Shani Wallis, TunnelTalk

Far north near the Arctic Circle, and about 1,000km north of Oslo, preparations are underway for launch of the first hard rock TBM to be used in Norway for more than 20 years. The refurbished 7.23m diameter Robbins machine will excavate some 12km of a new 19km of parallel headrace tunnelling needed to rebuild and increase the output of the existing Røssåga hydropower station.

The TBM will excavate two lengths of tunnel in a two-stage upgrade of the power installation. The first is a 7.4km long parallel headrace to the Nedre (Lower) Røssåga powerhouse to increase the performance of the six 43.5MW units that came online in 1955. The second is a 4.3km long tunnel that will increase the capacity of the trailrace of the Øvre (Upper) Røssåga installation to improve the output of its three existing 62.5MW units that started operation in 1961, and allow operation of a new 225MW turbine to be installed in a new cavern at the powerhouse. The NOK1.7 billion (US$285 million) upgrade project for Statkraft Energi AS, owner and operator of the plant within the Norway's 100% national Government-owned Statkraft Group, will increase the capacity Upper Øvre Røssåga installation to 475MW, and production output of the combined scheme by 200GWh.

Upper and lower Røssåga hydro alignment

The TBM will be used by Norwegian contractor LNS (Leonhard Nilsen & Sønner) after it was awarded the NOK700 million (US$116 million) civil works contract for the station upgrade in November 2012.

The machine is a refurbished Robbins TBM and one of the three new high-performance main-beam gripper machines that worked originally on the Kárahnjúkar hydropower project in Iceland between 2004 and 2008.

TBM alternative
From a traditional point of view for Norway, the excavation contract for the project's new tunnelling was based and tendered on a drill+blast specification. This would repeat also the drill+blast work undertaken to build the original plants in the early 1950s and 1960s. Believing in the advantages of the TBM technique, Robbins worked hard to promote the alternative with LNS and bring back TBM excavation expertise to Norway after a hiatus of 20 years.

"For this project, we worked with LNS and the client to promote the advantages of the TBM option," said Sindre Log, Mananger of Robbins Norway. "We concentrated on several obvious benefits. First, the mechanical excavation process reduces the risk of damaging the existing headrace tunnels that are about 100-200m parallel, and by association, the risk of having to take those operating headrace tunnels out of production. Second, the TBM application reduces the length of tunnel excavation in the reconstruction contract by reducing the number of accesses and working adits that would be needed to complete the long tunnels in the same time using drill+blast. A further significant advantage is that the reduced friction in a bored tunnel waterway allows a reduced cross section and a reduction in the amount of rock excavation as compared to a drill+blast water tunnel alternative. There is also the potential for higher production rates as compared with drill+blast headings. To realistically compete with mechanized tunneling on excavation rates, several intermediate working sites would need to have been constructed and this in turn would have increased operational costs for the contractor."

Near-Arctic location of the Røssåga hydro scheme

Ultimately it was said to be an economic choice. LNS is a skilled and experienced drill+blast contractor and could manage an efficient drill+blast operation, however, the TBM option was within close competitive comparison commercially, and the contractor and the client were convinced by the additional potential advantages.

The Robbins supply contract with LNS is a lease arrangement for the TBM, and for a continuous conveyor muck hauling system. "We will also supply all cutters, spares and a significant amount of field service support to the contract," explained Log. Given the diameter and length of the tunnel headings, a continuous conveyor system was the preferred option. A track system will be installed to support opertion of service and man-carrier trains. The tunnel is designed to remain largely unlined in operation with shotcrete being applied in critical zones.

Refurbishment of the TBM, first used under extreme conditions on the Kárahnjúkar hydropower project, was carried out in Italy and at the Robbins workshop in Solon, USA. The components have since been shipped to Norway and are being reassembled at the Vesterli work site using the Robbins OFTA (on-site first time assembly) process. This is designed to save time on TBM mobilization and have the machine ready for launch in November and before the Arctic winter weather sets in. "We are awaiting final elements of the new backup and of the continuous conveyor system and boring is scheduled to begin by about mid-November," explained Log when TunnelTalk connected with him at the job site. "The machine will excavate the 7.4km long Nedre Røssåga headrace first and will be recovered to the surface for an overhaul and inspection before starting the second 4.3km Øvre Røssåga tailrace."

Geological plan of the parallel headrace upgrade of the Røssåga hydropower installation

Both tunnels will be excavated at a slight uphill gradient of 0.02%. The TBM will also bore two access tunnels of about 500m each in addition to the main drives. These adits will be bored on a 1:9 decline and on a curve radius of about 500m.

The geology along the tunnel alignments consists mainly of limestone, for about one third of the distance, with mica gneiss, mica schist and some garnet mica schist and granite making up the remainder.

The limestone rock for the most part is medium strength at up to 150MPa, with the potential for karst features and heavy ground water ingress. Also identified are reaches of hard, massive rock of up to 250MPa in UCS, as well as zones of complex and altered geology. To prepare for these conditions the TBM is equipped with powerful drills and an innovative fully integrated real-time measurement system especially designed by Robbins for monitoring the drilling process. "This capability is included mainly to address the potential for meeting karst voids through the limestone," explained Log.

Working 24hr/day, six days/week, the 7.4km long Nedre Røssåga headrace drive is estimated to take 12 months of TBM boring towards a scheduled completion in Autumn 2014.

Ex-Kérahnjúkar Robbins TBM will drive the new hydro tunnels

Start of a new TBM era
Application of a TBM on the Røssåga project is the first after the golden era of hard rock TBM experience and development in Norway from the 1960s to the 1990s. Norwegian hydropower development started more than 100 years ago and the most productive period was between 1950-1990. To date there are some 258 underground hydropower stations with many hundreds of kilometers of headrace and tailrace tunnelling excavated by TBMs, matching the same, and more, excavated by drill+blast.

It was a downturn in hydro development 20 years ago, and failure of the technique being adopted for Norway's follow-on golden era of road and railway tunnelling projects, particularly undersea road tunnels, that the nation lost its wealth of experience and its confidence in the TBM technique. Success of the TBM alternative on this project signals perhaps a new golden era for TBM tunnelling in Norway.

Although LNS is the first to reintroduce TBM tunnelling and break the mould, other high profile projects are also adopting or supporting the TBM option. One of these is the Ullrikken railway tunnel, which is to be excavated beneath the city of Bergen and in an environment where excavation will have to meet strict noise and vibration limits. More high profile still is the Follo tunnel for the new railway link between Oslo and Ski. Some 19.5km of the 22km long underground link is to be excavated by four shield TBMs working two each in opposite directions from a single central access adit, installing behind them a single shell precast concrete segmental lining. These projects will confirm the feasibility and advantages once again for using TBM excavation for tunnelling projects in Norway.

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References

TBM tunnelling complete for Iceland's Kárahnjúkar power station - TunnelTalk, April 2008
TBMs will excavate the Oslo-Ski Follo Line - TunnelTalk, November 2012
Prequalification call for Oslo-Ski TBM link - TunnelTalk, May 2013
Europe hydro vision needs 165km of tunnels - TunnelTalk, November 2012
Norwegian Hydropower Tunnelling II - Norwegian Tunnelling Society Publication - TunnelTalk, Books and Reports
ITA/WTC 2010 Sir Alan Muir Wood Lecture - Tunnels and Underground Works for Hydropower Projects: Lessons learned in Norway and worldwide by Dr Einar Broch, Norwegian University of Science and Technology - YouTube video

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