Tunnel boring machines (TBMs) often play a crucial role during the construction of hydroelectric power projects. Whatever region the machines are deployed in, they are often faced with some unusual and challenging conditions. At a number of new hydroelectric schemes, the machines have proven to be up to the challenge, and below we take a look at the varied work of TBMs at projects in Canada, Switzerland and India.

Hard rock history at Niagara

Tunnelling success has come to the 10.2km long Niagara Power Tunnel, in Canada, which will be used by Ontario Power Generation to tap a proportion of the flow at Niagara Falls to divert to the Sir Adam Beck station, significantly boosting output. The tunnel excavation by the 14.4m diameter Robbins Main Beam TBM – named ‘Big Becky’ was completed in May this year.

Getting this far had proved to be a greater challenge than expected due to weaker geology along the vertical alignment, which had to be changed. What proved difficult was boring through the Queenston shale strata with TBM. Launched in late 2006, the TBM was to bore through a sandwich of near horizontal strata ranging that comprised limestone, dolomite, sandstone and also the shale. But, early on, there were delays and safety concerns due to extensive overbreak of the slate, in the excavated tunnel crown behind the TBM. Although the overbreak was an average depth of 1.5m it reached 4m in places.

Behind the TBM cutterhead, significant modifications were required to support the fresh excavation and further ensure worker safety. A revised ground support system was also introduced, including 9m long spiles and 4m long rock bolts as well as mesh, straps, and shotcrete. In addition, the tunnel was re-alignment out of the shale. Far behind the TBM, repairs involved re-establishing the profile of the profile. Then, all sections of the tunnel were ready for construction of the secondary lining to complete the 12.8m i.d. concrete-lined tunnel.

The power tunnel, originally scheduled for completed by mid-2010, is now programmed to finish by the end of 2013, and at 60% higher cost.

Niagara Tunnel Project is the third headrace tunnel running below the Falls. The tunnel will convey approximately 500m3/s to the Sir Adam Beck complex, increasing the exploitation of the Canadian share of the flow at the Falls, established by Treaty in 1950, and increase generation at the plant by about 1,600GWh of electricity per year.

The design-build tunnel contractor for Niagara Power Tunnel is strabag. Despite the excavation challenges, the TBM did, in sections, achieve rapid progress rate – two years ago of 153m in the best week and 468m in the best month, both in July 2009.

“There is a tremendous sense of accomplishment here. There were huge logistical challenges just delivering and assembling the TBM onsite, and the machine has now overcome significant geologic challenges as well,” said Mike Kolenich, Robbins Manager of Projects for Niagara.

As part of the historic breakthrough of the tunnel, the massive cutterhead from the TBM will be retained and kept on display in the city of Niagara Falls, Ontario.

Working under the Himalayas

India’s Hindustan Construction Company Ltd (HCC) awarded Seli the subcontract for TBM excavation and lining of a 14km long tunnel, which is a major feature of the Kishanganga hydroelectric project.

Located on the Kishanganga river, a tributary of the Jhelum river in the Baramulla district of Jammu & Kashmir, Kishanganga is one of the most important hydroelectric projects in India. Earlier this year, a specially designed Seli DSU TBM made its first official run in the tunnel project. The TBM was designed with special features and characteristics to bore difficult rock formations under the Himalayas, where unstable and squeezing ground conditions are expected. Main TBM features include: conical shield design, extended overcutting facilities, high cutterhead power, super high main and auxiliary thrust, and the capability to treat the ground ahead of the face through the shield and through the cutterhead.

With the Kishanganga TBM, SELI celebrated its 60th anniversary, marking the event by painting the TBM shields gold for this important occasion.

The TBM’s main specifications are as follows:

• 19” cutterhead design.

• 350mm of maximum total overcutting, adjustable depending on geology.

• 2520kW installed power to be able to rotate the cutterhead even in presence of collapsing faces.

• 43500 KN of auxiliary thrust.

The project is expected to be completed by January 2014.

Getting to grips with Nant De Drance

Located between the towns of Martigny and Chamonix in Valais, Switzerland, Nant de Drance is a joint venture between Swiss utility Alpiq (54%), Federal Swiss Railways SBB (36%) and municipal utility FMW (10%).

The project’s powerhouse will be built almost entirely underground in a cavern between two existing reservoirs, Emosson (1930m altitude) and Vieux Emosson (2205m altitude), on land belonging to the Valais border municipality Finhaut. A 5.6km long gallery will access the cavern at 1700m altitude in the Lower Valais.

The main headrace tunnel at the project, which is 5.5km long, will be driven from the main installation center of Le Châtelard. Two further accesses from Emosson will be required, 1.7km and 2.1km in length. The accesses will be driven forward under a gradient of up to 12%. Since January 2010 a Herrenknect Gripper TBM with a 9.4m diameter has been working its way through the hard rock. The S-522 has already driven forward to cover just under 1.8km within 12 months.