Sealing the subsoil at the Feldsee dam at 2,200m above sea level

Built in 1969, the reservoir is part of the Fragant power plant group in the Möll Valley and holds 2.1 million cubic metres of water. Two lakes, the Feldsee and the Wurtensee, are responsible for the reliable provision of electricity in Carinthia province. After only a few years of operation, cloudy seepage water was found in the dam base drains, suggesting that material was being discharged along with the flowing water underneath the dam body. A total of five different injection campaigns have been carried out over the past 50 years – none of them with any lasting success.

Solid bored pile wall in the high Alps

In 2019, the operating company Kelag resolved to carry out rehabilitation measures that were as extensive as they were extraordinary. The dam, which is founded on moraine debris, was fitted with a cut-off wall of secant bored piles connected to the asphalt concrete surface seal. This involved constructing a secant bored pile wall – with a diameter of 120cm and a depth of up to 35m – in the high alpine, rocky subsoil on the side of the existing cut-off wall facing the water. Several metres of gneiss and granite blocks were drilled through and each pile was embedded in the rock.

A total of around 5,800m of bored piles with a diameter of 120cm were inserted. The piles in the flanks were 5m to 8m long, and over 30m long in the middle of the dam crest. A drilling tolerance deviation of less than 10cm was required to ensure that the bored piles overlapped. The course of the drill pipe was measured for each pile to ensure this. The team succeeded in staying within a maximum pile deviation of 0.5%. The result was a wall in the subsoil that seals off the bottom of the reservoir.

Successful completion despite challenging building conditions

The high alpine location at 2,200m above the Adriatic and the resulting weather conditions, which are often quite harsh, posed a number of challenges for setting up the construction site, transporting the equipment and machinery to the site, the day-to-day logistics, and the construction site crews. The geological conditions and remnants of the previous injection measures made the work even more difficult. The tight construction schedule from March to August and the harsh weather conditions lasting into May made it necessary for the construction site team to work in shifts at high intensity and called for a high degree of responsiveness from PORR’s equipment department. Thanks to the team’s many years of expertise in specialist civil engineering, all the work was completed to everyone’s satisfaction in mid-August, one week before the scheduled end of construction.