# An Investigation into Bol Anchoring Performance during Tunnel Construction in Bedded Rock Mass
## 4 Field Application
### Bolt Arrangement at the Construction Site  
Bolts with lengths of 3.5 m with diameters of 22 mm were used in the field. The measuring bolts and vibration wire steel stress gauges were installed to measure the axial force of the bolts, as shown in Figure 11. Each measuring bolt was installed with three vibrating wire steel stress gauges with corresponding measuring lines. At the measuring station, the plug of each measuring line was inserted into the frequency recorder to record the value. Each vibrating wire steel stress gauge was calibrated indoors to obtain its reference frequency and calibration coefficient prior to its installation and use. The frequencies obtained from the field test were computed on the basis of a calibration formula to obtain the axial force of the bolt segments.  
Two site sections in the field ZK17+360 to ZK17+365 (\"ZK17\" means the 17 th kilometer of left line of DaoZhen Highway, \"360\" and \"365\" mean detailed distance(meters) in ZK17.) were selected to test the bolt axial force variations. These sites were the tunnel evacuation face and secondary lining, respectively. In each section, seven bolts were installed. To compare the impact of the anchoring methods on the anchorage effect, the ZK17+360 section adopted full-length anchoring, while the ZK17+365 section adopted the end anchoring scheme. The distribution method of the measuring bolts and site construction are shown in Figure 12. The test field simultaneously measured the periphery convergence and arch crown settlement of the two sections to analyze the support effect of the bolt.  
Figure 11: A measuring bolt.  
Figure 10: Field test: (**a**) Support parameters of the tunnel section; (**b**) Surrounding rock of the testing face.  
The periphery convergence was measured by a JSS30A convergence gauge, while the arch crown settlement was measured by a DSZ2 automatic compensated level.  
The ultimate axial force distribution of the measuring bolts at each section is shown in Figure 13. The positive sign represents compressive force; the negative sign represents tensile force. The figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress borne by bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. That is, under full-length anchoring, the bolts can provide greater support resistance and hence obtain a better anchorage effect. The axial forces of bolts in different parts of the test section also have certain regularities, as outlined below.  
The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation distribution of the tunnel's surrounding rock, as shown in Figure 14. Moreover, the maximum deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, this location frequently developed cracking and chipping, and it even extruded and deformed the lattice girder.  
The axial force of the bolt on the wall at each side of the test section was relatively small (5 to 10 kN), especially on bolts #6 and #7 on the wall at each side. These were installed after the evacuation of the lower bench, while the deformation of the tunnel's surrounding rock tended to be stable after  
Figure 12: Section distribution and site installation of the measuring bolts: (**a**) Distribution of the measuring bolts; (**b**) Installation of the measuring bolts.  
Figure 13: Axial force (KN) distribution of the test section bolts: (**a**) ZK17+360 section; (**b**) ZK17+365 section.  
evacuation of the lower bench. Bolts in this part did not engage their anchoring effects; rather, they became a kind of safety stock.