The Bolu tunnel is subject to considerable deformation due to stress-related failure in the rock mass in thrust zones and strike-slip faults that transect the tunnel. These deformations resulted in some serious geotechnical problems, which were overcome either by flexible or extremely heavy support systems. The heavy system was used to prevent or bring under control the large deformations that would take place during excavation, whereas the flexible method allows the deformations to develop without restraint. The heavy support system was used in thrust fault zones and in flysch zones in the Bolu Tunnel and produced more successful results in comparison to the flexible support system. A brief explanation of the tunnel collapse in the Duzce Earthquake is also provided. (C) 2002 Elsevier Science B.V. All rights reserved.