## 1 Introduction  
The permanent settlement caused by the cyclic loading of metro trains has greatly increased the operation risk and maintenance cost of urban metro tunnels [1, 2]. On the southeast coast of China, many metro lines cross the silt stratum with rich groundwater and complex geological conditions. In particular, the construction activities above the existing metro tunnels are increasing year by year due to the rapid development of the city [3]. Under the coupling effect of the above foundation pit excavation and the train vibration, the deformation and liquefaction of the silt surrounding existing shield tunnels must be studied. Currently a large number of laboratory tests [4, 5, 6, 7] and field tests [8] on the dynamic characteristics of silt under cyclic loading have been conducted. [PERSON] et al. (1991) pointed out that there is a threshold strain that determines the stable and unstable patterns of the clayey sand under undrained cyclic triaxial loading [9]. In some other research, the threshold stress has been used. When the dynamic stress amplitude exceeds the value, the plastic deformation and excess pore water pressure of the soil increase significantly as the number of cycles increases. When the dynamic stress amplitude is less than the threshold value, the plastic deformation and excess pore water pressure of soil are relatively small [10, 11]. Rapid increase in the excess pore water pressure may cause soil liquefaction [12]. In particular, current research focuses on the influence of fine particles on the liquefaction resistance of soil during earthquakes. [PERSON] et al. (2010) investigated the influence of fine particles on the dynamic properties of silt sand under cyclic loading. The test results reveal that the non-plastic fines have a significant impact on the cyclic resistance ratio (CRR) and pore water pressure accumulation [13]. [PERSON] et al. (2019) studied the effect of clay content and gradation on the liquefaction resistance of sand-clay mixtures through triaxial cyclic tests, and discovered that the liquefaction resistance decreased with the increase of clay content [14]. [PERSON] et al. (2021) systematically summarized the effects of fine particle content and plasticity index on liquefaction resistance of soil [15]. At the same time, the effect of principal stress rotation induced by dynamic stress paths on the deformation and stability of soils has been further studied [16; 17]. As the problem of mud pumping (subgrade fluidization and fines migration into the ballast voids) in high-speed railways becomes more prevalent, research into the effect of fine particle content on the dynamic response of high-speed railway subgrades under a train load is gradually increasing. [PERSON] et al. (2020) studied the cyclic load response of a high-speed railway subgrade with different kaolin contents [18]. The findings show that increasing the content of fine particles reduces the static undrained shear strength of soil mass, improves the plasticity index, and the fluidization resistance of the soil sample. However, considering that mud pumping occurs primarily on the soil surface, the confining pressure of the soil is limited to 20 kPa. Few studies on the development of permanent strain and pore water pressure of surrounding silt under long-term metro cyclic load have been carried out. It is particularly unclear how the fine particle content affects the dynamic characteristics of silt. The change in soil stress state caused by adjacent construction is an important reason for the deformation of the stratum and the existing shield tunnel [19; 20], which also has an impact on the cyclic deformation characteristics of the soil in the influenced zones. Therefore, the influence of surrounding disturbance should be considered when analyzing the dynamic characteristics of the soil around the tunnel under cyclic loading.