Rehabilitation of Salanfe Dam through Vertical Saw Cutting

The Salanfe gravity dam (height 52 m, length 600 m) is located in the Swiss Alps at an altitude of 1,900 metres. Regular inspections since it was commissioned in 1950 have revealed atypical behaviour linked to the swelling of the concrete. This behaviour is caused by an alkali-silica reaction (ASR): the sand and gravel used for the concrete contain poorly crystallised silica, which reacts by forming a gel that fills the hollow spaces; the environment is very basic (pH of around 13) and loaded with alkaline ions such as sodium and potassium. This phenomenon generates pressure inside the concrete, which gradually swells and then finally develops micro-cracks.

After studying the behaviour of the dam and on the basis of measurements and a digital model, Stucky SA proposed to proceed with vertical sawing to limit the progression of the deformations. The approach is based on the methodology for recalculating structures affected by alkali-silica reaction developed by the Laboratoire Central des Ponts et Chaussées (LCPC). The method is based on finite element modelling of the structure's hydric, thermal, chemical and mechanical behaviour. The calculations were carried out on 2D and 3D models, using the RGIB module of the CESAR-LCPC software package, based on the experiments and knowledge of the structure in 2007-2011.

While continuing to operate the reservoir, 22 vertical saw cuts were made on the upper part of the dam. The largest, in the middle of the dam, are 24 metres high. The width of the cuts is 11 millimetres. The concrete of the dam is first perforated to its full thickness at the lowest point of the saw cut. The diamond wire is inserted into the perforation and sawing then takes place from the bottom up to the crown. A seal and a protective film were then applied to the upstream and downstream ends of each cut to ensure watertightness. Due to the stresses present in the dam, the saw cuts are gradually closing. Specific measures have been put in place to continuously monitor the behaviour of the dam before, during and after the sawing work.

Services provided by Gruner Stucky Ltd

Engineering, from the feasibility study to the work supervision and commissioning

The services covered the entire project for the rehabilitation of the dam crest, the shaft and the valve chamber, the project for the auscultation system to monitor the works; the monitoring and management of the environmental impact study; the preparation of the building permit application file; the tender documents for recruiting contractors (technical specifications, price list, design), and works supervision for concrete sawing work.

• Sawing modelling with CESAR-RGIB software.
• Verification of seismic hazard before and after sawing.
• Justification of the solution to the supervisory authority (Swiss Federal Office of Energy - SFOE).

Back to overview