## 4 Conclusions  
This paper comprehensively presented a case study of field monitoring of the deformations and forces of both surrounding rocks and linings during the construction of a representative double-arched tunnel-the Gangkou tunnel. In the typical cross sections corresponding to three different classes of surrounding rocks (class III, IV and V), the internal displacements of surrounding rocks, contact pressures between surrounding rocks and primary linings, axial forces in rock bolts and internal forces both in steel arches and secondary linings were measured scientifically. The dynamic evolution and the spatial distribution characteristics of these monitoring data were analyzed and discussed carefully. The monitoring and analysis results show that the deformations and forces of both surrounding rocks and lining structures are directly related to the construction procedures, geological conditions and measuring locations in the double-arched tunnel. And according to the results, some reasonable suggestions were provided for the improvement of the tunnel construction. From this study, the conclusions are drawn as follows:  
1. The dynamic evolution of the internal displacements of surrounding rocks is closely related to the tunnel construction procedures. The internal displacements of surrounding rocks did not reach the stable state until the completion of the secondary linings. At different locations in the monitoring section, the internal displacements of surrounding rocks at the tunnel vault are greater than that at the tunnel spandrel, which are even greater than that at the side wall of the tunnel. The internal deformations of surrounding rocks are larger in the cross section with lower-level surrounding rock masses. 2. The contact pressure between surrounding rocks and primary linings converged rapidly and it generally reached its stable state within 5 days after the construction of primary linings. The monitoring results show that the existence of the middle wall is able to effectively transfer the surrounding rock pressures on the primary linings near the middle wall, resulting in the decreasing of the contact pressures at that side. The contact pressures between surrounding rocks and primary linings are greater in the cross section with lower-level surrounding rocks. In addition, based on [PERSON]'s theory, a simplified method was proposed to estimate the surrounding rock pressures of the double-arched tunnel and the calculated values are in better agreement with the measured values. 3. Single rock bolt bears pure tensile stresses which are relatively larger in the middle of the rock bolt and lower at the two ends. At different positions in the same cross section, the axial forces in rock bolts at the tunnel vault are higher than that at the spandrel, which are even higher than that at the side wall of the tunnel. Through calculation, the tensile stress of a single bolt is far less than its ultimate tensile stress, which shows that the rock bolt does not play its role to the full potential and it is necessary to take appropriate measures to strengthen the anchoring forces. 4. The internal forces in steel arches increased greatly at the very beginning. As the excavation proceeded, the growth rate gradually slowed down and eventually the forces tended to stabilize after the construction of secondary linings. By analysis of the internal forces at different positions in steel arches, it is found that the steel arch vault is the most disadvantaged loading position. 5. The secondary linings not only serve as a structure for sufficient safety margin but also bear certain loadings in the construction and long-term operation periods of the Gangkou tunnel. The lower the rock mass classification, the larger the internal forces in secondary linings. The internal forces in secondary linings are larger in the cross section with lower class of rock masses. **Acknowledgments:** The authors gratefully acknowledge the financial supports provided by China Railways Corporation under grant No. 2014G004-H for this research.