References
Salanfe Dam.
SWITZERLAND, 1992 - 1994
Grouting works on a deep cut-off curtain in rock.
The Salanfe Dam is situated in the Canton of Valais. Since it was commissioned in 1952, large leaks have been continuously observed on the north flank of the reservoir and these appeared to be associated with the presence of layers of weathered Tertiary rocks. The bed of the reservoir was operated at a reduced water level, 50% of its design volume, to avoid leaklage. The Project is famous for the 1,470m between the reservoir and its tail race discharge. To optimise the hydroelectric potential of the project, the owner design a monolinear grout curtain cut-off to the leaking northern flank. A consortium of companies constructed a 215m long access tunnel to a grouting tunnel, 620m long.
Grouting works
Consolidation grouting of the rock around the grouting tunnel was performed to deal with any relaxation and fissuring opened up during the excavation of the tunnel. This zone of consolidated rock acted as a link between the upper and lower grout curtains. Grout holes were drilled in radial fans, spaced 2.50m apart, along the length of the grouting tunnel. These were injected at relatively low pressure with a high-penetration, anti-bleed grout. In addition, the base of the grout tunnel was further consolidated by injection of a high stength grout. This encouraged the re-establishment of appropriate hoop stresses around the tunnel wich would have been reduced by damage to the rock near the tunnel perimeter during excavation.
The lower grout curtain had to penetrate the permeable Tertiary rocks through wich leakage was occuring. For the first 406m of the grout tunnel, this monolinear curtain was taken down to the underlying gneiss wich was know to be watertight.
High technology at high altitude
Four types of bentonite-cement grout were injected, two of which designed to give high penetration :
- RHEOSIL S, a grout based on CLK cement with microsilica and proprietry additive,
- C3S, a stable bentonite-cement grout.
Computerised management of grouting
Servo-control of the pumps
Recording of drilling parameters and water test results
Mesurement of hole deviation, processed by computer.
Salanfe Dam
A251.pdfTechniques
Grouting involves the injection of a pumpable product (slurry), which will subsequently stiffen, into the soil or into man-made material (masonry), in order to consolidate the soil or structure or make it impermeable, through filling all the voids it contains. The slurry can fill the voids in the ground, the cracks within rock, solution cavities (it is then referred to as fissure and permeation grouting) and/or displace the surrounding soils through a bottom-up process or by fracturing (compaction grouting or solid injection - see the section on the subject - and strain injection). Grouting with soil displacement may be used to prevent potential damage to the structure brought about by excavations (galleries and tunnels, major urban excavations, etc.) and this is called compensation grouting (see the relevant section).
Ouvrages
The sealing and stability of dams generally requires substantial grouting, sealing and drainage works. The techniques of boring, injection and drilling, which are the original activities of Soletanche Bachy, are now complemented by other procedures, including diaphragm cut offs, either in concrete, plastic concrete or slurry, and jet grouted cutoffs.
Voyage au coeur de l'ingénierie de pointe
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