Fast-moving floods threaten TGV train lines
© 2012 EPFL
Torrential storms can wreak havoc on train tracks. An EPFL laboratory has been commissioned by the French national train network, SNCF, to reproduce the consequences of extreme weather events at a very small scale.
Storms can have serious repercussions on the railway infrastructures. On some lines, flooding can erode the ballast – the bed of piled-up rocks underlying the tracks. The stability of the track is then at risk. In France, high-speed TGV trains, which travel at speeds of nearly 300 km/h, are particularly sensitive to this danger. EPFL researchers have been commissioned by the SNCF to study a scale model in order to better understand the consequences of this rare but dangerous phenomenon.
All the TGVs from Geneva, Lausanne, and Lyon pass through the village of Sarry in Yonne. In this sector, the traffic can get as frequent as one train every three minutes.
In 2000, this 2 km stretch, in which the tracks are lower than the surrounding land, was hit by an extremely violent storm. Torrential rainfall flooded the train line. Soil and vegetation torn up by the floodwaters was partially blocking the drainage infrastructures. The slope, combined with the bottleneck effect of a road bridge, served to accelerate the speed of the water, which reached the level of the tracks and eroded the ballast. Train traffic had to be interrupted for several hours to allow maintenance teams to make repairs.
The “Sarry” risk
This unusual event has led to a classification known as “Sarry risk.” The owner of the French rail network, RFF, and the SNCF conducted an inventory of sites at risk on the high-speed lines. And in 2010, they commissioned EPFL’s Hydraulic Constructions Laboratory (LCH) to conduct a research project to better understand how water erodes the ballast.
The decision to interrupt train traffic is a serious one for the SNCF to take, because it has many consequences. Repercussions spread quickly throughout the entire train network. “After Sarry, it was critical to scientifically verify our risk evaluation criteria. EPFL has already delivered a report that will enable us to modify the RFF and SNCF reference materials for the design of high speed lines.” says Cicely Pams Capoccioni, an engineer responsible for hydraulic risk for the SNCF.
Dangerous obstacles
EPFL engineers studied several different scenarios using their scaled-down model. They modified the slope, increased the speed and height of the water, and placed obstacles alongside the tracks, as is the case all along actual train lines. They discovered that certain objects, such as posts shoring up the line or pillars supporting road bridges, caused the flow to deviate into the foot of the ballast bed, exacerbating its erosion.
To conduct their case studies, the engineers reproduced a 1/3 scale model of 30 meters of the line, using several tons of miniature ballast along the entire length.
Simulating the passage of a train
“The ballast supports the entire structure. When a train passes, it vibrates slightly and transfers the weight and the vibration to the ground beneath. If the base is eroded, that causes the part supporting the tracks to collapse,” explains project leader Giovanni De Cesare.
To simulate the passage of a train, the LCH team is making the rails vibrate. “The flow of water is dynamic, the ballast static. It is critical to study the interaction of these two phenomena, and to see if that destabilizes the infrastructure even more,” adds De Cesare.
Potential interventions
The LCH’s next step will be to propose specific safety measures for the infrastructures, basing their ideas on measures used to prevent riverbank erosion. They will stabilize the ballast by securing it, and divert the flow before it reaches obstacles.
Looking forward
Today, Switzerland and Germany are exploring new ideas for building high-speed train lines, in particular in the Gotthard, Lötschberg and Grauholz tunnels. These sections are built on beds made of rubber. “But we don’t have any long-term experience with this yet. We don’t know how this material will behave over a 20-year period. We’ve been working with ballast for 150 years, and it’s a proven technique,” concludes De Cesare.
© AlpTransit Gottard SA
Images to download
