Cleuson-Dixence will go on line in October, achieving time and cost targets. Reaching this point has been a technical and organisational challenge for all involved
Cleuson-Dixence extension is a high head hydroelectric project in Valais, Switzerland. The project has an installed capacity of 1200MW. It sets a number of world records in terms of both gross head (1883m) and equipment.
The existing Grande Dixence dam impounds a reservoir which represents 1800GWh, or 20% of all hydro energy stored in Switzerland. The scheme, built in the 1960s, has more than twenty high altitude water intakes, adduction tunnels totalling about 100km and five pumping stations. The Grande Dixence gravity concrete dam still sets a world record with its height of 283m. However, the 780MW actual capacity of the three existing power plants of Chandoline, Fionnay and Nendaz is today insufficient.
The main purpose of the Cleuson-Dixence extension project is to increase capacity to 2049MW by constructing a new 3x423MW powerhouse. The plant will be the largest of its kind in Switzerland.
The plant owner is the regional utility Energie de l’Ouest Suisse (EOS), as majority shareholder, together with the Grande Dixence company, whose shareholders are EOS and other Swiss public utilities (BKW FMB Energi, NordostKraftwerke and Industrie Werke Basel). The Cleuson-Dixence Company was created to act as the engineering general management for construction.
The Cleuson-Dixence project includes:
•A new water intake in the existing gravity dam.
•A head race tunnel 15.9km long with a final diameter of 4.80-5m with internal water pressure of 20bar.
•An inclined steel lined shaft to the power house with a drop of 1624m over its more than 4km length.
•An underground power plant with three 423 MW five-jet vertical Pelton units.
The plant’s exceptional drop (1883m) represents a new world record, and allows economies to be achieved in civil engineering, hydroelectric equipment, sub-stations and power lines.
It also results in two other world records:
•Peak power capacity of the Pelton units of 423MW.
•Electrical apparent output per pole of 35.7MVA for the three generators.
Site works began in summer 1993 with the first two generating units scheduled for operation in autumn 1998. Total net investment amounted to SFr1.15B (1992).
Despite the difficulties, the planned completion of the two first units in October 1998 should hold, as should the total cost of construction.
With 150,000m3 of excavated rock and 1200MW of installed power, the under-ground power station at Bieudron is the largest power station in Switzerland. It consists of three Pelton vertical-shaft generators with a rated output of 423MW. The underground arrangement of the generators led the project manager to design large caverns whose stability called for extensive study, due to the poor quality rock.
Excavation and lining began in July 1993, and were mostly complete by June 1995. Structural concreting and construction of the generator foundations were completed in spring 1997. The power station is a complex feature with:
•A distributor access tunnel.
•A valve chamber, 80x9x12m, which is separated from the main cavern to limit a flood event.
•The generator cavern, 100x25x39m, which houses the three turbo-alternator sets and the auxiliaries rooms.
•A large tunnel for the cables and transformers. The latter are located in three 15m square caverns at the side of the access tunnel and in line with the turbo-alternator sets. They are isolated for fire protection reasons.
•A tail race tunnel.
It proved better to position the outfall, cable tunnel and transformer bays on the upstream side of the Rhone to minimise their length. This arrangement provides three independent accesses, ensuring flexibility during construction and offering emergency exits. A safety tunnel also connects the three main caverns.
The local rock formation is hard wafer sandstone alternating with semi-hard to hard calcareous sandstone schists with localised 1-10m intercalations of triasic shell (quartzite or anhydride). The formation is fairly uniform but physically anisotropic and fragile and must be maintained with special precautions during excavation.
The design of separate caverns was dictated by the exceptional drop and the need for reliability. The valve chamber must be separated in case of flooding, and the transformers are separated in case of fire. The structures are positioned to maintain their distance from the existing station, and to orientate the main caverns at right-angles to the foliation.
Blasting was the utilised excavation method and measures to limit vibration (10 mm/s) were used in order to preserve the soundness of the rock and avoid damage to the generators at the existing Nendaz power house.
It was in the generator cavern that the excavation work required the most careful timing. Boring a work tunnel provided roof access bypassing the excavation; this tunnel opened at the rear of the cooling reservoir. When the cavern had been crossed by a pilot tunnel, bored as far as its outlet in the cliffs, the cavern was executed in successive stages.
The valve chamber was executed in two stages: a full-section roof heading with support and lining; then excavation of the core using horizontal blasting.
Temporary support during excavation was provided by sprayed concrete with metallic fibre and brackets. Final support comprises sprayed concrete shells with passive grouted bars and active pre-stressed cables (main generator cavern).
The work programme had two separate priorities for the excavation work, each of them on a critical path:
•Excavating the distributor access tunnel and the distributor quickly, to provide access to bore the shaft.
•Beginning excavation of the main cavern quickly, and providing roof access first in order to achieve this. The roof access required the 380kV cable tunnel, then the by-pass tunnel and cooling reservoir to have been excavated previously.
Other constraints also had to be met:
•Excavation of the power station access tunnel in time to ensure that the spoil removal shaft was available.
•Concreting of the shielded bus-bar tunnels and outfall branches between the transformer bays and the main generator cavern, before the core was excavated from the latter, for excavation stability reasons.
•The power station and shaft working sites were physically separate and no access was possible, for example to excavate the valve chamber via the distributor access tunnel. The separation was decided as soon as the design work began, and proved to be an essential factor in ensuring that progress on the shaft boring operation was not disturbed.
During the first six months the excavation rates achieved by the consortium of contractors were much lower than those specified in its bid, delaying the schedule by four to five months. This was because:
•When work began in summer 1993, there was a lot of other underground work under way in Switzerland. Several sites had just opened and some completions were delayed. As a result, the consortium was unable to build experienced cohesive teams.
•The complexity of the Bieudron underground system required a blasting procedure specifically suited to the geometry of the structures.
•The installation of tie rods in the main cavern proved more difficult than anticipated. The procedure chosen by the consortium required a whole series of adjustments and trial and error procedures.
The consortium then did everything necessary to increase its supervision, and put together effective teams in order to achieve the required rates. However, much work was required to catch up on the accumulated delay.
During the same period, instability with convergence of more than 10cm occurred in the passageway connecting the transformer bays. Initially, the project provided for the tail race tunnel to be excavated below the access tunnel to the transformer bays. In view of the convergence measured, any excavation at the bottom of unstable walls could be risky and could cause serious additional delays. The work programme was therefore completely reorganised. Design changes and measures to speed up the work were introduced.
The main design changes were:
•The tail race tunnel was moved 50m upstream to avoid additional excavation at the foot of some unstable walls. This opened a separate site so the transformer bays could be concreted while the tail race tunnel was being excavated and concreted. This made excavation of the turbine pits easier; allowing the roof access to the main cavern to be maintained.
•An access tunnel section was added, connecting the roof access tunnel to the main generator cavern.
•The turbine pit excavations did not need to be supported by primary concrete which reduced the surface area of the formwork and the volume of concrete to be poured, saving time and money.
As a result of the design changes, the increase in costs caused by these difficulties and by the measures taken to speed up the work, was kept to a minimum.
It was thus possible to maintain the civil engineering work programme at the Bieudron power station. The additional investment required for the measures taken to speed up the work was relatively small (1.6%) compared with the cost which would have been incurred if the start-up date for the entire project had been delayed.
The successful implementation of the design changes and measures to speed up the work was due in large part to the excellent collaboration between the general and local work managements, the design engineers and all contractors at the working site.
|Main contractors and suppliers at Cleuson-Dixence scheme and Bieudron plant|
Energie Ouest Suisse and Grande Dixence.
General project management:
Steel lining works
Control, protection and command system: