## 2 Experimental Setup
### Drying-Wetting Cycle Process  
To explore the effect of the D-W cycles on the dynamic properties of GRS in the railway subgrade, the soil samples subjected to D-W cycles were used to conduct the dynamic triaxial tests. According to the environmental parameters, the severe D-W cycles of the railway subgrade of Xiamen in Fujian province caused by the summer monsoon climate conditions were simulated, which included the highest temperature of about 55 \({}^{\circ}\)C (that is close to that at the ground surface in summer) and the moisture content of approximately 10% (which is equal to the moisture content of subgrade in-situ). The prepared soil samples in OMC were all wetted to saturation and were then subjected to the D-W cycles. Each D-W cycle includes three phases, namely, drying, wetting and saturation. The first D-W cycle process for soil samples involves three steps: initial moisture content to saturation, drying to a 10% moisture content state, and then saturation again. The subsequent D-W cycles of a soil sample are considered to have undergone a D-W cycle as long as it dries from the saturated state to 10% moisture content and then wets to saturation. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} Soil \\ **No.** \\ \end{tabular}  } & \multicolumn{4}{c}{_Centa Size Functions/\({}^{\circ}\)._} & \multicolumn{4}{c}{_Liquid_} & \multicolumn{4}{c}{_Plastic_} & \multicolumn{4}{c}{_Hasticity_} & \multicolumn{4}{c}{_Confinement_} & \multicolumn{4}{c}{_Optimal_} & \multicolumn{4}{c}{_Maximum_} \\  & **d** \(<\)**0.875 mm** & **d** \(<\)**0.51 mm** & **d** \(<\)**0.25 mm** & **d** \(<\)**0.5 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** & **d** \(<\)**2 mm** \\ \hline C0 & 0 & 5.50 To find an optimum maximum number of D-W cycles, a series of preliminary tests were conducted with respect to 5, 7, 9, and 11 D-W cycles under the same testing conditions. The results indicated that their stress-strain curves of 7, 9, and 11 D-W cycles are very close. Thus, seven D-W cycles were used as the maximum D-W cycles from a time-saving point of view. In the present study, GRS samples with different D-W cycles were prepared via the following procedures (see Figure 1):  
(1) Drying process. The soil samples were put into the oven at 55 \({}^{\circ}\)C for about 6 h (to simulate real drying field conditions), then taken out and left for 24 h to cool down naturally to room temperature. Finally, the samples were weighed to determine the change in moisture content over time. The trial test results showed that the change of moisture content is less than 0.5%, and the moisture content remains around 10%, indicating that the samples at 10% moisture content are in completely dry conditions. (2) Wetting process. The samples were first pasted by the wet filter papers, then sprayed uniformly with water every 10 min using an electric sprayer. The wetting process is continuously monitored for soil sample quality and moisture content, and it is terminated when neither changes. (3) Saturation process. The saturation process includes vacuum saturation and back-pressure saturation. In vacuum saturation, triaxial saturators filled with the soil samples were placed in a barrel for vacuum saturation. The triaxial saturator is shown in Figure 1. Each triaxial saturator had a permeable stone at each end, and the specimen was placed between two permeable stones. During the saturation process, the specimen was saturated by exhaust and water absorption through two permeable stones. The procedure was as follows: First, the soil samples were vacuumed in the saturator with a vacuum pump for 2 h, then the vacuum pump valve was closed, and the water filling valve was opened to add water.