Traffic, 28.09.2023

The cause of the cracks in the Gotthard road tunnel remains unclear

Until the reasons for the damage to the Gotthard road tunnel ceiling are clearly understood, the excavation work for the construction of the second tunnel in close proximity to the existing tunnel is at a standstill.

On Sunday, 10th September, concrete parts suddenly crashed onto the road in the Gotthard road tunnel near the northern tunnel portal. Luckily, no one was injured, but the tunnel had to be closed immediately in both directions. This was a particularly bad time, as passenger traffic has also been interrupted in the Base Tunnel (the railway tunnel). Because a freight train had become derailed, railway traffic had been rerouted over the old mountain line.

Three days after the event, the Federal Roads Office (Astra) reported on 13th September that the cause of the crack formation in the intermediate ceiling was due to stress redistribution in the mountains, which had led to local pressure changes and placed additional stress on the tunnel in the affected section. This led to displacements and a 25-metre-long crack in the false ceiling. The Astra ruled out the structural condition of the tunnel as a cause.

It was also quickly speculated that the construction work in the nearby second tube of the Gotthard road tunnel could have had something to do with the shifts in the stress. Until the investigation into the causes was completed, Astra thereby stopped blasting in the immediate vicinity of the damage area. Since then, there has been a feverish search for the cause of the crack.

No influence on costs and deadlines
The causes of the shifts in the stress are still being clarified,” writes Astra spokesman Thomas Rohrbach. At the moment, it’s not possible to state the reasons or to specify a date by which we will know more. In connection with the construction of the second tube , the statement continues: “The last blasting action in the area took place more than 48 hours before the damage to the false ceiling slab. There was a practically identical event in 1985, i.e., five years after the tunnel was put into operation, when a crack also occurred in an intermediate ceiling slab after stress redistribution, with a practically identical damage pattern, although a little further into the mountain. This incident was also included in the investigation There was no work going on nearby at the time.”

The defective false ceiling in the Gotthard road tunnel is being removed                    Photo: Astra/zvg
“Jumbo drilling” (Bohrjumbo) in the second tube of the Gotthard road tunnel          
Photo: Ilona Schmid

According to Thomas Rohrbach, tunnelling work in the mountain in the immediate vicinity of the damage event will remain suspended until these clarifications have been completed. “All other work, including tunnelling work in Göschenen, is continuing and has never been interrupted. This has no influence on the total costs or the deadlines of the project.”

As the Astra spokesman explains, the work that has been stopped serves the so-called reverse drive (Gegenvortrieb) - from the inside of the mountain towards the north portal. Work from the north portal towards the south continues. These excavation and tunnelling works are converging. On the one hand, to precisely localise the exact transition from loose rock to bedrock and, on the other, to create space so that the 300-metre-long tunnel boring machine (TBM) can be installed next spring. This has to be done because a TBM is not suitable for loose material; it is only used in solid rock.

Steel structure to remain in place for the time being
The existing Gotthard road tunnel has been open to traffic again since Friday, 15th September. The damaged false ceiling panel was replaced with a steel structure in a remarkably short time. “This will remain in place until the old tube is renovated. The costs for this repair have not yet been determined,” writes Thomas Rohrbach. The night closures for regular maintenance, as they currently take place, would also always be used to record the condition of the individual building elements. “This also includes the inspection of the false ceiling as a central element of the tunnel. That was the case before the current damage.”