ABSTRACT
In cases where bolts are used, the mechanical states of the bolts are hard to be monitored since there are no space for traditional sensors in general. Such as in shield tunnels, shield segments are connected by bolts in both longitudinal and transversal directions.A new kind of sensor has been developed based on fiber optic technology. In this sensor, a pair of FBG optical fiber is used based on its characteristics of small volume, reliability, anti-electromagnetic interference and corrosion-resistant. In this design, two grooves are developed in both sides of the bolt body with a dimension of 4mm × 2mm (depth × width). The FBG optical fiber is installed inside the groove and finally fixed by epoxy and protecting coating.A series tests were carried out in the laboratory in order to validate the feasibility of the design and a special testing clamp system was developed to conduct the testing. In the laboratory testing, good linearity and repeatability results were obtained. The sensor can provide quite good performance in predicting the strain states of bolts with an error of no more than 0.75%. Besides, the linearity factors were also tested in terms of different bolts before field applications. The sensors were widely applied in the joint monitoring in Qianjiang tunnel, Hangzhou, China and some reasonable conclusions have been proposed.
