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  Tunneling through 48 Fault Zones and High Water Pressures on Turkey’s Gerede Water Transmission Tunnel

  The December 2018 breakthrough of a 5.5 m diameter hybrid-type Single Shield/EPB TBM at the Gerede Water Transmission Tunnel in Central Turkey was a feat of modern construction. The 9 km leg was the final section of the 31.6 km long water supply line bored through what is widely considered to be Turkey’s most challenging geology. The project was originally started with the contractor selecting three Double Shield machines, which were procured and supplied without Robbins involvement. When two of the machines became stuck and were unable to continue, the solution of the hybrid-type TBM was developed to complete the rest of the tunnel. The TBM was assembled and launched more than 7 km from the tunnel portal and successfully navigated 48 fault zones as well as hydrostatic pressures up to 26 bar. The machine also bypassed areas where the Double Shield TBMs had become stuck. The paper describes the machine and the unique design for the geological conditions, including the requirement to be sealable up to 20 bar hydrostatic pressure. When boring and during large water inflows, the TBM held back the water/muck and allowed time for pre-consolidation grouting. Despite the challenges, crews were able to achieve a best month of 484 m with a 285 m monthly average throughout tunneling. This paper will also discuss the performance of the machine in conditions above and beyond the TBM design pressure, and analyze the difficult geological sections and how the crew responded to the more severe fault zones. It will seek to make recommendations as to other projects in difficult and faulted mountainous rock conditions with high water pressures.

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