Matterock methodology

The major rockfall at Randa (Valais, Switzerland) in the spring of 1991 (30 Mo^m3) highlighted the scientific shortcomings in predicting such catastrophic events. It was therefore decided to develop a cliff monitoring methodology that would provide a better understanding of the phenomenon of cliff instability and anticipate potential rockfalls. This methodology, called MATTEROCK, was developed as part of the National Research Program PNR31 “Climate change and natural disasters” launched in 1992 by the Swiss NationalScience Foundation (SNSF). A second phase began in 2000. This time as part of a cross-border cooperation program: Interreg II C “Prevention of cliff movements and instabilities”. Funded by the SNSF and the canton of Valais, this new phase of the MATTEROCK project aimed to improve rockfall detection tools, in particular by exploring the possibilities offered by geomechanical analysis.

The MATTEROCK methodology is based on the premise that there can be no rockfall without discontinuities (faults, schistosity, etc.). The first step is to analyze and define the cliff’s structural layout by characterizing families of discontinuities. The structural layout is then compared with the topographic surface to identify cliff compartments (hazards) prone to sliding and/or tilting (automated with Matterocking). A hazard index qualifying the probability of failure is then assigned to each compartment according to its intrinsic factors (petrographic, lithological, structural and geomechanical characteristics), its sensitivity to external degradation processes (erosion, minor earthquakes, etc.) and to triggering situations (extreme physical conditions). Finally, hazard maps are drawn up on the basis of these hazards.

The hazard map is an indispensable tool for risk managers and land-use planners in mountain regions. The degree of hazard is generally qualified by the intensity and probability of occurrence of a catastrophic event. However, the particularity of a rockfall is that it is very rarely reproducible, unlike a flood event, for example. For this reason, MATTEROCK relegates the notion of occurrence to the background and relies primarily on a definition of the degree of danger, which considers not only the intensity of the phenomenon, but also the probability of the hazard breaking down and the probability of the object to be protected (homes, communication routes, etc.) being affected. By the end of the^2nd phase of the project, the MATTEROCK methodology had been applied to over 50 sites in Valais, notably in the Mattertal, Saastal and Simplon Valleys, in the Val de Bagnes, and in Italy.

The latest recommendations published by the FOEN(FOEN, 2016) consider that 6% to 8% of Swiss territory is affected by ground movements, which include landslides, mudflows and rockfalls in particular. In addition, the increase in bad weather in recent years has destabilized certain slopes that had never been reported or assessed as potentially unstable. The notion of climate change put forward by PNR31 in the early 90s now seems to be taking on its full significance.