The Grance-Dixence hydroelectric scheme is the flagship of hydroelectricity in the Wallis canton, with a 420 sq km catchment area, 35 glaciers, 80 water intakes and a 400 million cu m reservoir. It is formed by four hydroelectric plants (Bieudron, Nendaz, Fionnay and Chandoline) that produce nearly 25% of hydroelectric energy in the canton, which is 4% of the Swiss demand.
Located in the Swiss Alps, the Cleuson-Dixence hydroelectric scheme went into operation in September 2000. It consists of a 15,850 m-long headrace tunnel, a surge chamber and a 3,920 m-long inclined shaft leading to the 1,200 MW underground powerhouse of Bieudron. The design discharge of the scheme is 75 cu m/s. The power station is characterised by the highest drop in the world (1,883 m). The scheme is designed to produce peak energy.
Penstock rupture
On 12th December 2000, the penstock which brought water from the Grande-Dixence dam to the Bieudron power station ruptured at a point between Péroua and Condémines, 1,234 m asl and under more than 1,000 m of head. A fissure appeared on a longitudinal weld of the shaft lining and propagated in the steel of the adjacent segments. The rupture was approximately 9 m long by 60 cm wide. The released water rapidly reached the surface through rock joints. The ensuing rapid release of a very large quantity of high pressure water, pouring down the mountainside and turning into torrents of mud and rock, destroyed approximately 100 hectares of pastures, orchards, forest, as well as washing away barns and 17 chalets. Three people were killed. The volume of water released was estimated to be around 90,000 cubic metres. Since then, the Bieudron power station has been taken out of service, leading to a 1,200 MW reduction in capacity.
In 2003, Energie Ouest Suisse (EOS) Holding and Grande-Dixence founded Cleuson-Dixence Construction (CDC), with the goal to rehabilitate the power station. CDC’s mission is to plan and manage the studies and construction to re-commission the Bieudron power station in January 2010. The rehabilitation will cost CHF365 million. Work is ongoing 24 hours seven days/week and the project employs 180 people. Visit www.cleuson-dixence.ch
Penstock rehab
A feasibility study was carried out in 2003 by Verbundplan of Austria (now Pöyry) in collaboration with Stucky. The technically acceptable solutions for the power plant rehabilitation were studied, from the repair of the existing penstock to the building of a new shaft. Visit www.poyry.at and www.stucky.ch
Much investigation went into the accident resulting in the almost complete redesign of the penstock. Redesign considerations include operational control of maximum agreed design limits under all conditions of assembly, design and service conditions (including water hammer pressure spikes). The redesign consists in improving the pipe's lining as well as grouting around the penstock to reduce water flow between the penstock and the surrounding rock due to dynamic variation of the pipe diameter during operation (it expands due to water loading during operation then contracts when the load is removed, leaving a gap).
The selected solution that was chosen for the rehabilitation of the shaft is a L-shaped bypass under the failure zone, consisting of a 65 m-deep vertical embedded shaft and a horizontal penstock placed on supports in a tunnel (see map). Also, a self-supporting inner liner, made of S550 and S690 high strength steel, is placed over the entire shaft length, except in the zone of the accident. The space between the lining and existing shaft will be concreted. The existing liner will be maintained, but it will act only as a watertight barrier. The failure zone, where the rock around the penstock was badly damaged, has been specifically treated.
At Les Fontanettes, an upper exploration tunnel was dug to access to the upper part of the pressure shaft damaged by the accident and allow the geological investigation of the disrupted zone. Work started in August 2005 and ended in January 2006. This 20 sq m tunnel is also used for access to the penstock for the rehabilitation works. The entry to the tunnel is at an elevation of 1,260 m halfway along the penstock, between the Péroua and Condémines adits. The second step consisted in digging a cavern (cavern Fo I) around the damaged part of the penstock (the tip of the vertical leg of the L) so as to remove it on a length of 30 m. It implied extensive works to strengthen the ground (forepoles, steel supports, shotcreting and grouting of the rock mass).
The same process was implemented at a lower elevation (1,200 m) in Les Fontanettes to dig another exploration tunnel and access to the disrupted zone. At the junction of this tunnel with the pressure shaft (tip of the horizontal leg of the L), a cavern was also bored (cavern Fo III). There again, the penstock was removed in a length of 30 m. Excavation continued until connecting with the elbow of the L and bottom of the 65 m vertical shaft, where another cavern (Fo II) was constructed to make space wide enough for the penstock installation. The vertical shaft was dug from the bottom of the upper cavern (cavern Fo I) to the lower cavern (cavern Fo II). It is supported by steel arches, concreted and waterproofed.
The civil engineering works have been awarded to consortium LEDI (Losinger, Evéquoz, Dénériaz and Imboden). The engineering contract for the rehabilitation was awarded to a joint venture comprising of Electricité de France (EDF) as a leader, plus Stucky and Bonnard et Gardel. The contract includes the responsibility for the entire rehabilitation from the studies to the works on site. Visit www.losinger.ch and www.deneriaz.ch
Relining
The entire section is being repaired by providing a new lining for the existing penstock, which means introducing a new steel pipe inside the original conduit, from Tracouet to the distributor in the Bieudron powerhouse (4,050 m), except in the de-commissioned section, and in the new bypass. In 2005, a full scale template of the future steel segments was moved over the entire length of the shaft to validate the assumptions made for their installation. Relining of the penstock is currently underway.
The first steel pipes have started to be introduced in the existing shaft. The former penstock will be used only as formwork. A total of 12,600 tonnes of steel has been necessary to produce 409 steel pipes designed for a pressure of 200 bars, the world’s highest pressure for a hydropower plant penstock. LinzSion, Switzerland
bypass includes three elbows and is made up of about 30 pipes altogether. By the end of the year, the installation, welding, inspection of welds and concreting will have to be finished. The job consists in sliding 3 to 12 m-long 2.8 m-diameter thicker steel pipes into the bypass, from the end of the Fontanettes upper tunnel, 300 metres inside the mountain. This is done using an 11-tonne robot that moves forward inside the new shaft at a rate of one metre/minute. As of early August, 132 steel pipes had already been installed. Then, the gap around the penstock and the surrounding rock will be filled with 8 to 15 cm of concrete. 34-35/08.
