The 1979 Nice Harbour Catastrophe Revisited: A New and Consistent Trigger Mechanism

Gabriela Dan, Nabil Sultan, and Bruno Savoye
IFREMER, Plouzané, France

In 1979, a catastrophically event occurred on the Nice continental slope generating the lost of human lives and important material damages. A part of the new harbour construction collapse into the sea. Following the event, many studies have been done in the area, in order to describe the geological setting and to understand the trigger mechanism of the 1979 accident.

The aim of this work is to pass in review the geological setting and details related to the 1979 accident. The assessment of the slope stability, before and after the new harbour construction is also considered, by taking into account new available data such as submersible images, side-scan sonar data (SAR), coring and piezocone CPT data. Data from the piezocone tests shown the existence of very sensitive clay, which represents the possible failure surface.

Slope stability assessment using the finite element model (Sultan et al., 2001) has shown that the Nice continental slope was characterized by a metastable state even before the harbour extension. Under the new embankment loads, calculation results have demonstrated that creeping of the sensitive clay layer could be at the origin of the 1979 sliding. In addition, the exceptionally heavy rainfall occurred before the accident and thus the seepage of fresh water had probably accelerated creeping and the Nice slope failure.

Our modelling results showing a relatively long-term failure (creeping) are in good agreement with the official report mentioning that during the landfill operations, 110 reports of cracks, settlements, failures and embankment collapse have been recounted.