A construction site awakes from its slumber

The two-year pause in construction at the Gigerwald dam facility was caused by the threat of a power shortage in winter 2022/23. Now the bottom outlet of the Kraftwerke Sarganserland power plant’s upper dam is finally getting its upgrade. The time-critical work starts in late September 2024. Recommissioning of the pumped storage plant is scheduled for late May 2025.

Condemned to inaction, the two flame-red construction cranes on the Gigerwald dam have been awaiting their moment for two years. Now the slender arms are moving in every direction and breathing life back into the construction site on the craggy valley slopes of the Calfeisental. Matthias Kohler cranes his head and looks up at the 147-metre-high arch dam. The Kraftwerke Sarganserland Operations Manager is standing on the valley side of the major installation site, which is awakening from its slumber. Step by step, the hardy workers are returning to their stations with their equipment. ‘And the giant cable excavator is also back,’ says Matthias Kohler with a grin.

Barely arrived on site, the imposing dynamo is already flexing its muscles. ‘Without it, we would have a very hard time moving the heavier components.’ In parallel, the container camp has sprung back into life. Matthias Kohler: ‘The construction teams are working six days a week in double shifts, even in minus temperatures, so it’s important that they have comfortable common rooms.’ The Berggasthaus Gigerwald keeps the hungry workers fed. 

Construction halted to aid Swiss security of electricity supply

Between September 2024 and spring 2025, around 50 employees will complete what came to an abrupt halt two years ago. Following initial preparations in the summer months, Axpo pulled the rip cord in autumn 2022 and paused the CHF 25 million project. The reason was the threat of electricity cuts in winter 2022/23, which made flexible electricity production of the pumped storage plant essential, particularly at peak consumption times.

The unexpected electricity shortage was due to multiple factors. After Russia invaded Ukraine in spring 2022, the flow of gas to western Europe slowed dramatically, leading to record-low levels in European gas storage facilities. There was a shortage of gas for heating, but also for electricity production. Additional technical problems in some French nuclear power plants threatened to throttle winter electricity production further, creating additional uncertainty.

The halt to construction cost CHF 10 million. On the other hand, the electricity produced by the Kraftwerke Sarganserland power plants in the high-priced market environment of winter 2022/23 no doubt fetched attractive sums. ‘But for us, the focus was always on keeping the grid stable and doing our part to keep a functioning electricity system on Swiss soil,’ says Matthias Kohler. With an output of 300 MW, the Kraftwerke Sarganserland power plants are among the largest pumped storage plant facilities in the country, and the second-largest in northeastern Switzerland after the Linth-Limmern power plant (1000 MW) in the canton of Glarus. 

With the return to construction, the project is finally on its way again. Matthias Kohler: ‘We are literally taking the existing plans out of the drawer again in their existing form.’ Under the schedule, the intake structure for the bottom outlet and works water outlet are set for completion within seven months. 

Raising and separating the intake structures

Sand and gravel from inflows have left extensive sediment in the Gigerwald reservoir over the last 48 years of operations. ‘Many hydropower plants face this phenomenon,’ explains Matthias Kohler. With the dredging of the reservoir and other solutions rejected as unfeasible, the only workable option was to raise the intake structures.

The new intake structures for the bottom outlet and works water will be 18 and 23 metres higher, respectively – as high as an eight-storey apartment building. ‘This will ensure secure operation of the bottom outlet for the coming decades.’ The bottom outlet is the lowest closable opening in a dam facility and has to be ready to operate at all times – much like a bath plug.

The Kraftwerke Sarganserland power plant is also using the refurbishment as an opportunity to separate the bottom outlet and works water functions, which were previously combined in one intake structure. At Marti Tübbingwerk AG (a subsidiary of Marti Tunnel AG) in Balsthal, the project managers are having concrete components made that meet all the geometric requirements and quality standards. These elements will reduce the construction time. ‘The great thing is that we have found a customised solution here in Switzerland.’ Tubbing segments are traditionally used as an internal lining in tunnels. For the new intake structures, the specially cast elements will be assembled, bolted together and concreted on site. Matthias Kohler strolls over to the storage facility that houses 73 concrete elements, each as tall as a house. ‘They’re all one-offs. Not one of the elements is straight, they’re all twisted and crooked. That’s a challenge, and it requires a high degree of skill in prefabrication.’

Work starts underwater

Five serpentine curves overhead, the view opens up to the shimmering Gigerwald reservoir. For a few weeks now, the water level has been steadily falling. Nonetheless, the initial work will begin underwater. ‘We have to free the concrete cover of the intake structure from sediment.’ Easier said than done. ‘An employee of the construction company has constructed a special mechanism that can be used to operate a large excavator shovel with a crane.  Using the crane, the shovel grabs the gravel and then swivels to deposit it elsewhere in the reservoir.’

If the water level has sunk low enough to reveal the concrete cover by the end of September, the drilling work to open the cover of the existing structure can begin. ‘Up to that point, we will still be using the water inflows for electricity production.’ If the reservoir is empty, the plant will be definitively out of action. The work on the intake structure means that all natural inflows to the reservoir will be halted and channelled through the construction site. The lower Sarelli plant will use the water to produce electricity in reduced volumes throughout the winter. 

Maintenance window

Matthias Kohler is using the refurbishment of the Gigerwald dam facility as an opportunity for important inspection works on sections of the upper power plant. ‘We are working through a long list of maintenance projects,’ says the Operations Manager with a laugh. As soon as construction is sufficiently advanced, specialists will disassemble and upgrade the valves of the bottom outlet, which has been working steadily for 48 years. The facility will also get some new throttle valves with new electrical hydraulic controls. 

Hoping for a mild, dry winter

In the winter months, avalanches from the steep slopes surrounding the Gigerwald reservoir offer no cause for concern, as long as they don’t impact the access road to the reservoir. ‘Now the situation is different, of course,’ explains Matthias Kohler, looking up at the peak of the Gigerwald, which towers 2,292 metres above the western side of the reservoir. If it’s a very snowy winter, avalanches may threaten the access road and the routes within the site. ‘So we’re hoping we don’t have too much snow in winter.’ A specially convened avalanche commission will monitor the situation around the construction site and the access roads during the critical period and make arrangements as necessary.

Milestone at Christmas

Matthias Kohler aims to refill the reservoir in late April 2025. ‘We want to fill it with snow melt from the mountains.’ Whether the plan is workable or not will be revealed at Christmas. The time-critical dismantling work on the bottom outlet will continue until that point. ‘It’s simple – a delay in the completion of the bottom outlet will have a negative impact on the schedule for the refilling.’

If they get the green light in spring, it will take around seven days to fill the reservoir until the water level reaches the lower edge of the works water intake. The works water system will then go back into operation, and the machines in the Mapragg power plant will be ready for use once again. Matthias Kohler can then pump additional water up to the Gigerwald reservoir to get it shimmering in full splendour again as quickly as possible. If the pumped storage plant facility is on-grid again in late May 2025 as planned, the two red cranes will have long been stretching their arms out over new work sites.