The Bolu Crossing

Location: Bolu Mountain, Turkey

The Bolu Tunnels in Turkey lie along the Anatolian Motorway linking Istanbul and Ankara and forming part of the Trans-European Motorway Project connecting Eastern Europe with the Middle East.  The 25km long Bolu Crossing consists of a twin-tube road tunnel through the Bolu Mountain and approach viaducts.  The tunnels are 3.26km long, 40m apart and have cross sections ranging between 170m2 to more than 200m2 (equivalent to circular sections 15m to 16m in diameter).

Excessive post-construction movements were experienced along sections of the tunnel passing through low-strength clay gouge material, leading to failure of the support system. This resulted in long delays associated with the need to re-profile the lining and to devise a satisfactory solution for these sections.

GCG was engaged by the Client to provide advice on the ground conditions and selection of geotechnical design parameters so that a new design could be implemented, and comprehensive monitoring installed. Full-time site supervision was provided by GCG to enable rapid interpretation of the monitoring results and validation of the new design.

GCG was closely involved in evaluating the seismic design considerations for the tunnels and has direct experience of the performance of tunnels in two previous earthquakes (magnitudes 7.4 and 7.2).  Aspects of earthquake design included the collection of dynamic and small strain soil parameters, the mapping and assessment of faults and their activity, and the design of tunnels incorporating dynamic loads with special construction details to limit potential damage.