Abstract: The massive weight and size of prefabricated structures in underground metro stations, combined with multiple construction conditions, can lead to structural deformation, stress reconcentration, and ultimately, structural damage. To address these challenges, high-precision and high-spatial-resolution distributed optical frequency domain reflectometry technology is utilized to monitor the strain distribution within the sidewalls of underground prefabricated structures. The authors investigate the effects of various construction conditions, including sidewall assembly, roof slab installation, adjacent block assembly, and topsoil backfilling, on the deformation characteristics of the sidewalls. In addition, the effects of varying construction conditions on the strain field and bending moment distribution in the sidewalls are quantitatively analyzed. The contributions of each construction condition to the bending moments in the middle and end positions of the sidewall structures are also investigated. The results reveal the following: (1) Distributed fiber-optic strain monitoring indicates an obvious compression bending effect on the sidewalls during the assembly and topsoil backfilling stages. Among all construction stages, the topsoil backfilling process emerges as the most unfavorable for the stability of sidewall structures. (2) The support replacement and sidewall backfilling processes cause notable increases in the bending moment of the sidewall structure, with average increases of 42.8% and 19.3%, respectively. During the topsoil backfilling stage, bending moments in the B1 and B2 sidewalls increase by 510% and 252%, respectively, compared to the previous stage. (3) Bending moments at the ends and middle of the sidewalls continuously increase with the assembly and backfilling conditions, causing significant bending effects. The average contributions of the topsoil backfilling stage to the bending moments in the middle, top, and bottom of the sidewall are 70.4%, 69.4%, and 55.1%, respectively. The topsoil backfilling contributes the most to the sidewall bending moments. Significant attention should be paid to the influence of the top overburden soil pressure on the structural behavior of the sidewalls of the prefabricated metro station.

Key words: metro, underground station, prefabricated structure, optical frequency domain reflectometry, working conditions, mechanical characteristics