## 3 Monitoring Results and Analysis
### Axial Forces in Rock Bolts  
Figures 16 and 17 show the dynamic evolution of the axial forces in rock bolts at B and C locations in the K89+729 section during the tunnel construction. According to the field monitoring data, the axial forces in rock bolts present regular changes in stages: (1) When the installation of the rock bolts was completed, the axial forces in the rock bolts at different locations increased rapidly at a gradually increasing growth rate. After the construction of the primary linings, the values of the axial forces at each point in the rock bolts took up 60% of the final stable-state values and the growth rate of the axial forces began to slow down. This indicates that most of the axial forces in rock bolts were generated during the construction of the primary supports, owing to the fact that the sprayed concrete had not played its full role of supporting during this period, which then made the rock bolts bear the majority of the surrounding rock loadings; (2) After the construction of the primary supports, the axial forces in rock bolts remained on increasing slowly. From the completion of the primary supports to the completion of the second linings, the increments of the axial forces in rock bolts in this period accounted for about 30% of the ultimate stable values; (3) After the completion of the secondary linings, the axial forces in rock bolts increased slightly and then reached a steady state eventually.  
Figure 18 demonstrates the final distribution of the axial forces in rock bolts in different sections corresponding to class III, IV and V surrounding rocks (layouts of measuring points are shown in Figure 5). It can be seen from the figure that each 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 locations in the same section, the axial forces in rock bolts at the tunnel vault are larger than that at the right spandrel, while the axial forces in rock bolts at the right spandrel are larger than that at the side wall, which are even greater than that at the left spandrel of the tunnel. This indicates the initial load bearing effects of the middle wall significantly lower the axial forces in the rock bolts close to the side of the middle wall, therefore it is necessary to take measures to strengthen the anchoring forces. The stable values of the axial forces in rock bolts are larger in the cross section with lower class of surrounding rock masses, the stable value of the axial forces in rock bolts in the section K89+949 (class III) is about 12 kN, the stable value in the section K90+105 (class IV) is about 18 kN, whereas the value in the section K89+729 (class V) becomes about 25 kN. Among them, the maximum value of the axial forces in rock bolts is 25 kN and the tensile stress is 50.9 MPa by calculation (see Equation (8)). The body of the rock bolt (diameter: \(\Phi\)25 mm) used in the tunnel is made of hot-rolled ribbed bar (HRB) 335 round steel, of which the ultimate tensile stress is 168.4 MPa. It is quite clear that the actual bearing capacity of the rock bolt has not been utilized to its full potential, which is far less than the designed bearing capacity. Therefore, under the premise of ensuring the stability and safety of surrounding rocks, it can be acceptable to reduce the number of the rock bolts, to increase the bolt spacing and to strengthen the anchoring forces appropriately.  
\[\sigma=\frac{25\times 10000}{0.25\pi\times 25^{2}}\approx 50.9\,\mathrm{MPa} \tag{8}\]  
Figure 16: Time history plots of the axial forces in rock bolts at location B in the K89+729 section.