## 1 Introduction  
With the increasing shortage of underground space in cities, large diameter twin tunnels with near distances (i.e., smaller than \(0.25\) times diameter) are becoming more common. During tunnel operation, the vibration of a high-speed train disturbs the tunnel where the train runs and affects the structural stability and the train operation safety of the near-spacing adjacent tunnel. The diameter of the tunnel is the main characteristic that will affect the behavior of the tunnel. Therefore, the research of tunnels can classify various tunnels by the size of the tunnel's diameter. There are many tunnels with a diameter of \(d_{1}=5\)-\(10\) m under train vibration [1, 2, 3]. Studies with diameters between \(10\) and \(15\) m mainly focus on the construction stage of the tunnel [4, 5, 6]. The research focused on diameter \(d_{3}\geq 15\) m tunnels is inadequate. Studies focus only on the construction process of \(d_{3}\geq 15\) m tunnels [7, 8] and the seismic characteristics of \(d_{3}\geq 15\) m tunnels during their operation period [9, 10, 11]. The dynamic characteristics of \(d_{3}\geq 15\) m cross-river twin tunnels under train vibration are different from those of small diameter twin tunnels. However, there are still few piecesof research on the dynamic characteristics of a \(d_{3}\geq 15\) m tunnel under the vibration of a high-speed trains during tunnel operation. It is necessary to conduct specialized research on \(d_{3}\geq 15\) m twin tunnels, especially under train vibration. In the study of train vibration in a single tunnel, [PERSON] et al. [12] found that the final settlement of the subway tunnel in the saturated clay layer could reach 80 mm due to repeated train vibration. [PERSON] et al. [13] found that the 2.5-dimensional numerical model of the soil-tunnel system under a moving train load was more accurate than the 2-dimensional model. [PERSON] et al. [14, 15] conducted a series of studies on the dynamic response of a single tunnel under train vibration load by using analytical methods and both 2.5-dimension and 3-dimension numerical methods. [PERSON] et al. conducted a systematic study on the dynamic characteristics of single tunnel structure and its surroundings under train operation vibration [16, 17]. [PERSON] et al. used DEM to study the dynamic characteristics of tunnel segments and surrounding rocks under train vibration in a single tunnel [1,18]. Studies on train vibration in twin tunnels are as follows. [PERSON] et al. studied the influence of precision instruments in nearby laboratories under the vibration of Beijing Metro Line 8 and Line 15 [19]. [PERSON] and [PERSON] conducted a series of studies on the vibration response of trains under parallel-twin tunnels [20] and crossed twin tunnels [21]. [PERSON] et al. [22] studied the impact of high-speed train vibration loads on nearby damaged tunnels. It is necessary to re-construct and anchor the damaged tunnel to ensure the tunnel's safety. [PERSON] et al. [23] established a 2.5-dimensional model that can predict train-induced vibration in tunnels with any cross-section. After that, [PERSON] et al. [24] further proposed a three-dimensional analytical model for the interaction of subway vibration in parallel double tunnels. [PERSON] et al. [25] found that adjacent tunnels and water saturation had significant effects on soil vibration under subway vibration. From the research on train vibration in single and twin tunnels, closely spaced twin tunnels (smaller than 0.25 times diameter) have become more common. In contrast, the research on train vibration has mainly focused on single tunnels. The research on train vibration has rarely focused on twin tunnels, and has especially neglected twin tunnels with a distance smaller than 0.25 times diameter. This paper took the San-yang Road cross-river twin tunnels as the project background and established the small diameter (\(d=6.2\) m) and super-large diameter (\(D=15.2\) m) twin tunnels model by DEM.