## 3 Results and Discussion  
### Findings Related to the Performance of DOS for Rock Bolt Monitoring  
Part of the research effort was to further explore the capabilities and limitations of the use of the DOS as a tool for both in situ and laboratory investigations. As explored above, through tensile testing, the performance of the fiber for the application of capturing a full spectrum of loading was investigated, and results of the activation length was incorporated into the monitoring plan. This monitoring plan had a success rate of 75% for specimens under excess plastic loading (>160 kN) approaching ultimate state, and 43% survived shank failure. This allowed for observation of strain attenuations and insights into the active embedment length beyond bolt yielding. During this initial tensile testing, repeatability and performance at low level strains were also investigated to provide confidence in reported active embedment length observations. Even at loads below 1 kN (loads of 0.25 kN, 0.5 kN and 0.75 kN) with expected strain well below the accuracy of the sensor (expected strain is 4.6 \(\upmu\epsilon\) with the machine listed accuracy being +/ \(-\) 25 \(\upmu\epsilon\)), the location of the start or end of the sensor was still clear. The largest positional difference between the actual location in which the sensor is no longer bonded and where it was perceived in the data was 13 mm.  
Figure 6: Shear and displacement from axial strain depiction modified after [30; 31; 32; 33].  
Repeatability was also inspected by looking at multiple scans (20 to 30 scans at 4 random times) under no load for two randomly selected 5 cm lengths. Range, average and standard deviation were inspected. The largest shift in average was 8.4 \(\upmu\)e and 0.4 \(\upmu\)e for each segment. The largest range from the mean observed was 24.8 \(\upmu\)e and the largest standard deviation was 12.6 \(\upmu\)e. These results provided further confidence in the ability to detect and discern between low and zero loads.  
The use of fiber optics as a quality control instrument in a monitoring plan has been highlighted by research conducted in this group [18]. The need for verification of ground support performance is required due to grouting issues such as glowing [60]. This is often addressed through proof loading, in which the use of DOS can enhance insights provided to operators [55]. Due to an issue with the grout pump used for the first round of specimen creation, the opportunity to highlight this enhanced insight was provided. In Specim B-1-I there was a large air void roughly 20 cm in size located 40 cm into the embedment length. Because of this void, the same condition was retested with Specim B-1-ii. When viewing the two displacement load profiles, it is not evident that one failure is due to grout shearing because of the reduced grout. However, when viewing the strain results from the DOS, the presence of the voids is apparent. This can be seen in Figure 7. Due to the void's location, it generally did not appear to have significant effects on the strain attenuation. This is likely because there was still some grout present, which was able to transfer load until its capacity was reached and the void become apparent. This exemplifies some of the additional insight gleaned from this cost-effective monitoring solution.  
Figure 7: **(A)** Load-stroke displacement of both specimens; (B) strain response of specimen B-1-i, highlighting clear voiding; (C) B-1-i post-test analysis shows voids in line with strain results.