Abstract: A multilayer mechanical model comprising a surface-mounted optical fiber sensing element with a core layer, protective layer, substrate layer, and measured layer is constructed using optical fiber sensing technology. The strain transfer mechanism of the optical fiber is analyzed, and a strain transfer rate formula for a surface-mounted optical fiber sensing element is derived. The accuracy of the sensing element in detecting the structural shape of tunnel segments is verified. A 1∶20 scale model is employed to perform experimental investigations by measuring the strain variations in the segments under different water and earth pressures. Numerical simulations using ANSYS are conducted to compare the strain responses of the tunnel segment structure under two load scenarios: with and without water pressure. The results also reveal the influence mechanism of water-soil coupling pressure on tunnel segment deformation. The findings are summarized as follows: (1) The average strain transfer rate of the optical fiber sensing element reaches 95.1 % at a grating length of 10 mm. (2) Uniform water pressure stabilizes the deformation of the tunnel segment structure to a certain extent. (3) Under water-soil coupling pressure, the tensile and compressive strain changes are unsynchronized. The compressive strain change at the arch waist precedes the tensile strain change at the crown, indicating a higher sensitivity of the arch waist to water-soil coupling pressure. (4) In a water-pressure stratum environment, the displacement of the tunnel segment structure exhibits a fluctuating trend as the stratum load increases from 0 to 1.6 MPa. (5) When the load further increases from 1.6 to 2.4 MPa, the displacement of the tunnel segment structure follows a linear trend.

Key words: tunnel segment, optical fiber sensing, strain transfer, numerical simulation, scale model, structural shape perception