Grimsel: The dam enhancement goes through a new wall

“We, the Swiss, are always a bit of the pioneers,” smiles Ralf Grand. And the one who directed the work of the new Grimsel dam has enough to be proud: at the time when we hardly build in Switzerland of these impressive works, the Spitallamm replacement dam is one of those rare and spectacular projects.

We are in the Bernese Oberland, at 1980 meters above sea level, at a stone throw from the Col du Grimsel attended on this hot day in August by cyclists and bikers. The new wall of the barrier 113 meters high and a length of crown of 212 meters was mounted just in front of the old one, at the narrowest point of the valley where the turquoise waters of the lake are reflected.

The book stands at the same height as the old dam built in the 1930s. At the time already, we innovated with a curved dam, recalls Ralf Grand. And as at the time, the work on the new wall was completed in just six years. “We worked seven days a week, between mid-May and at the end of October, with a hundred workers on the site. During the day, two huge cranes were used for the formwork work, at night, we sank concrete. ”

The titanic work, which has just been inaugurated in June in the presence of the Minister of Energy, Albert Rösti, is not like the other dams. If everything goes as planned, it will be raised by 23 meters. The launch of the work is still open. This will almost double the volume of water restraint, going from 100 to 170 million m³and to produce almost 90% more electricity, estimates the operator KWO (Kraftwerke Oberhasli).

Strategic place for enhancement

The grimsel is one of the fifteen hydroelectric projects intended to add 2.2 TWh to the annual electricity production, especially during the winter, by increasing the height of the existing dams. Because climate change, which leads to the melting of glaciers, creates new lakes at very high altitude. The glacial lakes could then serve as restraint lakes for advanced energy production via pumping-turbine. An essential project for Bern, but which arouses in places the fear of environmentalists.

In the case of Grimselmost of the oppositions of environmental organizations have been lifted and the hope of starting with the elevation of the wall becomes more and more concrete. Because the place is strategic, notes Ralf Grand. “In recent years, glaciers have melted massively, with a record last year in energy production. And the region is very exposed to precipitation. In winter, it can snow up to 4 meters when we are only 1980 m. And in summer, it rains here more than in other regions. Our studies show that the amount of water in the lake will remain in the future almost the same, less in the form of ice, but more in the form of more frequent rains. ”

Who says enhancement also says development around. The road that goes up to the pass along the dam must be rebuilt, as well as the access bridge. The old dam will then be drowned under the waters. Just as was, in the 1930s, the former hospice at the bottom of the current lake, where workers of the first dam slept and whose ruins emerged when water drops in the spring, like ghosts of the past.

A double curvature wall

It is at the foot of the new dam that we take all the measure of technical and architectural prowess. By looking up, we see that the top of the dam covers us. “The new wall is double curvature, horizontal and vertical,” explains the director of the works. We needed much less concrete. It is the ideal form to be able to transmit water forces to the abutment. ”

The construction of a new dam just in front of an existing dam in service is a first. The site was therefore particularly scrutinized by the Federal Energy Office. “Two theodolitis on the left and on the right watched the wall every half hour at 40 points. And if the displacement had been greater than a millimeter during this half hour, the alarm would have been given. But it never happened. ” Filled, the dam, like all the others, undergoes the pressure of the water, which moves the wall a few centimeters forward. Again, very precise measures allow you to constantly monitor the work.

Rock compressed by the old glacier

In the bowels of the monster, three pendulums are suspended, from the top of the wall to its base, like a lead threads. Further on, the new valves of funds make it possible to empty the dam, a bit like the cork of a bathtub. Construction must also support extreme conditions, an earthquake or a flood as it arrives every ten thousand years or even damage to the bottom valve.

“The risk of landslide is minimal,” explains Ralf Grand. The glacier which was there during the last ice age was 1 kilometer in thickness. He compressed the rock. There is therefore no risk of cracks that would widen with the melting of permafrost. ” He smiles: “It is no coincidence that our ancestors have built here.”

From the top of the Col du Grimsel, our eyes are one last time on this brand new dam, with a feeling of work accomplished. On the flanks of Lake Azur, hikers are storming the mountain, which here seems more immutable than ever.

Family walk in Sidelhorn

The family hike starts from theHospice you ugly With the Sidelhorn cable car. You arrive at the foot of the Sidelhorn. The 5.6 km ride (count 2:30) begins with 300 meters of elevation to Huseneggg. The granite path offers breathtaking views on the Grimsel lake. In the distance stands the Finsteraarhorn and its 4274 m, the culmination of the canton of Bern. The march continues by passing through the Triebtenseewli and by descending via Bäregg to the Oberaar refuge. Contemplate Lake Oberaar, where the AAR has its source in the glacier. After regaining forces, you will return to the starting point with the Oberaar cable car.

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