Post grouting in rock was performed with polyurethane close to the pressure shaft valve chamber in the Fljótsdalur powerstation in autumn 2021 while the powerstation was in full operation. Sudden increase in leakage had been observed in the valve chamber during unit trips and associated water-hammering of the rockmass in the powerhouse area, in addition the total leakage in the tunnel system around the valve chamber, had gradually increased since commissioning of the powerplant in 2007 from 10 l/sec to 20 l/sec, going up to 50 l/sec during unit trips when the production is stopped. To prevent potential future problems caused by frequent transient state in the waterway, it was decided to perform grouting in the area. Detailed geological investigations of the rock mass between the valve chamber and the headrace tunnel gave premises for design of new grouting curtain and the choice of grouting material. Difficult conditions with cold water, highly permeable fracture zone and unpredictable changes in pathways resulted in challenging work where collaboration between contractor, client and consultants was essential. Roughly 17.000 liters of polyurethane was injected into the rock mass to seal the bedrock and the constant leakage was reduced down to 4 l/sec. Several unit trips have occurred since the work was completed and to-date, no increase in leakage has been observed.
KEYWORDS
Polyurethane; grouting, cold environment; hydropower plant
