## 2 Site Description and Experimental Section Monitoring
### Monitoring Results of Experimental Section  
During the construction of the experimental section, steel frame distortion, shotcrete crack, small scale tunnel collapse, and primary lining leakage were observed several times (Figure 5).  
The development of the surrounding rock pressure at DK68 + 220 is plotted in Figure 6. The final distribution of the surrounding rock pressure is shown in Figure 7. The positive value indicates a compressive force between the primary lining and rock mass. The stable distribution of the crown settlement of triple tunnels in the experimental section is shown in Figure 8.  
Figure 4: Installation of monitoring instrument: (**a**) Total station; (**b**) Double-membrane pressure cell.  
Figure 3: Layout of monitoring points for each tunnel (monitoring instruments at the knees were installed later).  
Figure 5: Tunnel damages in the experimental section: (**a**) Steel frame distortion; (**b**) Shotcrete crack; (**c**) Small scale tunnel collapse; (**d**) Primary lining leakage.  
Figure 6: Development of surrounding rock pressure at section DK68 + 220: (**a**) Left tunnel; (**b**) Right tunnel; (**c**) Middle tunnel.  
Figure 7: Distribution of surrounding rock pressure at section DK68 + 220 (Unit: kPa).  
According to the monitoring data, we noted the following trends:  
1. For each tunnel, with the stress release of the surrounding ground, the surrounding rock pressure jumped up sharply in the earlier periods. The pressure increments during the top heading excavations primarily accounted for 80% of the final pressure value. It could be inferred that the steel frame and the shotcrete immediately bear rock load after installation.
2. The crown and shoulders were the least favorable parts with considerably larger surrounding rock pressures ranging from 220 to 550 kPa. This trend was also validated by more structural failures, and more small scale tunnel collapses observed at the crown and shoulders.
3. The middle tunnel was found to be the most unfavorable tunnel among the triple tunnels. The mean surrounding rock pressure and the mean crown settlement of the middle tunnel were 275.9 kPa and 60.9 mm, considerably higher than that of the left tunnel (139.7 kPa and 38.8 mm) and the right tunnel (182.9 kPa and 33.6 mm). This behavior might be expressed by that the middle tunnel was excavated in the weak surrounding ground, which had already been severely disturbed by the leading tunnel excavations.
4. Pillar width and excavation sequence greatly influenced the multiple interactions among triple tunnels. Due to the pillar width of 1.65D, the left tunnel was hardly affected by the right tunnel excavation. Conversely, the left tunnel is heavily influenced by the middle tunnel excavation because of the rock pillar of 0.38D. This finding also showed that, with the decrease of pillar width, the rock pillar was more severely worsened and weakened by multiple excavation disturbances.  
In conclusion, due to the complex and intense multiple interactions of triple tunnels, the roof conditions of triple tunnels were considerably deteriorated; the rock mass, especially the rock pillar, might be already weakened and damaged ahead of the tunnel face. The original supports could not meet the safety requirement of triple tunnel constructions. Due to the extreme importance of the station and the high durability of station operation, proper pre-construction techniques must be used.