The Nant de Drance Hydropower Scheme – brief overview of the civil works

With the rapid development of on-grid renewable energy sources, and the programmed decommissioning of a number of nuclear power stations in mainland Europe, the need for large-scale, adaptable electrical energy storage and generation solutions in the near future is self-evident. Modern pumped hydro storage offers a technologically proven and efficient solution to reduce volatility and match supply and demand on grid. Located in the Valais region of Switzerland, the Nant de Drance project involves adding a 900 MW pumped-storage power plant to an existing hydropower scheme by connecting the two existing reservoirs of Emosson and Vieux-Emosson. Projected capacity is to be achieved by installing six vario-speed pump-turbines of 150 MW each and by heightening the upper, smaller Vieux Emosson dam in order to double its storage volume. The power plant is expected to produce 2'500 GWh per year and commissioning is to take place progressively between 2018 and 2019. The aim of this article is to give a brief overview of this exceptional project from a civil engineering point of view, and some of its key features.

Project background

The existing scheme The Nant de Drance project is being constructed within an existing hydropower scheme which has been developed continuously over the past 100 years: • The first reservoir built on this site was Barberine, which was developed from 1920 to 1926 by the Swiss Federal Railways (SBB) to power its electrical rail network. • The Vieux Emosson reservoir, located 2’200 m a.s.l. above Barberine, was constructed by SBB between 1952 and 1955 to provide an additional 10 million m3 of storage capacity to the scheme. Power generation over this head difference (future Nant de Drance) was not considered economical at the time. • From 1967 to 1975, Emosson Dam (180m high, 560 crest length) was constructed by a Franco-Swiss joint venture (EDF and ATEL), drowning the Barberine scheme. A large network of underground adits and tunnels was constructed in parallel to increase the hydrological catchment area. [caption id="attachment_41535" align="alignright" width="300"]

Figure 1 -Schematic of the existing Emosson hydropower scheme[/caption] A summary of the existing scheme is illustrated in Figure 1. A view of the scheme from Vieux Emosson Dam is given in Figure 2, above right. Nant de Drance The Nant de Drance project consists of a 900 MW pumped-storage power plant connecting the two existing reservoirs of Emosson (1'930 m a.s.l.) and Vieux-Emosson (2'225 m a.s.l.) in the Alps in the southwest of Switzerland. The project was initially designed for a total capacity of 600 MW, provided by four variable speed pump-turbines of 150 MW each. During the detailed design phase, the installed capacity was increased to 900 MW by adding two pump-turbines of 150 MW and by raising the height of the upper, 45 m high Vieux Emosson dam by 21.5 m, thereby doubling its storage volume. In order to build and access the main powerhouse cavern (KMA), the adjacent transformer cavern (KTR), the power waterways and the valve chambers, a 14 km long system of access tunnels was excavated. The main access tunnel, which connects the main entrance (located in the village of Châtelard at 1'100 m a.s.l.) and the main powerhouse cavern (located at around 1'700 m a.s.l.), has a length of 5.6 km and a slope of more than 10 per cent. It was excavated using a 9.45 m diameter hard-rock tunnel boring machine (TBM). The tunnel was completed at the end of August 2012. The remaining access tunnels, with cross-sections of 46 or 52 m2 and slopes of up to 12 per cent, were excavated using the drill and blast method. A cross-section of the scheme is given below. [caption id="attachment_41536" align="alignright" width="300"]

Figure 3: Cross-section of the hydraulic system and access galleries[/caption] The underground powerhouse, located 600m below the surface, will house reversible Francis pump-turbines with variable speed technology. Key figures: Projected cost: 2 bn CHF (1.8 bn €) Installed capacity: 900 MW (6 x 150 MW) Annual production: 250 GWh/yr Head: Min 250 m, Max 395 m Max flow rate: 2 x 180 m3/s Start of works: Autumn 2008 Commissioning: 2019 Key players in the project: Nant de Drance is a joint venture between a number of Swiss utilities and power firms, as well as Swiss Federal Railways (SBB). Shareholders are summarised below: Company: Share in Nant de Drance Alpiq: 39% SBB (Swiss Federal Railways): 36% IWB (Basel Utilities Company): 15% FMV (Valais Power Company): 10% Designers The principal consultants involved with the design of Nant de Drance are summarised below: AF Consult: General Consultant, HEM BG Consulting Engineers: Powerhouse and switchgear caverns, HSVE Pöyry: Waterways, access tunnels SRP and PRA: Spoil management Stucky: Vieux Emosson dam heightening Contractors The principal contractors involved with the design of Nant de Drance are summarised below: GMI (Joint venture Marti Implenia): Civil works GE Hydro: Electro-mechanical works Andritz Hydro: Steel lining works ABB: Electrotechnical works ABAG: HSVE general contractor

Notable features of the Nant de Drance scheme

This chapter outlines some of the particularities of the scheme, difficulties encountered and engineering solutions devised to overcome them. Geological conditions and access tunnels The powerhouse caverns are located in metamorphic rocks of sedimentary origin belonging to the crystalline platform of the Massif des Aiguilles Rouges (Mont-Blanc area). Overburden at the axis of KMA is about 600m. The caverns sector was investigated by five core-boring exploratory drills up to 660m deep. The lithologies encountered, all of which present excellent geomechanical properties, are slaty gneisses, micaschists, metagraywakes (shales rich in chlorite) and paragneisses (coarse grained gneiss). These rocks are in good condition, hard, laminated to interbedded. The main lamination plane dips 70 to 80°, at a strike angle almost perpendicular to the cavern axis. It has no effect on the stability of the rock, which is globally good to very good. The fracture state can be described as poorly fractured to unfractured. Fractures are generally closed, with cristallisation of quartz, epidote and calcite. Observed fractures are weakly persistent. Six types of fractures were observed. From a hydrogeological standpoint, several fracture flows have been encountered. Individual inflow discharges encountered are very low, never exceeding 0.1 l/s. Caverns and access tunnels were excavated using the drill-and-blast method, except for the main access tunnel, which was excavated using a 9.45m diameter hard-rock tunnel boring machine (TBM) (Figure 4). [caption id="attachment_41537" align="alignright" width="300"]

Figure 4: Mounting the TBM cutter head at Châtelard Portal[/caption] Powerhouse caverns The powerhouse cavern (KMA) is 32m wide, 52m high and 194m long and is connected to the transformer and switchgear cavern (KTR), which is 20m wide, 15m high and 130m long, located in its vicinity. They are among the largest of their kind in Europe. A view of the fully excavated KMA is given in Figure 5. [caption id="attachment_41538" align="alignright" width="300"]

Figure 5: Fully excavated powerhouse cavern, summer 2014[/caption] Pressure shafts The scheme comprises two power waterways, each of which consists on its upstream side of a 200m long concrete-lined pressure tunnel (Ø = 7.70 m), followed by a 434m deep concrete-lined vertical shaft (Ø = 7.00 m) and an 130m long steel-lined pressure conduit (Ø = 5.50 m) including the bifurcators (Ø = 3.20m). The pressure shafts were excavated using the raise-drill technique, which involved the following steps: • Exploration borehole drilled from top to bottom; • Attachment of drill-bit and excavation from bottom to top (Figure 6);

[caption id="attachment_41540" align="alignright" width="300"]

Figure 6: Raise drill for pressure shaft excavation[/caption] • Widening of the excavation from top to bottom (drill-and-blast method). Downstream intakes Nant de Drance has to be constructed while maintaining the Emosson in operation with minimal disruption. For this reason, the time window for executing the downstream intakes was too short for in-situ construction in the bed of the lake. The solution that was devised was to build them at high water level (Figure 7), and then float them using immersed box techniques to their final location (Figure 8). The hydraulic connection between intakes and waterways was only completed once the downstream gates were fully commissioned. [caption id="attachment_41541" align="alignright" width="300"]

Figure 7: Construction of downstream water intake[/caption] Heightening Vieux Emosson dam [caption id="attachment_41542" align="alignright" width="300"]

Figure 8: floating downstream water intake across Emosson lake[/caption] Due to the scheme power increase which was decided during detailed design, it was necessary to increase the height of Vieux Emosson Dam (final height 66.5m) in order to double the reservoir volume. By doing so, the dam was converted from a single curvature to a double curvature arch dam. This heightening was carried out in the following steps: • Demolition of upper 20m of dam in order to correct its geometry; • Reconstruction of upper 40m to new crown level; • Grout injections and surface treatment. The heightening operation is illustrated below. [caption id="attachment_41543" align="alignright" width="300"]

Figure 9: July 2013, heightening of Vieux Emosson dam[/caption] Spoil management Managing the muck generated by the Nant de Drance project is an arduous task: more than 4.2 million tonnes of muck will be produced during construction, including a considerable volume of spoil containing high amounts of radioactivity and arsenic (both found naturally in the bedrock), which require special treatment. With limited backfilling options and a view to minimising environmental impact, the following measures were worked into the project from the outset: • Production of all concrete for site requirements using aggregate generated from spoil (600,000 tonnes of spoil reused), see Figure 10; • Use of spoil to backfill and revitalise an existing quarry, see Figure 11. [caption id="attachment_41545" align="alignright" width="300"]

Figure 10: production of concrete aggregate from spoil[/caption] Progress of the works The key dates in the construction of the Nant de Drance scheme: Start of the works: autumn 2008 Start of dam heightening: spring 2013 Start of powerhouse concrete works: summer 2014 Completion of concrete works: summer 2016 Delivery of first spiral case: summer 2017 Commissioning: gradually, from 2018 to 2019

Conclusion

Given the current energy production context and future trend, the ability to store and generate electricity rapidly on-grid is going to be increasingly important. By its ability to absorb or inject up to 900 MW of power, Nant de Drance is going to be a key instrument for stabilisation and regulation on the Swiss and European grid. [caption id="attachment_41546" align="alignright" width="300"]

Figure 11: backfilling of existing La Gueulaz quarry with spoil (backfill capacity 350,000m3)[/caption] This article presented some of the key features and characteristics of the scheme from a civil engineering standpoint, challenges encountered by the project and solutions devised to overcome them.

Acknowledgements and references

The authors would like to thank Project Client Nant de Drance SA for its support in writing this article. For more detailed information regarding modelling of the excavation and structure, the reader is referred to the following articles: 1. Nilipour, N., Garin, E., Ihly, T., Seingre, G., 'Adding a 900-MW pumped storage power plant between existing reservoirs in the Swiss Alps', Proceedings, HYDRO 2012, 29-31 October 2012, Bilbao 2. Garin, E., Nilipour, N, Fournier-Bidoz, L., Kohler, P., 'Geomechanical design of a large deep underground powerplant', Proceedings, HYDRO 2012, 29-31 October 2012, Bilbao 3. BG Consulting Engineers ltd., 'SCIA User Contest 2015: Nant de Drance', 16.02.2015 4. Kazerani, T., Nilipour, N., Garin, E., Seingre, G., 'Application of numerical modelling for large-scale underground excavation in foliated rock mass', Proceedings, EUROCK 2015 & 64th Geomechanics Colloquium, Schubert Ed. Authors: Patrick Heck, CEng MIEI MICE, graduated with a degree in civil, structural and environmental engineering in 2007 from Trinity College Dublin, Ireland. Heck started his career in geotechnical engineering, first with Buro Happold in the UK, and from 2010 with BG Consulting Engineers in Lausanne, Switzerland. He joined the BG Hydropower Business Unit in 2014, and has been involved in the design and construction of several hydroelectric schemes in Switzerland. Patrick is currently design coordinator for BG for the Nant de Drance powerhouse. Nima Nilipour, civil engineer, MSc, graduated in 2001 from Iran University of Science and Technology in geotechnical engineering. In 2003, he obtained his MSc in hydraulic schemes at Swiss Federal Institute of Technology in Lausanne (EPFL) and since then has been working in the design of several large hydropower schemes and large dams in Switzerland and abroad. In 2009, he joined BG Consulting Engineers and is in charge of the Hydropower Business Unit of the BG Group. In parallel, he obtained his MBA in 2010 from HEC Lausanne. He is currently lead designer of the Nant de Drance powerhouse. Gérard Seingre, civil engineer, MSc, graduated in 1990 in civil engineering from the Swiss Federal Institute of Technology in Lausanne (EPFL). He benefits from vast experience in the field of underground construction. He has actively participated in the design and construction of major tunnels in Switzerland and abroad, as well as hydroelectric power schemes. He is currently the site manager at the Nant de Drance Hydropower project. He is in charge of the tunnelling course at the Swiss Federal Institute of Technology in Lausanne and animator of the ITA Working group on long tunnels at great depth.