Breakthrough of the headrace tunnel at the Yamanli II hydroelectric project in Turkey's Adana Province has been achieved on schedule.
In December 2013, a 4.3m Robbins Double Shield TBM emerged successfully after boring the 9.3km tunnel. The hard rock TBM broke through on time, due to excellent advance rates and minimal downtime for the duration of the project, Robbins said.
The machine, operated by contractor NTF Construction Co., reached high monthly advance up to 782.8m, with an average monthly rate of 472.7m 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 8km 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.
Some modifications were required during machine operation, specifically the pea gravel conveyor system, bridge-front supporting legs, and the surveying system.
The project took place at a remote jobsite roughly 200km 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 78MW to power up to 78,000 homes.
