of the adjacent CCBP and ACWL under train dynamic loads, and thus, interface shear stress can be generated. Repeated shear action on the interface between CCBP and ACWL will cause the initiation of interface stress. In Northern China, the annual temperature difference is high, and the temperature changes rapidly in spring and autumn. For example, in Harbin the annual temperature difference exceeds 50 \({}^{\circ}\)C and the daily temperature difference is about 15 \({}^{\circ}\)C in winter. The air temperature changes will cause temperature changes in the ballastless track, and concrete materials tend to deform under temperature loads [17]. The thermal expansion coefficients of asphalt concrete and cement concrete are different, and hence, interface shear stress can be generated by the temperature-stress difference between the adjacent CCBP and ACWL under the fluctuating temperature load [18; 19; 20]. The interface damage accumulates under the cyclic temperature loads and the propagation process of interface cracks is accelerated because of train dynamic loads. The interface cracks offer entry channels for rainwater to infiltrate into the subgrade body which can result in serious freeze-thaw damage in cold regions [21]. The interface cracks can not only weaken the entire structural integrity of the ballastless track subgrade, but also reduce the running stability and derailment safety of the high-speed railway. Therefore, the high-speed railway engineers have paid great attention to the interface safety of the ballastless track and have conducted several research works on the interface delamination mechanism of the typical track subgrade structure [14; 15; 16; 21]. But there has been almost no research on the interface damage of the novel ballastless structure which utilizes ACWL under fluctuating temperature loads and train dynamic loads. The combination of laboratory and field full-scale shear tests and finite element numerical simulations offers excellent possibilities for investigation of the interface properties of layered structures. [PERSON] et al.