LANDSLIDES IN THE COAL-BEARING SERIES OF THE ARC VALLEY, FRANCE

The right bank of the Are Valley bears a number of huge landslides loc ated between Saint-Michel-de-Maurienne and Modane. They mainly affect the coal-bearing series of the ''Brianconnais facies belt'' which is f amous throughout the Western Alps for being prone to slope movements o f many types and sizes. This paper describes the three major slides: t he Poucet, Francoz and Bonvillard slides. Two of these slides extend f rom the very crest down to the valley bottom, with elevation differenc es ranging between 1400 and 1700 m. These slides have long been identi fied because of their geomorphological expression. However, they have never been studied in detail owing to their size and a lack of practic al interest. Indeed their displacement has been sufficiently slow to r esult in only minor damage to the railway and road in the area. Recent ly, preliminary studies for a deep tunnel on a section of a new motorw ay (construction of which has just started) required that significant subsurface investigations be carried out. Geophysical surveys and asso ciated exploratory borings have provided a great deal of information o n the thickness of the landslides and shows them to be essentially of the planar type.