## 4 Effect of Burial Depth of Utility Tunnels  
### Effects of Burial Depth on the Structural Seismic Response  
This section studies the effect of burial depth on the seismic performance of underground structures by analyzing the deformation, internal forces, and uplift of the utility tunnels, with drift ratio, bending moment and shear force, and vertical displacement of the structure as the measure. The drift ratio is the inter-story displacement of underground structures normalized by height. The Chinese Code for Seismic Design of Buildings (CCSDB) [29] and Chinese Standard for Seismic Design of Underground Structures [30] are mostly consistent in taking 1/250 as the limit value for the elastic-plastic drift ratio of underground structures [31]. As it is shown in Table 3, the drift ratio of the utility tunnel in the earthquake increases with the burial depth, but in general, the effect of burial depth on the drift ratio is not significant. Because of the high stiffness of the structure, the drift ratio is much smaller than the threshold value in the code in all cases. Figure 11: Time histories of vertical displacement of the midpoint of the top slab and the surrounding soil in the 2 m burial depth model. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Burial Depth (m)** & 2 & 2.5 & 3 & 3.5 & 4 \\ \hline
**Drift Ratio (\%)** & 0.0111 & 0.0127 & 0.0139 & 0.0165 & 0.0172 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Maximum drift ratios of the structures in the five numerical models during the earthquake. Figure 10: Vertical displacement and displacement vector of the 2 m burial depth model at the end of the earthquake. Figures 12 and 13 show the envelope of the maximum and minimum moments and shear forces during the ground motion of the left wall for the five burial depth conditions, as well as the residual moments and shear forces, in addition to the design capacity values of the wall. The data in the figure show that the wall remains almost elastic in all conditions, and only in the 4 m burial depth condition, the shear force at the bottom of the wall briefly reaches the shear capacity and approaches the yield state. The residual values show that the distribution of bending moments on the wall is concave, and the bending moment values are larger at the junction of the wall and the top and bottom slabs, and these parts are susceptible to tensile damage; the shear forces at the bottom and top of the wall are larger and susceptible to shear damage. Meanwhile, the shear forces and bending moments distributed on the walls increase significantly with the increase in burial depths, the distributions are more uneven, and the growth of the internal forces at the bottom and top junctions with the slabs are significantly greater than that in the middle of the wall. Figure 14 shows the vertical displacement of the ground surface at the end of the earthquake when the utility tunnel is located at different depths. The structure uplifts and the soils near both sides of the structure settle; the uplift on the left side of the structure is larger than the right side due to the inhomogeneity of the input ground motion and the incomplete symmetry of the utility tunnel. With the increase in the structure covering  
Figure 12: Maximum and minimum envelopes, residual values, and design capacity value of the left wall moment distribution during the earthquake, the data in (**a–e**) are from numerical models for 2 m, 2.5 m, 3 m, 3.5 m and 4 m burial depths, respectively. Figure 13: Maximum and minimum envelopes, residual values, and design capacity value of the left wall shear force distribution during the earthquake, the data in (**a–e**) are from numerical models for 2 m, 2.5 m, 3 m, 3.5 m and 4 m burial depths, respectively. depth, it can be seen that both the uplift and settlement values decrease significantly.