Long-term monitoring of reinforced earth structures using distributed fiber optic sensing

Reinforced earth structures are commonly used to construct earth structures with high inclination angles. The stability and safety of such constructions is highly dependent on the strain distribution of the geogrids inside the earth structure. In this article, we report about the development and implementation of a structural monitoring system to measure the strain distribution of geogrids with high accuracy and high spatial resolution. The system was developed for the Semmering Base Tunnel project in Austria but can also be applied to other reinforced earth structures. The Semmering Base Tunnel is one of the core infrastructure projects in central Europe. Prior to the start of the actual tunnel excavation, comprehensive preparatory works in the alpine landscape were necessary. One of these works included the relocation of a river stream at the disposal site Longsgraben. To establish this relocation, a reinforced earth structure with a total length of more than 1.3 km and heights up to 25 m was constructed. To evaluate the stability of this structure, a monitoring system was developed. This monitoring system is thereby based on external geodetic and internal distributed fiber optic measurements. For the latter, about 2 km of Brillouin sensing cables in several sensor sections were installed in the project area. In this paper, we report about the laboratory experiments to determine strain and temperature coefficients for the conversion of the measured Brillouin frequency shifts into strain or temperature values. Furthermore, the long-term behavior of the monitoring structure is analyzed for the period of 1 year after installation.