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. We  present  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  analyses. We  demonstrate  that  with the developed monitoring system internal deformations can be measured reliably and with high precision.