The stress–strain analysis of the entire construction process of underground diaphragm wall based on self-sensing FRP bar monitoring

The diaphragm wall, as a key supporting structure in bridge anchorage projects, faces complex and variable stress conditions during construction. Therefore, ensuring structural safety and performance monitoring is crucial. Traditional rebar monitoring methods, due to poor corrosion resistance, insufficient real-time capabilities, and maintenance difficulties, cannot meet the high precision and reliability requirements of modern underground engineering. This study applies self-sensing technology to fiber-reinforced polymer (FRP) materials by embedding optical fiber sensors within the FRP, enhancing real-time monitoring of stress and strain during the construction of diaphragm walls. This technology has been successfully implemented in the Shiziyang Bridge project, enabling real-time monitoring of stress and strain in the anchor diaphragm wall. The study adopts a quasi-distributed optical fiber monitoring scheme, combined with wireless transmission and a cloud platform for remote data acquisition and analysis. The results indicate that the self-sensing FRP bar shows excellent stress and strain monitoring capabilities at various stages of diaphragm wall construction. In stages 1-3, the stress curve transitions from tensile stress to alternating tensile and compressive stress. The shallow and mid-layers exhibit tensile stress, while the deep layers experience compressive stress. The maximum tensile stress recorded is 35.8 MPa, and the maximum compressive stress is -20.3 MPa, mainly due to pressure imbalance caused by soil excavation and the decreasing groundwater level. In stage 4, the upper stress gradually decreases, while the lower stress transitions from tensile to compressive, with the maximum tensile stress at 13.9 MPa and the maximum compressive stress at - 37.7 MPa. These changes are attributed to the completion of the liner and bottom slab construction, backfilling of the soil, and the increased self-weight of the upper structure. In stage 5, as construction progresses, the stress curve forms an M-shape, with compressive stress gradually decreasing. The maximum tensile stress is 2.3 MPa, and the maximum compressive stress is - 11.8 MPa, mainly influenced by the increasing tensile force applied by the stay cables. As of November 14, 2024, the monitoring data show that the tensile strain in the shallow layers remains unchanged, while the compressive strain in the middle and deep layers is gradually decreasing.