The 7.4km long headrace tunnel is being built for the Røssåga Hydroelectric Project, and marks the first use of a TBM in Norway in 22 years. Increased demand for hydropower, and longer tunnel lengths, in a country that has specialised in drill and blast, is ushering in new interest in mechanised excavation. Given that Norway builds between 80 and 100km of tunnels per year, the market is primed for a TBM renaissance.
Tunnel boring past and present
"Norway has a long history of tunnelling — in fact there are 1.2m of tunnel for every Norwegian in the country," said Frode Nilsen, managing director of Røssåga contractor Leonhard Nilsen & Sønner (LNS). That long history includes a focus on drill and blast tunnels throughout the country’s famously hard and abrasive rock. "We have specialised in drill and blast in Norway, and we are able to do it very efficiently, with very few people at the face, for a low cost. It is a myth though, that this is all done in competent rock. Many foreign contractors think that if you have hard rock, then the conditions must be good, but often we encounter fault zones and other poor quality rock."
In the 1980s and 1990s, a surge in hydroelectric power plant construction drove demand for TBMs to tunnel long head and tailrace tunnels through rocky conditions – approximately 260km of tunnel were driven by TBMs over several decades.
At the Svartisen hydroelectric project, a total of six Robbins Main Beam TBMs were deployed to excavate 57km of tunnel through incredibly hard rock ranging from 100 to over 300MPa UCS. To complete the excavation, Robbins developed what were the largest disc cutters in the industry at the time – 19 inches (483mm) in diameter, and installed high-powered motors in each machine to allow between 7,000 and 9,000kN of thrust. The innovative machines achieved then-world records between 1989 and 1991, including a best week of 360.5m and most material excavated in 24 hours, at 1,309cu.m. Since that time, TBMs have been absent from Norway.
The lack of TBMs may have been a combination of factors, including an absence of governmental support for many hydroelectric projects, and more demand for road tunnels. "Drill and blast has always been the dominating method in Norway. This is due to the fact that in Norway we are always excavating a large number of different types of caverns for various purposes and in lots of different sizes. And for this purpose conventional drill and blast fits much better. TBMs are likely not a relevant option to make these caverns. Further, the typical horseshoe-shaped drill and blast cross section is likely more adapted to the needs of road tunnels specifically," said Eivind Grøv, Adjunct Professor at NTNU and Chief Scientist at SINTEF, Norway’s largest independent research organization.
The resurgence of Norwegian TBMs at present is a combination of new types of tunnels being proposed and governmental initiatives. "After more or less 20 years standstill in hydropower construction in Norway there is suddenly a boom in this market. This is mainly due to "Green Certificates" issued by the government, which guarantee a set price for all new power plants in production before 2020. In addition, some of the older hydropower plants are in dire need of refurbishment. Building a new tunnel is much more cost effective than taking the plant offline for one to two years for remodelling," said Sindre Log, civil engineer and general manager of Robbins Norway.
Longer tunnels are also driving change: Planned tunnels such as the Follo Line consisting of 20km parallel tunnels are bringing in outside contractors. These international firms are proposing TBMs as the most cost effective method, bringing it to the forefront of new tunnel construction in Norway.
Potential for production
In competent hard rock conditions, TBMs are capable of high rates of excavation – a fact that appeals to Norwegian contractors looking at long distance tunnels. In fact, hard rock machines such as those used at Iceland’s Kárahnjúkar Hydroelectric Project have similarly dealt with incredibly cold temperatures and hard, abrasive rock with resounding success. At that project, three Robbins Main Beam TBMs were launched in temperatures as low as -25oC, excavating volcanic rock of 300MPa UCS or more with extensive water inflows. Despite the challenges, the machines achieved world records, including 115.7m of rock excavated in 24 hours.
While success stories abound, there are still some barriers towards a broader acceptance of TBMs in the Norwegian market. The perception of risk involved in bringing in new technology is one that needs to be addressed. Recent applicable experience by Norwegian firms may also be an issue: "Building up of competence by all parties involved in a sub-surface project is key, including the consultants, the owners and the contractors. This also needs to happen within academia, which is often on the front line with research and development projects," said Grøv.
Røssåga’s TBM choice
When engineers looked at the design of the Rossaga tunnel, a 7.4km long headrace tunnel plus a 450m long access tunnel through hard rock, they began considering alternative excavation methods. Designed to provide an overhaul and addition to existing power stations, the Røssåga Hydroelectric Project involves building a new powerhouse, headrace and tailrace tunnel to increase annual generating capacity by 70MW. The project was originally tendered as drill and blast. However, once contractor LNS submitted an alternative TBM solution, the project owner identified the benefits of the TBM method, and asked for alternative TBM solutions from all bidders, eventually leading to the contract being awarded to LNS.
The 7.2m diameter Robbins TBM chosen by LNS was in fact the refurbished record-breaking machine used at Iceland’s Kárahnjúkar tunnels. "I am most proud that LNS has been able to bring TBMs back to Norway after 22 years since the last TBM project. And the reason we managed to do that was because of a very close and good cooperation with Robbins," said Nilsen.
Mountainous Assembly
Getting the machine assembled and launched in the mountainous terrain of northern Norway, however, required some serious logistics. The contractor opted for Onsite First Time Assembly (OFTA), making it the first time the assembly method had been used in Europe.
First developed in 2006 at the Niagara Tunnel Project, Robbins’ OFTA method allows for components of the machine to be shipped directly to the jobsite, so that the entire machine is assembled for the first time on location.
This method results in reduced shipping and manpower requirements, with proven time savings of up to five months and cost savings up to USD 4M for large diameter hard rock machines.
The solution to assemble the Røssåga TBM using OFTA was strategically planned to expedite the project schedule. "The logistics were complex for bringing the TBM, conveyor system, spare parts, and cutters from all over the world to almost ‘the top of the world’," said Nilsen. Good weather resulted in bare road conditions and allowed delivery before winter snow arrived. Altogether, more than 90 loads were delivered to the remote jobsite without any major setbacks.
This included the heaviest component, the centre cutterhead, weighing in at 62,000kg. Five months after the first part was delivered in September 2013, the Main Beam machine started boring. The machine, dubbed ‘Iron-Erna’ after the country’s Prime Minister Erna Solberg, was launched on January 16, 2014.
Hard rock challenge
As of April 2014, the machine has excavated several hundred meters of tunnel. "The main problem is the rock condition, which is currently 275MPa quartzite. Boring is very slow. We may come into limestone 100 to 200m from now, but for the time being we are in close cooperation with Robbins to find the best cutter for these very difficult conditions," said Nilsen.
Laboratory testing of the unexpectedly hard and abrasive rock is underway, and will help to identify the best cutting tools for the job. Certain sections may be even harder than 275MPa. "We think this may be some of the toughest material ever seen in a hard rock TBM tunnel. We are also analysing the quartz content."
In addition to heavy duty cutters, the machine is outfitted with a Measurement While Drilling (MWD) system to analyse ground ahead of the face. Probe drilling is measured manually or automatically. The MWD system is then used to analyse the rock in detail (hardness, water content, rock mass properties, etc) and can be used to generate 3D-models of the rock mass in order to decide on the rock support.
Probe drilling also has the capability of being a continuous operation with a new innovation being tested on the TBM. Drills at angles of five to eight degrees allow for probing and grouting of the periphery on all types of TBMs, and for drilling through the face even when the TBM is not boring. The benefit of the system is to reduce the overall construction time. How much time can be saved depends on the extent of probing and grouting and the capacity of the drilling and grouting equipment, but estimates have savings as high as 20 to 30 per cent. The TBM is currently on schedule for a breakthrough in summer of 2016. The newly renovated powerhouse is slated to go online in spring 2017.
With more road, rail, and hydroelectric projects planned, the Norwegian market is expected to boom in the next five to ten years. "The Norwegian tunnelling industry went sky-high in 2013 with a record-breaking production of more than 6 million solid cubic meters of rock. As far as can be seen from the political scene, there is a drive towards increased infrastructure development in Norway, for road and railway construction and hydropower development," said Grøv.
Grøv’s work at the Norwegian University of Science and Technology (NTNU) in Trondheim may play a part in the uptick of TBMs used in hard rock. Engineers there are hoping to extend cutter life through hardened steel mixtures — a research collaboration by academics, steel manufacturers, the Norwegian government, BASF and Robbins.
The project, known as FAST-Tunn (Future Advanced Steel Technology for Tunneling), aims to investigate tool steel mixtures in the hopes of finding a stronger material that can withstand the tough conditions seen on recent hard rock projects. "In hard rock, thrust is very important. The higher the forward thrust, the higher the advance rate; this is not a linear but an exponential function. To be able to push harder, better bearings and better cutter rings are needed. One can make steel that can withstand very high thrust, but then it becomes brittle and abrades faster. A good resistance against high abrasion and increased thrust is the optimal design," said Amund Bruland, Professor in the Department of Civil and Transport Engineering at NTNU, and FAST-Tunn researcher. The research collaboration has been largely successful, and is currently in its last year. "We are sure that we are on the right track as far as achieving steel qualities that can provide the improvements that we are aiming for through this project," said Grøv. With academic buy-in and improvements in TBM tunnelling through extremely hard rock, mechanised boring has a bright future in Norway. For Grøv, subsea TBM tunnelling and more universal machine designs that can handle a variety of conditions are exciting prospects, as long as the technology can be adapted for the Norwegian market. But for Nilsen, the adoption of TBMs is clear-cut: "In Norway, we have deep wallets, high mountains, and deep fjords. We need a lot of tunnels"
