The National Road RN 91 has been threatened for about twenty-five years by a huge landslide, located 25 km south-east to the town of Grenoble (France). If several million cubic meters of rock fall down, the debris will dam the valley. Then the failure of the dam by overtopping and rapid erosion might result in a catastrophic flood and dramatic consequences for human life, environment and economy throughout the valley. The paper presents the hazard assessment based on geological and hydrological surveys, including small scale hydraulic tests, as well as the risk evaluation that has been performed. The risk management relies first upon a high level monitoring and an emergency plan; various mitigation strategies have been considered.


In 1980, rock falls including blocks of limited size occurred on the National Road RN 91, in the valley of the Romanche river, approximately twenty kilometers southeast of Grenoble town (Figure 1). The starting point of the rocks, called "Les Ruines de Sechilienne", is located 300 m above the valley bottom, one kilometer downstream from Sechilienne village, in the French Alps.

(Figure in full paper)

Such rock falls are frequent in mountainous areas and nobody paid special attention to the event of 1980 as most of the slope was covered by a dense forest. But in 1985, new rock falls with large size blocks occurred and the road had to be closed during several days. An emergency monitoring was decided, during day and night. Soon afterwards a barrier of concrete blocks was set up along the road at the toe of the slope; the blocks were surmounted by detection wires connected to red lights located at both ends of the danger zone, one hundred meters long.

The first geological surveys showed that the unstable area, not easily accessible and wooded, was not just a matter of one cliff generating some rock falls, but included a potential volume of a few million cubic metres.

From the moment the landslide hazard was identified, the risk management was based on a monitoring system associated with an emergency plan. This system, initially composed of geodetic and extensometer manual measurements (cables stretched through fractures), was gradually developed and improved.

In 1985–1986, as a first prevention measure, a diversion of the RN 91 was built in the valley floor, at the toe of the slope opposite to Les Ruines, with temporary bridges at each side of the exposed area. The heavy trucks had to keep using the old road. To avoid the potential wanderings of the river dammed by debris in case of a rockslide, a diversion channel has been dug; it is protected by an earth dyke, supposed to be able to retain 1–2 million m3 of debris. The final bridges for the road diversion were built a few years later.

The main features of the hazard and risk evaluation linked to the moving rock mass are presented below.

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