New Diamond Layout Method for Tunnel Displacement Monitoring

A monitoring method based on the characteristics of weak grating sensors is proposed for long-term monitoring of the uneven settlement and horizontal displacement of segments in shield subway tunnels during the operation period. The cable type weak grating sensor is fixed on the wall of the shield tunnel in a diamond patten after prestretching. By monitoring the deformation of the weak grating sensor, the tunnel deformations corresponding to different tunnel structure modes can be distinguished and calculated to meet the real-time and precision requirements for engineering. According to the similarity ratio principle, a 1∶5 tunnel displacement model is designed and manufactured to simulate the local expansion, settlement and horizontal displacement of the tunnel segments. The grating sensors are also fixed on the wall surface of the tunnel model. Sensitivity tests of the weak grating sensors are first conducted before displacement monitoring. Then, the weak grating monitoring results with different diamond layout angles are compared. The following step is to monitor the vertical settlement and horizontal displacement of the tunnel model, which are compared with the actual displacement values. The results show that the strain of weak grating with the diamond layout is larger than that with a linear layout under the same settlement condition, which means the sensitivity of the diamond layout method to settlement change is better than that of traditional linear layout. Furthermore, 10°~20° is the optimal angle interval of the diamond layout for weak grating, for the monitoring result error is low. Additionally, the inversion equation of weak grating wavelength change and segment displacement is established based on the test data of diamond layout with an angle of 15°.