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Hard Rock Machine Makes Excellent Time in Tough Turkish Ground
In December 2013, a 4.3 m (14.1 ft) Robbins Double Shield TBM emerged triumphant after boring a 9.3 km (5.8 mi) headrace tunnel in Turkey’s Adana Province for the Yamanli II Hydroelectric Project. The hard rock TBM broke through on time, due to excellent advance rates and minimal downtime for the duration of the project.
The machine, operated by contractor NTF Construction Co., reached high monthly advance up to 782.8 m (2,568 ft), with an average monthly rate of 472.7 m (1,551 ft) over the course of the 14-month project. Rates were achieved in ground conditions of mainly limestone with some clay–filled fissures. Average rock strength was 60 to 70 MPa, with a maximum strength of 120 MPa. Despite the hard ground, cutter expenses were kept extremely low as a result of few replacements, proper cutter maintenance and a well-trained crew. Mustafa Akgül, Tunnel Chief at NTF Construction provided one such example: “The 28th gauge cutter ran more than 8 km throughout the bore in limestone up to 120 MPa without any change”.
The machine also passed through two fault zones with clay inflows; at machine launch, only one fault zone was anticipated. Probing and pre-grouting allowed crews to prepare in advance for the additional fault zone and other difficult ground conditions encountered along the way.
In addition to challenging ground, the machine also experienced unexpected power cuts. This led to delays resulting in long machine and PLC startups. Extended day and night shifts allowed the machine to catch up: “The machine performed very well with minimal downtime and made up the initial delays, finishing right on time,” said Dursun Yıldız, Machinery Manager of NTF Construction.
Some modifications were required during machine operation, specifically the pea gravel conveyor system, bridge-front supporting legs, and the surveying system. Akgül spoke about the adjustments, saying, “Only three modifications were undertaken during [the bore], and Robbins Field Service conducted them smoothly. Their cooperation with NTF personnel was strong and continuous”. 
The project took place at a remote jobsite roughly 200 km (124 mi) from the city of Adana. The mountainous region contains a valley, in which many civil works projects are currently taking place; the Yamanli II project is the third hydroelectric project in the area.  The completed tunnel will source water from the Göksun River, with an annual generating capacity of 78 MW to power up to 78,000 homes.
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