## 1 Introduction  
Granite residual soil (GRS), a product of the in situ weathering of igneous parent rocks, is widely distributed in the humid tropical regions of southern China [1; 2], especially in the Fujian province. GRS covers approximately three-fifths of areas in many cities such as Xiamen, Fuzhou, Quanzhou, etc. Since the essential mineral grains of GRS are quartz and kaolinite, while the secondary mineral grains are feldspar, chlorine and other ferric minerals, GRS is characterized by high mica content, loose structure, water sensitivity, and low cohesion [3; 4; 5]. GRS can satisfy most engineering conditions under dry conditions, but its mechanical and compressive properties change drastically after exposure to water. However, the degradation of the mechanical properties for GRS in the railway subgrade is strongly influenced by the rise of moisture content, which causes an interlayer of coarse and fine particles due to the release of contacts between the soil particles subjected to cyclic loading [6; 7]. Owing to the subgrade interlayer, the reduction of the shear strength and compressibility of GRS results in severe damage to the roadbed, such as uneven settlement, track irregularity and mud pumping of the railway subgrade. The cyclic loads, such as seismic actions, periodic waves, tide forces, and traffic or train loads, have a significant influence on the dynamic properties of soils. During cyclic loading, the pore water and soil particles in saturated soils assigned to the load will vary continuously with time, and different fines contents will change the capability of pore water to bear the load [8]. Several studies reported that the fines contents can significantly affect the dynamic properties of the soil [9; 10]. This is due to the fact that fine particles change the contact style and skeleton structure between soil particles [11; 12]. When the fines content is low, these fine particles exist with coarse particles on the skeleton nodes to play the role of a ball, making the contact surface between coarse particles reduce the inter-grain friction. When the fines content becomes high, the soil tends to a dense state, and the static friction between particles increases, which then prevents the misaligned slip between particles [13]. Moreover, the GRS subgrades are commonly subjected to seasonal drying and wetting (D-W) cycles in southern China because of the subtropical climate [14; 15; 16]. After repeated D-W cycles, GRS is prone to cracking due to a series of changes, such as reduced fine particles, coarsened pores, and loose structure, among other characteristics [1; 17; 18]. The reciprocating migration of the pore water, caused by both cyclic loading of train vibration and D-W cycles, drives a further movement of the internal fine particles and contributes to the development of original cracks [19; 20; 21]. The occurrence of cracks and changes in pore space affect the soil water characteristic curve (SWCC) and the hysteresis curve of unsaturated GRS [22; 23; 24; 25], and the shear modulus can be predicted by matrix suction with the help of the SWCC curve. All these factors not only accelerate the formation of coarse-fine grain interlayers of GRS but also cause irreversible structural damage and fatigue failure of the subgrade. Therefore, it is important to understand the dynamic properties of GRS under D-W cycles for different fines contents of GRS. The hysteresis curve describes the dynamic stress-strain relationship curve of soils under cyclic loadings. Its morphology provides a concise visualization of the dynamic soil performance, such as the dynamic deformation, viscous hysteresis, stiffness change, and energy loss. The hysteresis curve and backbone curve together constitute the viscoelastic dynamic constitutive model. It is a well-known fact that dynamic parameters such as the shear modulus \(G_{d}\), damping ratio \(\lambda\), and accumulative plastic strain \(\varepsilon_{sp}\) are important indexes to evaluate the safety and stability of subgrade constructions.