## 2 Experimental Model Tests
### Test Cases  
Tunnels with and without the isolation layer were the experimental and control groups, respectively. The whole loading process was divided into two steps: the first fault movement was achieved by the jack and then seismic excitation was input by the shaking table system.  
Specifically, turning on the four jack oil pressure switches, the tunnel and the surrounding rock at the hanging wall were lifted with a vertical displacement of 1 mm and fixed by the jacks. The ones at the footwall were fixed with zero displacements. Next, two kinds of seismic records, El-Centro and Kobe waves, were successively applied to the tunnel model. Figure 7 presents the acceleration-time histories and Fourier spectra of the input motion. Based on the time similarity ratio of 1:4.08, the El-Centro and Kobe waves were scaled in time to the durations of 8.32 s and 8.32 s, respectively. The peak acceleration of the El-Centro and Kobe waves was adjusted to 0.4g to investigate the effect of strong excitation. The dominant frequencies of El-Centro and Kobe waves were 8.81 and 6.65 Hz, respectively. The input seismic excitation in different directions produces different  
Figure 5: Layout of rubber-sand isolation: (**a**) isolation layer installation, (**b**) relative position of isolation layer and tunnel-surrounding rock.  
Figure 6: Sensor layout: (**a**) longitudinal section, (**b**) cross-section.  
response characteristics [52]. Specifically, the input motion was applied in three directions: orthometric, longitudinal, and perpendicular to the tunnel axis. They were denoted as x-, y-, and z-directions, respectively. It should be emphasized that the z-directional motion occurs in a perfectly vertical direction, orthogonal to the tunnel axis. Thus, a total of 6 loading cases for each group are shown in Table 4. At the end of each working case, white noise with a peak acceleration of 0.05 g was used to check the sensitivity of the sensor and the degree of damage to the model.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**Number**} & \multicolumn{2}{c}{**Creep-Slip**} & \multicolumn{2}{c}{**Input Motion**} \\ \cline{3-6}  & & **Displacement/mm** & **Type** & **Direction** & **PGA/g** \\ \hline \multirow{8}{*}{\#1 (without isolation layer)} & \#1-1 & 1 & El-Centro & x & 0.4 \\  & \#1-2 & 1 & El-Centro & y & 0.4 \\ \cline{1-1}  & \#1-3 & 1 & El-Centro & z & 0.4 \\ \cline{1-1}  & \#1-4 & 1 & Kobe & x & 0.4 \\ \cline{1-1}  & \#1-5 & 1 & Kobe & y & 0.4 \\ \cline{1-1}  & \#1-6 & 1 & Kobe & z & 0.4 \\ \cline{1-1}  & \#2-1 & 1 & El-Centro & x & 0.4 \\ \cline{1-1}  & \#2-2 & 1 & El-Centro & y & 0.4 \\ \cline{1-1} \cline{2-6} \#2 (with isolation layer) & \#2-3 & 1 & El-Centro & z & 0.4 \\ \cline{1-1}  & \#2-4 & 1 & Kobe & x & 0.4 \\ \cline{1-1}  & \#2-5 & 1 & Kobe & y & 0.4 \\ \cline{1-1}  & \#2-6 & 1 & Kobe & z & 0.4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mechanical characteristics of the isolation layer.  
Figure 7: Seismic excitation used in model test: (**a**) accelerogram of El-Centro, (**b**) Fourier spectrum of El-Centro, (**c**) accelerogram of Kobe, (**d**) Fourier spectrum of Kobe.