## 1 Induction  
Unballasted track is the most important structural form of China's high-speed railway, which has the advantages of good smoothness, good stability, long service life, good durability, and less maintenance work [1]. The ballastless track itself has a certain flexural rigidity, which has a certain resistance to the initial settlement of the lower subgrade. However, under the action of multiple factors, e.g., environment, improper construction and repeated load of high-speed train, the subgrade of unballasted track will appear uneven settlement and other disasters to a large extent. It causes stress concentration and cracks in high rigidity track slab, which leads to the decline of long-term service performance of track slab, and then affects the comfort and safety of train operation [2]. Engineering cases related to railway settlement have been reported; for instance, due to subsidence of subgrade caused by traffic loads, the train speed on Tokaido Shinkansen Line in Japan is limited. A high-speed railway in west Germany once stopped for a time due to serious damage to the roadbed after operation [3]. After 8 months of operation, the subsidence of Shanghai Metro Line 1 increased to 30-60 mm and increased by 140 mm in four years [4]. The uneven settlement of subgrade leads to the irregularity of the upper track structure, leading to the stability and safety of high-speed train operation. At present, when dealing with the settlement problem of the unballasted track foundation of high-speed railways, the main treatment methods are the replacement and reinforcement method, the compaction pile reinforcement method, and the geosynthetic material reinforcement method. [PERSON] et al. put forward the use of lightweight foam concrete as the roadbed filling to control the settlement of the subgrade in special soil areas and established a large-scale model of unballasted subgrade filled with lightweight foam concrete. It was found that the strength of lightweight foam concrete with target density of 500-800 kg/m\({}^{3}\) can meet the requirements of both the static and dynamic conditions of ballastless track subgrade [5]. In order to improve the applicability and service life of ballastless track of high-speed railway, [PERSON] et al. proposed epoxy asphalt concrete as the lower structure of railway and studied the mechanical properties of materials and structures by material test and numerical simulation, and found that all the three EACs are suitable for the high-speed railway substructure, and the EACs could improve the cracking resistance and deformability of substructure. [6]. According to the characteristics of collapsible loess foundation in Zhengxi Passenger Dedicated Company, [PERSON] et al. studied the adaptability and effect of subgrade Pile-sheet structure, cement-soil column punching pile, cement-soil compaction pile, and CFG pile to treat collapsible loess foundation, which provided a guidance for practical engineering application [7]. Through the model test of oblique jet grouting pile with different angles, [PERSON] et al. obtained that the bearing capacity of the composite foundation of the oblique jet grouting pile was obviously higher than that of the natural foundation, and the research results could be used for the railway base line disease treatment works [8]. [PERSON] et al. designed a new full-scale laboratory test facility for railway track beds and founded that the settlement of geotechnical grid reinforced subgrade was reduced compared to the original subgrade [9]. [PERSON] et al. used the geotechnical grid to reinforce subgrade and verified that the geogrid could meet the requirements of controlling foundation settlement by monitoring the settlement of foundation [10]. All of the above methods can effectively alleviate the settlement of high-speed railway subgrade. However, there are two problems in these methods. First, the repair time is long and the damage to the original structure is large, which is not suitable for the subsidence disease of the existing high-speed railway roadbed.