assessing the difference in measuring bolt stress: a comparison of two optical fiber sensing techniques

Monitoring the load level of the rock bolts is of great importance for assessing risk. Based on the mechanical transmission mechanism of grouting bolts, the bolt pull-out test was carried out in lab. The performance of the bolt under pull-out loading was measured using the pulse-pre-pump Brillouin optical time domain analysis (PPP-BOTDA) and fiber Bragg grating (FBG) sensing technologies. The distribution characteristic of axial stress along the bolt was analysed in combination with the measurements obtained by the two sensing technologies. The relative standard deviation for repeatability errors in the determination and the setting time of resin grout was investigated. The results show that the distribution of axial stress is nonuniform along the anchorage section. The maximum value of axial stress on the bolt is closed to the pull-out side. The relative standard deviation for repeatability errors obtained by PPP-BOTDA is less than that obtained by FBG. The comparison of the measurements obtained by the two methods indicates that when the drawing force is greater than 20 kN and the axial stress is more than 10 kN, the two methods have better consistency. In the field application, it is necessary to estimate the deformation of matrix, leaving at least 500 minutes for resin bond to work.