Anthony Monnier: Etude du danger de chute de blocs le long de la ligne ferroviaire Aigle – Les Diablerets

Anthony Monnier
Supervisor: Prof. Michel Jaboyedoff
Expert: Andrea Pedrazzini

This Master memory focuses on the risk of rockfall along the Aigle – Sépey – Diablerets railway line, in the Swiss Alps.

This memory breaks up into four main parts: Identification of the rockfall sources, risk definition by trajectometric analyses and field observations, the creation of hazard maps, the calculation of the damages cost and the individual risk, protection measures proposals and a calculation cost/effectiveness of these measures.

The most dangerous zones along this railway line have already been protected a few years ago, on the basis of a detailed study and general studies. But it remains three zones classified in 1st priority which were not secured. These three zones are the subject of this memory.

We can find indicative hazard maps in Switzerland, generated by computer based on topographic and geological criteria, which do not have the force of law. They have only an advisory role for the landscape planning.

Securizing the Swiss railway lines against the rockfalls consists in protection measures and monitoring measures in rare cases. There are four principal types of protection against rockfall : the rockfall screen, the rockfall net, the barrage and the gallery.

The hazard identification consists in a consultation of the preceding studies and an investigation of the people directly concerned by the rockfall hazzard, field observations as well as localization of the already fallen blocks and the discontinuities families so being able to determine the mechanisms of rupture. Treatment of the DEM by Matterocking© can confront the field observations with the data-processing modelings. The use of the DEM requires an error evaluation, by confronting it with altimetric measures taken with a differential GPS.

Once the rockfall sources located, it’s time to determine the hazzard, consisting in the propagation and the intensity at each place of the study perimeter. The size of the blocks is estimated according to the fallen blocks and the scanline method. Rotomap3D©, a trajectometry software allows to determine the main rockfall corridors and then extract the profiles which are processing by Rocfall©, a two dimensions trajectometry software.

The results of these simulations make it possible to define the intensity of the danger according to the energy mobilized by rockfalls as well as the propagation and the hazzard of the scenarios which can occur on the three studied zones.

Two types hazard maps are produced: One is based only on the energy mobilized the rockfall simulations, the other uses the same the criterias than the preceding but it includes some conventions and its polygons are smoothed in order to allow a better legibility.

In order to calculate the cost of the potential damage generated by rockfalls in these areas and the individual risk of a person travelling by train, it is necessary to define some scenarios, their consequences, their recurrence time and the localization of their occurrence, according to the characteristics of the rockfalls and topographic criterias.

The protection measures suggested here consist only of rockfall nets. Two alternatives are proposed: a total protection and a protection decreasing to the maximum the risk for a minimum investment. The utility of these alternatives will be evaluated using a calculation cost/effectiveness.

Lastly, a railway line as that studied does not have a much traffic, which makes protection measures less or not profitable. A study way would be the installation of a rockfalls monitoring system. These systems are mainly developed in Canada. The most elaborate of them uses some geophones in order to be able to detect several natural phenomenas as the rockfalls but also exploitation incidents like a derailment or a broken rail.