Abstract: Based on the Brillouin optical frequency domain analysis (BOFDA), the distributed optical fiber monitoring technology has been preliminarily applied in geotechnical engineering. The strain monitoring results show that there is a certain strain loss at the pretension section of the optical fiber with time after the pretension of the optical fiber. The strain loss is mainly caused by the fatigue characteristics of optical fibers. The strain loss caused by the fatigue of optical fibers is obviously disadvantageous to engineering monitoring to a large extent, which needs to be eliminated in practical monitoring. For the distributed optical fiber monitoring technology, two strain optical fibers (HY material, new strain optical fiber and polyurethane strain optical fiber) are selected to conduct point-fixed pretension in the laboratory ( ), and strain loss monitoring is carried out for 8 months. By comparing and analyzing the difference of strain loss between the two kinds of optical fibers, we obtain the variation rule of strain loss with time and the rate change of strain loss of optical fibers with the increases of temperature and humidity under pretension state. Given this, a point-fixed pretension method for tunnel deformation monitoring is studied using the distributed optical fiber monitoring technology, which provides theoretical basis and construction guidance for engineering application.

Key words: BOFDA technology, tunnel monitoring, optical fiber pre-stretch, strain loss, optical fiber fatigue