Sylvenstein Reservoir headrace tunnel

The Sylvenstein Reservoir lies 10km to the south of the town of Lenggries, in the Isar Valley in Bavaria. In the gorge between Sylvenstein and Hennenköpfl, a 48m-high embankment dams the River Isar, creating a state-owned reservoir which serves as flood protection for the river. During the sixty years that it has been operating, the reservoir has exerted heavy loads on the hydraulic steel structure, the more so after the dam structure was elevated, raising the reservoir level. The 2013 flood put particular pressure on the structure, and so the sluice tunnel, headrace tunnel and numerous flood relief structures were in need of comprehensive renovation.

The key elements being renovated were the sluices and their drive mechanisms, along with various associated components including armouring for the tunnels and recesses, walking surfaces, steps and concrete platforms. PORR had already renovated the sluice tunnel a couple of years earlier – between August 2016 and July 2017. This prior experience was one of the reasons Wasserwirtschaftsamt Weilheim selected PORR for the technically complex, intricate renovation work to the headrace tunnel. Work began in November 2018, and in June 2020 the team successfully completed the renovation.

According to the renovation plans, the existing 8m-wide, 42m-deep wet shaft was to remain in place. However, the top and bottom sluice gates and their drive mechanisms were to be replaced. The team began by removing all the steel parts that were no longer needed. The sluice gates, 4.5m high and 2.8m wide and weighing some 36 tonnes, were moved using electromechanical winches. These were positioned in a cavern that was only accessible via a steep and narrow – just 3m wide – route blasted into the rock. The bottom sluice gate was removed first – this was a two-part roller sluice gate with a hydraulic drive unit installed in the centre. About 560m³ of interior scaffolding, plus a gantry with a 45m-long electric chain hoist, were installed for the dismantling and demolition work. The equipment was used to lift out the steel parts, slice them up and transport them away for stripping. Once the new bottom sluice gate had been manufactured and installed with the hydraulic steel construction, work began on demolition of the top sluice gate.

One major challenge PORR faced was concreting the cavern floor. Due to tricky structural conditions, the concreting had to be done in a number of stages. To ensure characteristics suitable for the structural loads, approximately 2,300kg ( ≈ 260m) of CFRP fins had to be glued into crosses on the underneath of the cavern floor and anchored in place.