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Excavating Turkey’s Most Challenging Project: The Gerede Water Transmission Tunnel

At the Gerede Water Transmission Tunnel in Central Turkey, a 31.6 km long water supply line has been designated a national priority due to severe and chronic droughts in the capital city Ankara. Drawing water from the Gerede River (and conveying to Çamlıdere Dam), it will be the longest water tunnel in Turkey once complete.

But completing the tunnel has been an obstacle in itself. The project has been called the most challenging tunnel currently under construction in Turkey, and with good reason. Out of three standard Double Shield TBMs originally supplied to bore the tunnel, two became irretrievably stuck following massive inflows of mud and debris. In 2016, a hybrid type “Crossover” machine was launched to excavate the final 9 km of tunnel, but to do so it would need to cross dozens of fault zones and withstand intense water pressures up to 20 bars.

To accomplish this, the machine was designed with a number of unique features including the ability to be sealed up to 20 bar pressure. In the event of a large water inflow, the stopped TBM would hold back the water/muck and allow time for pre-consolidation grouting. The machine is also equipped with a bottom screw conveyor and a unique cutterhead design that facilitates effective muck transportation in both hard rock and mixed ground, among other features. This paper discusses the performance of the machine in exceedingly difficult conditions, and outlines the challenges yet to be overcome and how they may be surmounted. The paper also covers the unique aspects of this urgent Turkish projects and the unique logistical requirements of assembling and launching a machine deep within an existing tunnel.

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