## 3 General Characteristics of Seismic Response  
### Liquefaction Characteristics of Soils  
To provide a comprehensive understanding of the seismic response of a non-homogeneous liquefiable soil-underground utility tunnel system, this section analyses the liquefaction characteristics of the soil, and the internal forces and uplift of the structure during the earthquake for the numerical model with a 2 m burial depth. Control points are set in the far-field soils at different depths, where points A and D are located at the midpoints of the top and bottom non-liquefiable soils, respectively, and points B and C are located at different depths of the liquefiable layer. As can be seen from Figure 7, there is a significant attenuation of soil acceleration in the model compared to the input ground motion. The main manifestation of soil liquefaction is the accumulation of pore water pressure and the reduction in effective stress. The pore water pressure ratio, \(r_{u}=\frac{\Delta u}{\sigma_{v0}^{2}}\), the ratio of excess pore water pressure to initial vertical effective stress, is used in this study to represent the liquefaction of the soil [7]. Figure 8 shows the pore water pressure ratio (EPPR) of the soil at point B at \(-4\) m depth, point C at \(-8\) m depth in the far-field liquefaction layer, and point M under the center of the structure. Under seismic loading, the pore pressure ratios of the soils at points B and C in the far field do not reach 1.0, indicating that the soils have not yet liquefied here, but the pore pressure ratio at point B reaches 0.83 at 6.3 s and the pore pressure ratio at point C approaches 0.8 at 8.2 s, indicating a significant increase in excess pore water pressure in the soils. The pore water pressure of the soil below the structure grows rapidly: at 2.3 s, the pore pressure ratio reaches 1.0, and the soil liquefies; at 2.3 s to 6.5 s, the pore pressure ratio is relatively stable; at the later stages of ground motion, the excess pore water pressure of the soil beneath the structure dissipates rapidly, and the pore pressure ratio decreases. Figure 6: Input ground motion acceleration time history and response spectrum. When the soil liquefies, it causes a certain amount of water to flow upward, the development of excess pore water pressure below the structure generates vertical unbalanced forces, and the structure is uplifted by the action of the surrounding liquefied soil, as shown in Figure 10. The soil on both sides of the structure moves toward the bottom of the structure to form a circulation loop during the earthquake. Figure 11 shows the vertical displacement of the middle point of the top slab and the soil around the structure (located at the same horizontal height as the top slab) during the earthquake, where the uplift of the structure is accompanied by the settlement of the surrounding soil. At about 2 s the pore water pressure ratio of the soil below the structure reaches 1.0, and the structure starts to float; from 2 s to 10 s, the vertical displacement of the structure increases significantly accompanied by strong ground motion; from 10 s to the end of the earthquake, the seismic acceleration amplitude decreases, the excess pore water pressure partially dissipates, the pore pressure ratio decreases, and the uplift of the structure gradually stabilizes. \begin{table}
\begin{tabular}{c c c c} \hline  & **Location** & \
\begin{tabular}{c} **Moment** \\ **(kN\(\cdot\)m\({}^{2}\)/m)** \\ \end{tabular} &
\begin{tabular}{c} **Shear Force** \\ **(kN/m)** \\ \end{tabular} \\ \hline Left side wall & LW-1 & 64.6 & 170.9 \\  & LW-2 & 73.2 & 112.5 \\  & LW-3 & 64.5 & 63.4 \\  & LW-4 & 51.1 & 162.6 \\  & LW-5 & 97.2 & 275.9 \\  & LP-1 & 7.7 & 8.3 \\  & LP-2 & 5.4 & 9.6 \\ Left internal partition & LP-3 & 3.7 & 9.5 \\ wall & LP-4 & 5.6 & 8.3 \\  & LP-5 & 10.6 & 9 \\ \hline \end{tabular}
\end{table} 
Table 2: Distribution of bending moments and shear forces of structural members in the 2 m burial depth model.