Geotechnical Approach
Research Studies by T. Lebourg and B. Fresia, Geoazur, UNS, Nice, France.

Thursday 10 December 2009 by Thomas Lebourg

traduction [English ] [français ]

The site was the subject of several core drilling surveys performed by ERG in 2001 and Simecsol in 2002-2003.

A general geometry of the landslide basement is pointed out:

  • a Cretaceous marlstone-limestone rocky substratum that is topped by Eocene stony sandstone with a variable power according to drillings,
  • loose deposits from the Lower Eocene reworked by the recent and ancient movements. These sandy clay deposits show important variations of facies in length and depth with lenticular shapes (see the figures beside).

The interface between the two lithologies results in:

  • a high cohesion contrast,
  • a difference in permeability and behaviour regarding fluids, which is determining in gravitational processes.

Thus the subsurface waters circulation is capable of being concentrated at the interface level and creating an increase of interstitial pressure fitting to a sliding surface. Furthermore some drillings indicate the presence of polished and striated surfaces in clay, underlying ancient rupture zones at various depths (-5 to -15 m). Also, levels comprising limestone blocks can be observed in some drillings (C7, C17, C18, C20). The existence of limestone blocks in the clay beds is explained by the collapse of the prevailing cliffs, that witnesses the gravitational reworking of the slope.

As indicated on the map some drillings were implemented with inclinometric records (C1 to C4) enabling to highlight deep deformations:

  • -5.00 m in C1
  • -11.5 m in C2
  • -9.00 m in C3
  • -11.00 m in C4

Those ruptures logically get organized with the deformations observed during the 2000 and 2001 movements showing a local mobilization of the clay sands along around a ten meter thickness.

Location of the drillings by Simecsol (2002-2003)

Cross sections figurative of the slope movement according to the core drillings by Simecsol (2002-2003)