The metal arches, known as piers, were installed in 1993 to completely restore the efficiency of the Gravagna Viaduct by permanently supporting the spans by means of large steel portals whose imposts, as they move apart, rest on the pier bases. Made of steel and weighing approximately 400 tonnes each, they range in length from 45 to 62 metres.

The three portals supporting the North carriageway, which has already been demolished, were removed by means of explosives on 2 separate occasions at night on 19 and 20 June 2024, ensuring safe conditions for operators and users, closed to traffic. The charges were placed at five points inside the metal caisson. The cuts were made in the midspan section of the portal’s buttressed arches in the horizontal and vertical directions to allow them to fall to the ground by gravity. The part of the arch most in direct contact with the road deck was weakened so as to prevent the involvement of the counterpart portal on the opposite carriageway, as well as minimising the stresses transmitted to the portal hinge joints and, consequently, to the piers. Outside the area where the cut was made, protection was provided by alternating sandbags and water.

A monitoring system consisting of approximately 90 inclinometers and around 90 accelerometers positioned on the piers and decks of both the North and South carriageways was installed. The objective was to monitor the effects of the demolition on both the piers and the southbound deck, to verify that the shock wave did not create excessive stress on the structural parts still to be demolished. Monitoring was installed in the days before the demolition of the arches, after which load tests were carried out, and repeated after each demolition as a further check.

Given the high weight of the demolished arch portions (approximately 160 tonnes), technical estimates were made of the effects produced by the fall of the portals after demolition with explosives. The arch segments, as they fell to the ground, could have generated vibrations that may have affected the pier foundations or other structural elements. It was therefore planned to dampen the effect of the fall by placing dunes of inert material in between, which absorbed the vibrations induced by the impact on the ground.

KEY NUMBERS:

• Approximately 60 people were employed, involved in the preparatory, execution and road network support phase
• Approximately 3000 cubic metres of inert material for the creation of dunes to dampen the effects of the arches falling to the ground
• 400 kg of explosives
• Several kilograms of gelatine for blasting the hinge joints