2015 – Remedial works to reduce seepage under Visegrad HPP dam

Nikifor Petrovic, Sladoljub Pezerovic

Dam rehabilitation works at the Visegrad Hydropower Project on the River Drina in Bosnia and Herzegovina were completed in October 2014 after two years of very challenging and collaborative effort between the client, designer and contractor.
The successfully accomplished remedial works programme was a highly complex geotechnical intervention. The dam was constructed on a karst foundation extending up to 200 m below reservoir floor level. Rates of seepage through the foundation increased over time, from 1.4 m3/s following first impoundment in 1989, to 14.7 m3/s in 2009.
The rehabilitation works comprised:
 Preparatory works (site installation, work platforms, conveyer belts, electricity and water supply, drilling and grouting equipment installation);
 Site investigation works (drilling of boreholes, measurements of inclination, geo-physical carotage, downhole video, underwater camera recording);
 Installation of monitoring equipment and implementation of real time recording system;
Installation of inert material into a sinkhole within the storage area and into the bore holes located upstream of the dam; and
 Grouting of the foundation area using different grout mixes and grouting methods.
During rehabilitation works the main achievements were:
 A total of about 37,300 m3 of inert material (granular materials with different fractions from 0 to 32 mm) was installed into the foundation cracks and caverns. This was a significant achievement due to very complex geological conditions and resulted in a seepage reduction through the foundation and improvement of the overall safety and stability of the dam.
The total consumption of grouting material was in access of 2,500 tonnes of cement, bentonite, sand and additives.
After completion of the work, seepage of water through the foundation was reduced to about 4.5 m3/s.
Keywords: Seepage, remedial works, dam, grouting, inert material.

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