# Study on Mechanical Properties and Application of a New Flexible Bolt
## 6 Engineering Application
### Monitoring Scheme and Results  
To master the bearing law of flexible bolts and the corresponding control over the roadway surrounding rock, it is of necessity to monitor the axial load of the bolts, the surface displacement of roadway, and the development law of surrounding rock fractures. Two monitoring stations for bolt axial load (a1 and a2), one monitoring station for surface displacement (b), and one monitoring station for borehole camera images (c), are set in the testing zone.  
Figure 16 shows the monitoring results. Figure 16a shows the results of axial load of flexible bolts. As shown in the dynamometers a1 and a2, the preload is 80.5 and 84.5 kN, respectively. Within the distance of 10 m to the excavation face, the axial load increases sharply by 20%. Since the bolts are in high-stress quasi-elastic stage, the bolts come to function rapidly in response to any deformation in the surrounding rock of the roof, thus bringing the early roadway deformation under control [27]. Within the distance of 80 m to the excavation face, the axial load increases slightly. During this period, the roof deformation develops at a quite limited rate. The dynamometers a1 and a2 show that the axial load is fixed finally at 109 and 104 kN, respectively. The development law of axial load demonstrates that flexible bolts can adjust to the surrounding rock deformation and thus achieve the final goal of bringing it under control.  
Figure 15: Support design of the testing roadway.  
Figure 16b shows the monitoring results of surface displacement in the roadway. Similar to the axial load, the surface displacement also rises sharply at first, then experiences a mild increase, and tends to be stable in the end. The sharp rise in roadway displacement occurs within the distance of 20 m to the excavation face; the mild increase appears within the distance of 90 m to the excavation face. From this analysis, it can be known that compared with bolt stress, roadway deformation possesses some degree of delay. The final subsidence of the roof is 15 mm and the rib convergence is 23 mm.  
Figure 16c presents the monitoring results of borehole camera on the roof. This borehole is set in the middle of the testing zone. Through observing the situation of borehole wall, the fracture development in the roof surrounding rock can be deduced indirectly. As shown in the figure, there exist one case of serious damage and two annular fractures within the depth of 1.0 m in the roof. Within this area (top-coal), deformation and damage occur easily under the action of stress. Within the depth of 1.0-2.5 m, there exist two tiny fractures, and beyond the depth of 2.5 m, no fracture or separation layer is observed.  
With the support of flexible bolts, the control effect on the roadway surrounding rock is improved remarkably, as can be seen in Figure 17 (the photos taken on the scene). The even transmission of roof stress improves the roof pressure on the ribs, which in turn lessens the damage on the ribs and realizes a virtuous cycle.  
Figure 16: Results of the roadway monitoring: (**a**) axial load of flexible bolts, (**b**) surface displacement in the roadway, and (**c**) borehole camera images and corresponding analysis.  
Figure 17: Field photos of testing roadway: (**a**) roof and (**b**) rib.