ABSTRACT
La Frasse Landslide is a large slide with a volume of 42 million m3 that intercepts a main road in its lower part. In this zone the long-term slide velocity was about 60 cm/year and several crisis episodes due to long rainfall conditions had led to affect the road platform. Detailed investigations have demonstrated that a drainage gallery equipped with vertical drainage boreholes would improve the stability of the slope. This gallery was carried out between 2007 and 2009 and its efficiency was monitored by a Robovec laser distance meter system, allowing the recording of the displacements of 14 points in the most active zone. The first results were quite satisfying, as the average velocity of the fastest point dropped from 60 cm/year to 2.4 cm/year. But it was necessary to check the behavior of the slide after a critical rainfall episode, similar to the ones considered in the hydro-mechanical modelling. This event occurred during the first half of the month of May, 2015, when a long and intense rainfall affected the area, but no acceleration phase was noted.
