During the excavation of the southern section of the Gotthard Base Tunnel, the rock mass response to tunneling activities induced by two open, gripper TBMs described unusual magnitudes of displacements in the near-field domains of the heading without evidence of major failures of the surrounding medium. The sedimentary rock mass encountered along the whole tunnel belongs to two typical gneiss-families, which are well known to geologists and engineers since many underground facilities have been mined out in this region starting by the late 40s; nevertheless, the behavior of the opening turned to the unexpected as soon as the chainage 13’500 has been reached, where the mechanical excavation was carried out in a biotite-rich gneiss with sub horizontal weakness planes with relatively low shear strength of the surfaces compared to that of the intact rock and a overburden slightly exceeding the km mark at the same location. Following the usual notions of engineering mechanics, prevention of displacements is accompanied by increase in the state of stress in and around the support units preserved to control near-field rock deformation, which turned out to be a major issue by managing the whole contract of the southern section of the Gotthard Base Tunnel. In fact, the result of such a ductile rock mass was to overstress the steel ribs supporting the 9.0 m circular excavation and finally to shrink the drifts so much, that the requested cross section were not more assured. The increase of the deformations took place with an unforeseen delay with respect to the mining of the tunnel in so far as the concrete invert was totally damaged by shifting up to 20 cm. As a result, securing and replacing of this final lining work caused a time and cost expensive intervention for the last 3 km of tunnel for widening the opening according to the contractual specifications.