The project consists of two double-barrel tunnels at “Uriol” and “Petit Brion”. Uriol has a Jurassic-era lime-scale massif, while the Petit Brion crosses a lime scale quartz-clay massif.

Both tunnels were excavated mainly through full-section explosives. Support systems were built by availing of the Austrian procedure which envisages nailing and the subsequent use of spritz beton.