The 1.01km tunnel is the longest tunnel on the project.
Concrete work in the cavern has been under way since the beginning of September and excavation in the cavern is expected to be fully completed this week.
In August, the Porr tunnel construction team broke through from the cavern to the Schwarzenbach underwater tunnel and raise boring work began on the Murg shaft a few days later.
Porr is converting the more than 100-year-old run-of-river power plant into a pumped storage hydro facility for EnBW Energie Baden-Württemberg.
Porr says the project remains on schedule to be commissioned in 2027.
