Monitoring of reinforced earth structures at the new Semmering Base Tunnel

Reinforced earth structures are extensively used at the disposal site Longsgraben of the new Semmering base tunnel. These structures were monitored during construction using the observational method according to Eurocode 7. Geodetic measurements to targets on the surface of the structure were regularly performed in selected cross sections. However, surface deformations give only limited information about the operation grade of geogrids due to the inhomogeneous strain distribution within the object. IGMS developed a fiber optic measurement concept for the reliable measurement of strain of geogrids within earth structures. The monitoring system was specifically developed for the Semmering project and uses distributed fiber optic sensing based on Brillouin scattering. About 2km of sensing cables were installed in the project area. Unique anchors were designed to reliably transfer deformations of the geogrids to the sensing cables. The stability of the anchors was verified in detailed laboratory investigations. The measured Brillouin frequency shifts are converted into strain values using calibration functions derived from laboratory experiments. Finally the measured internal and external deformations are compared to predicted deformations from the finite element analysis. In total 2km of sensing cable were installed in the reinforced earth structure. All segments of the cross sections can be measured at the same time from a central monitoring station. We developed a software for an automatic classification of the segments and calculation of the temperature compensated strain values using the strain coefficient determined in the laboratory.