The Bolu Crossing, Turkey

Location: Turkey

Client: The Turkish Highways Administration, and Yuksel-Rendel

Dates: 1998- current

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. There were long delays associated with the need to re-profile the lining and to devise a satisfactory solution for these sections.

GCG was asked 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 which enabled rapid interpretation of the monitoring results and development and validation of a new design. Site support by GCG was also continued to cover the rebuilding of damaged sections and construction of new sections to the new designs.

GCG was also closely involved in evaluating the seismic design and construction and retrofit of the tunnels and following two Turkish earthquakes of magnitudes 7.4 and 7.2. This experience is described in a separate job sheet. 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 and permit displacement along an active fault crossed by the tunnels.


Menkiti, C. O, Miles, R. and Mair, R. J. (2001) “Tunnelling in Complex Ground Conditions in Bolu Turkey”, Proceedings of the Underground Construction 2001 Symposium, London, 18th – 20th September 2001.

Invited lecture, Institution of the Engineers of Ireland: Tunnelling in Complex Ground Conditions in Bolu, Turkey – 7th October 2002