## 1 Induction
Second, the traffic must be blocked when the roadbed is repaired, which brings a great burden to the railway line planning. Grouting reinforcement method [11; 12] not only reinforces the roadbed but also does not affect the normal traffic, which is very suitable for the application of traffic infrastructure maintenance. For the high-speed railway subgrade reinforcement and lifting, the slurry needs to have the characteristics of good expansion, high strength after consolidation, and fast condensation speed [13; 14; 15; 16; 17]. At present, the cement slurry commonly used in the engineering has the disadvantage of small expansion force and fragility after hardening, which makes the long-term use effect of roadbed repaired unsatisfactory [18; 19]. In recent years, polyurethane polymer chemical slurry has developed well, which not only has the characteristics of high fluidity, high strength, and fast condensation speed but also has the characteristics of good control of expansion pressure, uniform uplift of pavement, and stable long-term use effect. Therefore, it has been widely used in road and airport runway reinforcement projects, for instance, [PERSON] et al. determined the layout and construction of grouting holes according to the construction technology and verified that the preventive reinforcement effect of polymer grouting is good by the FWD and GPR [20]. [PERSON] et al. studied the polymer constraining force at channel bottom by numerical simulation and found that the degree of common pavement structure parameters based on slab corner filling polymer materials can be obtained with density of 0.303-0.372 g/cm\({}^{3}\), density of polymer material at the edge of the plate is 0.263-0.315 g/cm\({}^{3}\), density of polymer material at middle plate is 0.207-0.25 g/cm\({}^{3}\)[21]. [PERSON] et al. investigated the influence of fuel-resistant polymer on the consistency and performance properties of asphalt binder and hot mix asphalt (HMA) mixtures and found that 1% PMB is an optimal proportion of Elvaloy in asphalt concrete mixtures, yielding maximum MR values [22]. However, the uneven settlement of high-speed railway has very high requirements on the uniform lifting and the dynamic characteristics of the roadbed after lifting. At present, there is little research on polymer grouting technology in settlement and lifting repair of high-speed railway. Only [PERSON] et al. [23] tested the dynamic cumulative settlement of subgrade and the deformation and shrinkage of polymer under cyclic loading in large cycles through large-scale physical model tests. However, it does not involve the influence of the physical and mechanical properties of polymer specimens on settlement of high-speed railway unballasted track. Therefore, it is urgent to investigate the mechanism of polymer subsidence and uplift in railway subgrade, improve the grouting technology and theory, and provide technical support for engineering application. In this study, considering the physical and mechanical properties of polymer filling, the obtained law is closer to reality so as to provide technical support for the selection of parameters in engineering practice. In detail, the expansion force characteristics of different density polymer materials and the stress-deformation curves at corresponding density were tested in laboratory to provide the mechanical parameters of polymer. Then, the spatial dynamics model including high-speed train, unballasted track, and subgrade was established by ABAQUS. The structural and mechanical properties of polymer lifting layers with different thicknesses and densities under train loads were analyzed. Finally, the effectiveness of polymer grouting repair was verified by comparing with the effect of cement slurry repair.