On 18 December 2018, A Robbins Crossover XRE TBM – a hybrid between a Single Shield Hard Rock Machine and an EPB – completed what would once have seemed far-fetched. In the mountains outside of Ankara in Central Turkey, the TBM had bored the final 9km leg of the 31.6km-long Gerede Water Transmission Tunnel by navigating 48 fault zones and hydrostatic pressures up to 26 bar. The notoriously difficult conditions had previously caused two double shield TBMs from a different manufacturer to become stuck in massive inrushes of mud and water.
The tunnelling success story highlights what today’s equipment is capable of, and how far the Turkish tunnelling industry has come in tackling its own incredibly difficult geology.
History in the Making
While TBMs are today a staple of underground work throughout Turkey, that wasn’t always the case. Nuh Bilgin, Professor of Mine and Tunnel Mechanisation at Istanbul Technical University and Chairman of the Turkish Tunnelling Society, says the change was a gradual one.
“I was first involved in an underground project in 1982, which involved three years of drillability studies in the Zonguldak Coal Field. At that time 10km of tunnels (roadways) were driven each year in Zonguldak coalfield using the drill and blast method in very hard and abrasive rock formations with a mean daily advance of 1.2m/day. At that time the contractors in Turkey were not acquainted with mechanised excavation.”
Several years later, mechanical excavation had made its way into the country. “My first experience with mechanized tunnelling was with shielded roadheaders in 1986,” continues Bilgin. “The roadheaders were instrumental in cleaning the polluted Golden Horn in Istanbul, the results of which were published in Tunnels & Tunnelling International in 1988 and 1990. We developed a rock cuttability index for the prediction of cutting rate of the machine, which is now widely used for the prediction of roadheader performance.”
The country’s first instance of TBM tunnelling was one year later in 1987, using a 5m-diameter Robbins Double Shield TBM on the 2.4km-long Dalyan-Moda Wastewater Tunnel in Istanbul. But further acceptance of TBMs would take more than a decade as financial constraints and a developing economy placed limitations on tunnelling project funding.
“Contractors became interested in using TBMs after a series of short courses was given by Levent Özdemir from the Colorado School of Mines on ‘Mechanical Excavation Technologies in Underground Technologies.’ The use of TBMs was further boosted after an excavation laboratory was established in our Department at Istanbul Technical University in 2002,” says Bilgin. The project was sponsored by NATO for the “Development of Rapid Excavation Technologies for the Turkish Mining and Tunnelling Industries” as part of its Science for Stability Programme. With a project budget of almost USD 1M, Bilgin was named the coordinator of the project and Özdemir the consultant to NATO. Its first project was on full-scale cutting tests for the rebuilt Dalyan-Moda machine while it bored the Tuzla Sewerage Tunnel. The now well-established laboratory has released many research papers over the past two decades and has been selected by the European Union Scientific Council as one of the top ten best laboratories in Turkey.
“I am glad to say that today in Turkey, TBMs are standard machines for metro, water, wastewater and some railway projects. However, the drill and blast tunnelling method is still used in almost all road tunnels,” added Bilgin.
Turkey’s Biggest Challenge Yet
Recent Turkish tunnels, including the Kargi Kizilirmak HEPP, where a 10m Double Shield TBM overcame multiple fault zones and difficult geology, underscore the challenges of tunnelling in Turkey. Perched between the European and Asiatic land masses, Turkey can be thought of as a geologic patchwork, comprised of older rocks mixed with younger igneous and volcanic rock. More than 80 per cent of the country’s land surface is rough and mountainous, making both agriculture and travel a challenge. Adding to the obstacles, Turkey is seated on an active tectonic belt bounded by the North Anatolian Fault and the East Anatolian fault. Tunnelling in such a zone is difficult at best, and nearly impassable at worst.
The latest and perhaps most ambitious project in Turkey, the Gerede Water Transmission Tunnel, underscores the numerous geologic challenges. The long conduit is necessary to alleviate chronic droughts in and around Ankara by conveying water from the Gerede River to Çamlidere Dam, which provides potable water for the Ankara city water system.
A 5.6m Robbins Crossover XRE TBM was supplied to complete the final 9km of 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. “Three double shield TBMs were originally working on this project–the first section with lower overburden and less tectonically disturbed ground was completed successfully. However, due to increasing overburden and more tectonically disturbed areas, problems in the second and the third sections of the project started soon after boring. Due to a sudden water and material inrush, the segments collapsed in section 3 of the tunnel causing irreversible damage to the third TBM. The job owner and the contractor were in shock. The second TBM also stopped due to safety reasons,” says Bilgin.
The Kolin/Limak JV had to develop a new strategy given the unexpected ground conditions, and consulted with the owner, the State Hydraulic Works or Devlet Su Isleri (DSi). “DSi encouraged the contractor to find a solution to the problem. The contractor was keen to get a TBM more robust and more resistant to the changing geology and high water pressures consistently around 10 bars. In some part, I assisted the contractor to select the machine and discuss the technical future of the machine,” says Bilgin.
“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,” says Yunus Alpagut, Robbins representative in Turkey.
The specialised 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. “Most surprising for me was that the TBM was assembled inside the tunnel under heavy environmental conditions,” says Bilgin. The inner assembly chamber was 7km from the portal and the excavated section of tunnel was experiencing inflows of 600 l/s (159 gal/s), which made the logistical transport of materials a challenge.
Despite the challenges, the machine began boring in summer 2016 and within the first 50m 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 consistently high water pressures, 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. “The Crossover XRE TBM performed very well in all geological formations of the tunnel section and statically held back 26 bar water pressure although designed and manufactured for only 20 bar water pressure,” says Alpagut.
The high water pressures made tunnelling a slow process in sections, however. “As the TBM progressed and increased its distance from the portal each successive month the difficult geological conditions and the travel time to reach the TBM within the tunnel caused significant effects on TBM performance. But despite these constraints, the tunnel excavation was still completed,” says Baris Duman, Project Manager for the Kolin – Limak JV. In fact, the tunnelling operation achieved a best day of 29.4m, best week of 134.6m and a best month of 484m while averaging 285m per month.
“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,” says 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.”
Bilgin adds, “The last month of boring was a great success. I believe that the breakthrough was not only the success of the TBM but also for the tunnelling sector in Turkey and all the players.
Technical assistance from the manufacturer, skilled tunnelling staff, encouragement from DSi and the courage of the contractor in taking on this challenging project are all factors. We are proud of this success in the Turkish tunnelling sector.”
Lessons Learned In Difficult Conditions
Gerede is certainly not the first tunnel in difficult ground, nor is it the last, with projects such as the Bahçe Nurdagi High Speed Railway ongoing in extremely hard and abrasive rock. As for what the Gerede project means for the future of tunnelling in Turkey, Bilgin notes several key takeaways.
“I think learning always has its costs. Past TBM projects have shown that in order to achieve success in difficult ground or in very hard and abrasive rock formations, robust machines equipped with the latest technology must be coupled with the experience of the machine manufacturer. These things are a necessity for rapid and efficient tunnel excavation.”
He cites recently published papers on the Bahçe Nurdagi, Gerede and Kargi tunnels as good case studies of the process. As for his more than four decades of experience in the tunnelling industry, Bilgin sees further lessons learned: “What I’ve learned on tunnel projects is that understanding the geology is essential and must be paired with detailed site investigations. Lastly selecting the correct, robust and reliable machine and an experienced tunnelling crew are the keys for success.”
