# Seismic Response Analysis of Concrete Box-Type Subgrade in High-Speed Railways
## 1 Introduction  
China is located at the junction of the world's two major seismic belts--the Circum-Pacific Seismic Belt and the Eurasian Seismic Belt. It is squeezed by the Pacific Plate, the Indian Plate, and the Philippine Plate, making it prone to earthquakes with historical records of earthquakes spanning over 3000 years. According to the _\"China Seismic Hazard Map\"_[1], more than 50% of China's land is in regions with an earthquake intensity greater than 7 degrees, resulting in a peak ground acceleration exceeding 0.1 g for most of the national territory. As shown in Figure 1[2], nearly 10,000 km of China's \"Eight Vertical and Eight Horizontal\" high-speed railway network traverse areas with high seismic intensity, posing challenges for both the construction and operational maintenance of high-speed railways in the face of earthquake damage. The box-type subgrade structure is a novel subgrade structure proposed by Chinese engineers in recent years, which replaces traditional subgrade fillers with concrete structures [3, 4]. It has the advantages of small land occupation and high bending, as well as torsional stiffness, offering significant potential for widespread applications [5]. It may also be considered a sustainable option to traditional filler subgrades as it reduces the need for natural aggregates [6]. Earthquakes can be considered random loads with immense destructive power for high-speed railway analysis purposes. If the infrastructure design does not adequately consider the effects of earthquakes, the consequences can be extremely severe. Therefore, it is of great theoretical significance and engineering value toconduct a seismic analysis of the box-type subgrade structure, both for its promotion and structural optimization. Currently, there have been no reported studies on the vibration response characteristics of high-speed railway box-type subgrades under seismic action. Considering that the box-type subgrade uses concrete structures to completely replace traditional subgrade fillers, and it is structurally similar to traditional simply supported box girders, the present study focuses on the seismic response analysis methods of simply supported box girders to study the box-type subgrade. Scholars at home and abroad have conducted extensive research on the response of bridges under seismic action. [PERSON] et al. [7; 8; 9] have specifically studied the structural buckling of box culverts and the interaction between the structure and soil, while also considering the support provided by an elastic half-space, rather than a simple Winkler support. [PERSON] et al. [10] studied the dynamic response of bridge structures under various combined seismic excitations. [PERSON] et al. [11] discussed the impact of wave propagation effects on the seismic response of railway bridge systems based on the absolute displacement input method (ADM). [PERSON] et al. [12] explored the impact of track structures on the seismic response of seismic isolation railway bridge systems. [PERSON] et al. [13] proposed an integrated bridge-line analysis model based on the longitudinal constraint effect of the ballastless track system. [PERSON] et al. [14] analyzed the influence of track system constraints on the lateral seismic collision effect of simply supported beam bridges. [PERSON] et al. [15] established a refined spatial model of the CRTS Type-III slab track system on a bridge and conducted research on the dynamic response of the CRTS Type-III slab track system on a bridge under multi-dimensional seismic action. [PERSON] [16] used fiber beam element modeling to analyze the nonlinear elastoplastic seismic response of railway portal piers. [PERSON] [17] conducted a design study on the key structures of large-span steel box portal piers, determining the feasibility of engineering application of portal pier structures. Figure 1: Chinese earthquake records and railway lines. This paper primarily focuses on the basic requirements for seismic design of box-type subgrades.