## 3 Calculation Results and Influence Factors Analysis
### Ground Settlement of Special Points Under Different Excavation Steps  
When analyzing the influence of different excavation steps on ground subsidence, the ground zeros above the left and right tunnel axes are selected for double-line tunnels with different excavation intervals. For the tunnel excavated at the same time, due to the symmetry of the model, the settlement of the ground zero point above the left and right axes is the same. Therefore, the ground zero point above the left line is selected for analysis, and the maximum settlement point (the zero point above the symmetry axis) is added, as shown in Figure 7. Here, the lines marked by L, R, and C represent the land subsidence at the zero point of the left line, the zero point of the right line, and the zero point of the symmetry axis, respectively. For each tunnel, this section considers three tunnel spacing conditions of 1.0D, 1.5D, and 2.0D. As shown in Figure 7a, the ground settlement curve of a continuously excavated double-line tunnel at zero point of the left line and zero point of the axis of symmetry consists of two distinct phases. The initial stage is characterized by rapid growth, while the subsequent stage is marked by stability. In the initial rapid growth stage, the two-point settlement curve demonstrates a rapid increase in response to an increase in excavation step length. During this stage, the settlement curve exhibits a linear relationship. In the ensuing stabilization phase, the settlement at both points exhibited relative constancy. As the lateral tunnel spacing increases, the land subsidence on the left axis and the symmetry axis gradually decreases. The maximum ground settlement at the zero point above the left axis of the tunnel is located at steps 15, 14, and 13 for the 1.0D, 1.5D, and 2.0D tunnel spaces, respectively. The findings indicate that settlement achieves its maximum at a faster rate as tunnel spacing increases, and the extent of soil disturbance is more pronounced when tunnels are laterally spaced closer together. For the double-line tunnels with spacings of 1.0D, 1.5D, and 2.0D, the maximum ground settlement at the zero point above the axis of symmetry occurs at the 15 th excavation step. This indicates that the excavation step affects the ground settlement of the synchronously excavated double-line tunnels in an area of about 45 m. Furthermore, the maximum ground settlement at the zero point of the left and  
Figure 6: Comparison of transverse ground subsidence curves of synchronous excavation double-line tunnels under 1.0D, 1.5D, and 2.0D tunnel intervals. right alignments or the zero point of the axis of symmetry increases with the decrease in the spacing of the tunnels. As illustrated in Figure 7b, the settlement curve of the zero point above the left line and the right line of the double-line tunnel with an excavation interval of 24 m can be divided into three stages. The initial phase is the left-line excavation stage, the subsequent phase is the double-line simultaneous excavation stage, and the final phase is the stable stage. In the left-line excavation stage, as only the left line is excavated, the settlement curve of the left-line zero point remains constant under different tunnel spacings. The disruption of the left-line tunnel excavation has the potential to induce a specific degree of settlement at the zero point of the right line. This settlement is observed to diminish with an increase in tunnel spacing. In the double-line simultaneous excavation stage, the left-line tunnel is influenced by the post-excavation of the right-line tunnel.