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Studying permafrost to understand rockfall

Permafrost is ground which has been frozen for at least two years. Its formation, thickness and eventual disappearance are closely related to climate change. Depending on temperature in mountainous areas, permafrost variations can lead to rockfall. An example of this is the disappearance of the famous Bonatti Pilier on the Dru in 2005. In 2013, the Petzl Foundation contributed to a research mission to study permafrost in rock walls.

SEPTEMBER 2014

© collection ADRGT

  • Project partner: Landslide research development association (Association Développement Recherche Glissement de Terrain)
  • Country:  France, Upper Savoy
  • Project type: Gaining knowledge
  • Budget: €7,100 in 2013

Permafrost is often accompanied by the presence of ice, which acts as cement between two sides of a rock formation, particularly in cracks and fissures. If the temperature of the ice rises, it loses its strength and rockfall occurs. Unlike glacial retreat, permafrost warming is an invisible phenomenon. However, when the rock is homogeneous, we are now able to measure the temperature inside the walls at various depths using temperature sensors.

In 2005, the Aiguille du Midi in the Mont Blanc mountain range was chosen as a pilot site for performing such measurements in the field. A meteorological station was set up at the top by Météo France, as well as automatic digital cameras to monitor changing snow conditions. Snow can change rock temperature due to its insulation properties.

To observe permafrost in real time and at depth, three boreholes were drilled into walls oriented to the northwest, northeast and south. A chain of fifteen temperature sensors was inserted into each hole to a depth of ten meters. This allowed the researchers to observe the changes in the permafrost, caused by climate factors. The long-term aim is to study how temperature affects rock instability, particularly when this is due to the presence of fissures that make the rock fragile.

In 2013, with partial funding from the Petzl Foundation, a study conducted by the Association Développement Recherche Glissement de Terrain and the EDYTEM laboratory (University of Savoie - CNRS) examined the internal structure of the northwest wall of the Aiguille du Midi, which was observed using ground penetrating radar. This electromagnetic measuring device scans the rock to reveal its internal structure. The radar operator descended with the device, which was held firmly against the wall, so that the entire length of it could be measured. The radar-operator abseiled down in particularly difficult conditions.

Two profiles of the wall have been made. Data acquisition was immediately sent by wifi. This information will be processed to obtain an image of the internal structure, which will show the increase of fracturing and will enable potential slippage plans to be drawn up. In the long term, we hope to understand how these fractures affect the evolution of temperature within the rock and so anticipate potential rockfall sites.

The researchers, Héloïse Cadet and Florence Magnin, explain:

© collection ADRGT "In the mountains, measurements cannot be random. Everything must be prepared in advance when you're hanging on the end of a rope. We must anticipate hazards, such as wind and cold.
We got a set of unique, clear and accurate data that we could compare with rock temperature data. For us this is a real source of satisfaction!
In addition, this project has allowed us to work in a magnificent place."


Photos: ADRGT

Uploaded in september 2014


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