The Gerede Water Transmission Tunnel is arguably one of the most difficult projects attempted in the world of tunneling. Dozens of fault zones and intense water pressures up to 20 bar are just a couple of the challenges to overcome. Prior to Robbins involvement, three standard Double Shield TBMs, originally supplied by a European manufacturer, attempted to bore the tunnel. Of the three, two became irretrievably stuck following massive inflows of mud and debris. In 2016, a Robbins Crossover XRE machine was launched to excavate the final 9 km (5.6 mi) of the 31.6 km (19.6 mi) long water supply tunnel. Due to severe and chronic droughts in the capital city Ankara, the water line has been deemed a national priority. Once complete, the supply line will draw water from the Gerede River, and will be the longest water tunnel in Turkey.
Turkey is in a tectonically active region controlled on a grand scale by the collision of the Arabian Plate and the Eurasian Plate. At a more detailed level, a large piece of continental crust almost the size of Turkey, called the Anatolian block, is being squeezed to the west. The block is bounded to the north by the North Anatolian Fault and to the south-east by the East Anatolian fault. Geology in the fault zones tends to be highly variable and unstable.
At Gerede, the Anatolian Fault Zone has certainly presented many obstacles. Geologic testing and borehole samples showed a mix of volcanic rock including tuff, basalt, and breccia, giving way to sedimentary formations like sandstone, shale, and limestone, all punctuated by fault zones that contained clay and alluvium. The Crossover machine will also be facing an aquifer system that could cause high-pressure water inrushes of up to 20 bar.
Due to the geology, the Gerede machine required a convertible cutterhead optimized for hard rock. The cutterhead was designed for ease of conversion between hard rock and EPB modes, and cutter housings can be fitted with either disc cutters or tungsten carbide tooling. In addition, the cutterhead is designed to operate in a single direction. The setup, which is used on all XRE machines, allows for greater efficiency while excavating, with lower power requirements and less chance of regrind. To cope with difficult ground, the Gerede machine was also equipped with special gearing.
In order to protect the machine from the expected high water pressure, an extensive sealing system was put into place. Around the main bearing, there is an outer row of six seals and an inner row of three seals. Between each seal, the cavity is filled with grease to ensure a constant pressure. In the event that the machine is shut down and an inrush of water overtakes the machine, a pressure sensor will detect this presence of water and flush the seal system with grease in order to continually protect the seals. The articulation joint and the gripper and stabilizer shoes are sealed off in the same manner.
Perhaps one of the most important parts of the Gerede TBM design is the screw conveyor. Because of the potential for massive amounts of water, the machine must have a sealed screw conveyor. However, running rock through a screw conveyor can be highly abrasive. To account for potential wear, the screw has been designed with replaceable wear plates along its entire length. The screw itself is also made up of short sections that can be removed and replaced if needed. Multiple access hatches are included for maintenance of the wear plates, while two large, removable outer casings can accommodate the change-out of entire screw sections.
Due to the unpredictable ground conditions, it is necessary to detect and grout off zones of concern wherever possible in order to protect the machine from loose ground and water pressure. The machine utilizes a standard array of twelve Ø100 mm ports angled at 7° that are equally spaced around the rear shield. Each port is sealed by a ball valve until it is needed for probing. Ten of the same-sized ports are also located straight through the forward shield for probing and grouting. Six additional hatches are built into the pedestal at the front of the machine. The hatches can be used with a pneumatic percussive drill in the center section of the cutterhead.
The Robbins XRE TBM was launched in summer 2016, using some components from the original Double Shield TBM back-up, as well as the remaining segments being stored for the project. Crews excavated a bypass tunnel to one side of the stuck Double Shield TBMs and the Robbins TBM components were walked in through the south portal. The machine was assembled using Onsite First Time Assembly (OFTA) in an underground launch chamber. Water flow made it difficult to the materials to the machine, but this was overcome by designing custom flat cars equipped with hydraulic lifts to transport bigger sections of the TBM through the tunnel to the build chamber.
The Robbins machine began boring at a slight angle to the rest of the tunnel, bypassing the stuck machine before gradually meeting up with the original tunnel alignment. The machine was required to be used in EPB mode as it encountered water pressures up to 23 bars, alluvium, flowing materials, and clay. 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 became routine after advancing 50 meters (164 feet) past the section that buried the original Double Shield TBM. To date, the machine has bored over 25 percent of the remaining tunnel length and has already crossed through the material that caused the Double Shield machine to become stuck.