During the construction of the new Ionian national road, a deep-seated instability phenomenon was triggered when excavating the South portal of the Piscopio I tunnel. Results from the extensive monitoring system and from the tunnel face surveys indicated that both geometry and kinematics of the observed phenomenon were governed by the specific geo-structural settings characterizing the slope involved in the excavation. The reconstruction of the geo-structural settings allowed the definition of appropriate stabilizing measures. They consisted mainly in the implementation of deep drainage systems, in particular two large diameter wells equipped with long radial drainage pipes, aiming at lowering the pore water pressures at critical depth of the unstable mass. The effectiveness of these mitigation works resulted evident from the almost immediate reduction of the displacement rate, allowing the completion of the tunneling works and the safe opening of the road.