ABSTRACT
According to the Austrian Guideline for Geomechanical Design of Underground Structures, the system behaviour is defined as the behaviour of the compound system resulting from rock mass and selected construction measures (excavation and support method, auxiliary measures). In tunnels with low overburden, the stress redistribution, combined with poor quality of the ground can lead to instability of the face and the crown. Shear failures can develop progressively without significant sign (Schubert & Vavrovsky 1994). This can lead to serious overbreak of the crown or even to a chimney type failure in case of poor ground and low confining stress (Feder 1981). Therefore, deformations should be carefully controlled to minimize the risk and scale of collapses, as well as surface settlements.
