Excavation of Turkey’s longest water tunnel came to an end on Dec. 18, 2018. To get there, a 5.56 m (18.2 ft) diameter Robbins Crossover (XRE) TBM and the contractor JV of Kolin/Limak had to overcome dozens of major fault zones and water pressures up to 26 bar. The completed national priority water line is set to go into operation in March 2019.
The 31.6 km (19.6 mi) long Gerede Water Transmission Tunnel is an urgently needed project due to severe and chronic droughts in the capital city Ankara. Its final leg, a 9.0 km (5.6 mi) section of extremely difficult ground including sandstone agglomerate, limestone and tuff, was just one section in the middle of a tunnel widely considered to be the most challenging ever driven by TBMs in Turkey.
“I’ve had the chance to study and visit the majority of mechanized tunneling projects in Turkey since the 1980s. The Gerede project is one of the most challenging projects among them,” said Dr. Nuh Bilgin, Professor of Mine and Tunnel Mechanization at Istanbul Technical University and Chairman of the Turkish Tunneling Society.
The Robbins XRE TBM was called in to complete the tunnel, which was at a standstill after using three Double Shield TBMs from another manufacturer. Those machines encountered incredibly difficult geology including massive inrushes of mud and water. The Kolin/Limak JV had to develop a new strategy given the unexpected ground conditions. They contacted The Robbins Company, who suggested a Crossover (Dual-Mode Type) TBM for the remaining section of tunnel.
“The Crossover TBM provided great ease and versatility during the entire project with frequently changing ground conditions. The TBM was equipped with features such as increased thrust, two-speed gearbox, and modular screw conveyor. It was capable of giving the necessary responses in different geologies, which was our most important asset in achieving our goal,” said Barış Duman, Project Manager for the Kolin/Limak JV.
“The challenging part for us was to design and manufacture a TBM that could complete the difficult section of the Gerede Tunnel where two other competitor TBMs had failed,” said Yunus Alpagut, Robbins representative in Turkey. The specialized machine was designed to statically hold water pressure up to 20 bar, a failsafe that none of the standard Double Shield TBMs had been equipped with.
A convertible cutterhead was also provided that was designed for ease of conversion between hard rock and EPB modes, and with cutter housings that could be fitted with either disc cutters or tungsten carbide tooling. To cope with difficult ground, the Gerede machine was also equipped with the Torque-Shift System, multi-speed gearing allowing the machine to function as either an EPB or a hard rock TBM. This function is done by adding another gear reduction – heavy duty pinions and bull gears accommodate high torque at low speed, allowing the machine to bore through fault zones and soft ground without becoming stuck.
The Crossover machine was assembled in spring 2016 after crews excavated a bypass tunnel to one side of one of the stuck Double Shield TBMs. An underground assembly chamber allowed the machine to be built in the tunnel using Onsite First Time Assembly (OFTA).
“The logistics of getting components through the existing tunnel were the most challenging thing. The assembly chamber was 7 km (4 mi) from the portal. The water inflow of 600 l/s (159 gal/s) made it difficult to get the materials to the machine,” said Glen Maynard, Robbins Field Service Site Manager.
Despite the challenges, the machine began boring in summer 2016 and within the first 50 m (160 ft) of boring had successfully passed through the section that buried the original Double Shield TBM. The machine was required to be used in EPB mode as it encountered water pressures up to 26 bars, alluvium, flowing materials, clay and a total of 48 fault zones. Water pressure was lowered by draining the ground water through the rear shield probe drill ports, which were equipped with normally-closed ball valves. Probe drilling was done on a routine basis to get through the ground conditions.
“Together with the difficult geological conditions the travel time to reach the TBM within the tunnel had effects on TBM performance. Despite this constraint, the tunnel excavation achieved a best day of 29.4 m (96.5 ft), best week of 134.6 m (441.6 ft) and a best month of 484 m (1,588 ft),” said Duman.
“We had many challenging areas with water and high pressures up to 26 bar along with alluvial material in fault zones. Ground pressure on the shield body caused squeezing conditions in clay. In these regions, we were able to quickly pass through by keeping the TBM advance rate, cutterhead rpm and screw conveyor rotation speed at the ideal level. Ultimately, we think our decision to select a Crossover TBM was correct,” continued Duman.
With tunneling complete, the pipeline is on track to open in March 2019. The tunnel will convey water from the Gerede River to Çamlıdere Dam, which provides potable water for the Ankara city water system.