Advanced Observation Techniques for Sophisticated Shallow Tunnel Projects

The article presents experiences gained during construction of the approximately 3 km long urban Lainzer Tunnel (LT31), driven with side-wall galleries under shallow overburden. The structure's safety is assessed mainly on the basis of 3D deformation analyses, comparison of predicted and observed system behaviour, evaluation of shotcrete stress, and online monitoring of surface settlements. Displacements of the tunnel face, correlating with pre-face surface settlements, are monitored by an innovative, fully automatic face monitoring system (OBM). A correlation between volume loss (surface) and volume increase (face) could be established. A test field comparing fibreglass anchors (GRP) with IBO face bolts revealed - contrary to expectations - no appreciable reduction of surface settlement due to GRP anchors. However, temporary inverts constructed in the side wall galleries in combination with IBO face bolts support turned out to be effective in reducing horizontal convergence and resulting settlements of the ground surface in a sensitive residential area.