Illsee dam

Rehabilitation and upgrading of the dam to new seismic standards


Switzerland, 2011-2013

General description

The concrete of the dam, at an altitude of 2500 m, was subject a chemical reaction causing swelling due to aggregate–alkali reaction. As a result there was deformation, with structural cracks appearing in the curved part of the dam: the upper part partially separated from the lower part. The rehabilitation project involved installing permanent anchors in the top part of the dam to ensure safety with respect to earthquakes. The site was accessed (for both men and equipment) only by cable car, while the anchors were helicoptered to the site.


- 62 vertical anchor rods (2000 m of drillholes)

- Grouting of ninety 50-mm Gewi bars (1700 m of drillholes)

- Drilling of 60 drainage holes (600 m)

- Replacement of 400 m2 of concrete on the dam face

- Installation of anchors at the top of the dam and permanent anchors on the right bank to ensure seismic safety.

- Relaxation of stress due to swelling by cutting vertical slots in the dam.

- Modification of the static stress of the curved part of the gravity dam.

- Readjustment of the spillway.

- Consolidation of the concrete on the downstream face (sprayed concrete).

- Drilling of drainage holes.

- Renovation and maintenance of equipment.


PraderLosinger SA (35%) / Imboden (25%) / Evéquoz SA (20%) / Dénériaz (20%)

Project owner

Argessa AG

Key figures

4.3 km

of drill holes



2,500 m

altitude of the dam


Associated Projects

Mont Cenis-Fréjus Tunnel
Mont Cenis-Fréjus Tunnel

Upgrading of tunnel to high-speed TGV gauge

More info
Simplon tunnel
Simplon tunnel

Upgrading of a 19.8-km-long stretch on the Lötschberg Tunnel route

More info