Geology and the Genoa bridge collapse

In Genoa, part of the important A10-highway bridge 'Polcevera' (locally known as 'Morandi', so named after the engineer who planned the bridge) collapsed Tuesday during a thunderstorm. Today 39 victims are confirmed, 16 survivors were saved from the debris, 9 are severely injured, and 10-20 people are still reported missing.

The bridge was built between 1963 and 1967 and planned by Italian engineer Riccardo Morandi, who planned bridges also in Venezuela and Lybia.

Newspaper issue from 1964 showing the project for the Polcevera viaduct.

At this time the cause of the collapse is unclear. Speculations range from thunderstorm damage, material fatigue of the 50 years old bridge to a very unlikely case of terroristic act.

Collapsed section of the Polcevera viaduct after Spiegel Online

Geology can play a role in statics and dynamics of a bridge. The A10 connects Italy to France and follows the coastline between the Ligurian Sea and the Alps.  The limited space and rugged terrain demands the construction of many tunnels and bridges. The Morandi bridge crosses the  Polcevera river bed and an industrial zone and connects the city of Genoa with its harbor and Nizza/France, making it one of the most important routes in the region. 

Unconfirmed are claims of a landslip on the base of one of the bridge's pylons, triggered by the heavy rain, causing the collapse. A published video seem to show the pylon collapsing only after the highway deck. Photos of the ongoing rescue attempts also don't seem to show damage on the base of the collapsed pylon.

Also unconfirmed are claims of possible subsidence movements of the underground, destabilizing one of the pylons. According to the geological map the underground is composed of marine and alluvial sand and conglomerates, filling a river vally incised in siltstone-formations.

Geological map extract of Genoa showing the A10 crossing the river , blue: alluvial sediments, green: siltstone.

Such terrain can be problematic for a bridge's foundations. A changing water table can cause erosion and resulting underground cavities, followed by collapse and locale subsidence movements on the surface over time. However, many other factors, like construction type of the foundations, play a role in case of a collapse. At the moment there is no evidence to support this scenario.

 Photo from 2016 showing the collapsed pylon and also renovation works at the Polcevera viaduct and river.

A rupture of the highway deck caused by material fatigue, the bridge was constructed in a time when traffic was less intense as today, is favored by most interpelled experts, but only forensic investigations in the coming months may reveal the true cause of the collapse. On Friday it was speculated, that one of the suspension ropes broke. Reinforced concrete is vulnerable, especially near the sea, as water and salt accelerates the corrosion of the iron parts.