March 2019 is set to see operation begin of Turkey’s 31.6km long Gerede Water Transmission Tunnel, following the successful December 2018 breakthrough of the contractor JV Kolin/Limak’s 5.56m diameter Robbins Crossover (XRE) TBM. To complete boring on this the longest water tunnel in the country, the JV had to overcome dozens of major fault zones and water pressures up to 26 bar.
The project is urgently needed due to severe and chronic droughts in the capital city Ankara. Its final leg, a 9km 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 mechanised tunnelling 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 Mechanisation at Istanbul Technical University and Chairman of the Turkish Tunnelling Society.
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The Robbins XRE TBM was called in to complete the tunnel, after incredibly difficult geology including massive inrushes of mud and water had scuppered previous attempts to bore the tunnel.
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 specialised machine was designed to statically hold water pressure up to 20 bar. 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. 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 to 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 a previously stuck Double Shield TBM. An underground assembly chamber allowed the Robbins 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 7km from the portal. The water inflow of 600 l/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 50m of boring had passed through the troublesome section. 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 29.4m, best week of 134.6m and a best month of 484m,” 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 tunnelling 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.