## 6 Conclusions  
Numerical modeling and analysis of double-line tunnels under complex geological conditions were carried out. Based on the surface settlement curves, the effects of different tunnel lateral spacings and longitudinal excavation spacings on the surface settlement were analyzed. The relationship between the surface settlement at the characteristic points of double-line tunnels and the excavation step of the tunnels was quantitatively analyzed. The horizontal deformation patterns of the three tunnels were investigated. The surface settlement data for different tunnel spacings were analyzed by linear fitting. The conclusions are as follows. * For the double-line tunnel with continuous excavation, as the tunnel spacing increases, the width of the ground settlement trough gradually increases, and the position of the maximum settlement point gradually moves to the left-line tunnel. The excavation of the right-line tunnel will cause secondary disturbance to the soil. With the increase in  
Figure 10: The relationship between the ground transverse settlement ratio and tunnel spacing in double-line tunnel construction. tunnel spacing, the influence of the right-line tunnel excavation on the left-line tunnel is weakened. * For the cross-excavated tunnels with different excavation intervals, the disturbance of the soil is larger when the post-excavated tunnel is just excavated, and the ground settlement is larger than that of the continuous excavation of the double-line tunnel. With the increase in the distance between the two tunnels, the rise of ground subsidence caused by the excavation of the right tunnel will be weakened or even reversed. The size of the staggered excavation distance has no significant effect on the final ground lateral settlement. The transverse settlement value of the tunnel ground under continuous excavation and staggered excavation decreases with the increase in tunnel spacing. However, the curves of settlement value with tunnel spacing under different excavation distances are parallel, and the difference in the settlement change rate is very small. * For the synchronous excavation of double-line tunnels, the ground subsidence curve shows a 'V'-shaped symmetrical distribution trend. With the decrease in tunnel spacing, the maximum settlement of ground increases gradually, and the width of the settlement trough decreases gradually. * The influence of tunnel lateral spacing on ground subsidence is greater than that of longitudinal excavation spacing. * For the synchronous excavation of double-line tunnels under different excavation steps, the ground subsidence of the two zero points gradually increases and, finally, tends to be stable when the longitudinal excavation range is 45 m. Under different excavation intervals of double-line tunnels, the settlement curves of two zero points show three stages: left-line excavation stage, double-line synchronous excavation stage, and stable stage. In the process of staggered excavation, the ground subsidence of the left-line tunnel and the right-line tunnel will affect each other, and the tunnel excavated first will have a greater impact. For the double-line tunnel with tunnel space \(\geq\) 2.0D, the influence of the post-excavation tunnel on land subsidence is small, and the excavation influence range of single-line tunnel landscape is about 45 m.
* The horizontal ground deformation of the single-line tunnel and synchronous excavation double-line tunnel is centrosymmetric around the zero point of the symmetry axis. However, the ground horizontal deformation zero point of the continuous excavation double-line tunnel will shift to one side of the first tunnel, and the maximum horizontal deformation is greater than that of the synchronous excavation double-line tunnel. At first, the maximum horizontal deformation of the double-line tunnel is larger than that of the single-line tunnel. With the increase in the tunnel spacing, the maximum horizontal deformation of the double-line tunnel will be smaller than that of the single-line tunnel.