Section 853 “Interventions on the North Portal Vigana area” consists of a twin-tube tunnel and two caverns, one of which is partially under the surface of the highway. The external works consist of trenches dug perpendicularly to the slope of the highway. The underground part starts with the external cavern, 232 m long, under the embankment of the highway for 48 m. The first part of the excavation is carried out using the jet-grouting technique. The cavern proceeds in the rock with partial face excavation and finally it branches out into two tubes. The link (277m), like the eastern cavern, starts under the highway embankment after which it proceeds in rock until it is transformed into the western cavern (405m).