Abstract: To explore the boundary effect of the strain transmission between embedded optical fiber sensor and tunnel lining, the study was conducted from three aspects: theoretical, experimental studies and field applications. The results were verified in actual engineering. The strain transfer model of optical fiber in the concrete lining was constructed, and the mechanism of optical fiber strain transfer was analyzed. The strain transfer efficiency of optical fiber was then calculated and compared that with the numerical modeling results, which verified the accuracy of the mechanical calculation model. The concrete lining of tunnel was simulated with the reinforced concrete beams and distributed optical fibers were embedded on the surface of the concrete beams. Two groups of tests were carried out in this study. The test beams were loaded in stages and the optical fibers were tested by BOFDA technology. The test results show that the embedded fiber optic sensor has a boundary effect. The two ends of the beam structure are the low efficiency strain transmission area of the optical fibers, which cannot fully transfer the strain of the test beam. The middle part of the test beam is the high efficiency strain transfer area of the optical fibers, which can completely transfer the strain of the test beam. Based on the research results, the engineering application research was carried out. A distributed optical fiber sensor was installed in the tunnel lining in the CRD construction method section of the subway tunnel of the Beijing New Airport Line using the embedded fiber technology. The monitoring results show that the boundary effect has little influence on the monitoring results. The placement method of the distributed embedded optical fiber is feasible. This research can provide a reference for the application of distributed optical fiber technology in underground engineering structure monitoring.

Key words: distributed optical fiber, coupling performance, tunnel monitoring, BOFDA technology