ABSTRACT

The deterioration of coastal cliffs, such as the Viviers-Basques', is more and more considered in France because of potential risks. In order to analyze the influence of the mechanical and hydraulic properties on the jointed rock mass behaviour, this cliff has been modelled using a distinct elements method. For this purpose, we have considered the geology, the geometry (lithology, jointing) and hydrogeology of the rock mass established thanks to in-situ measurements. According to in-situ observations and to kinematic analysis, the failure type is plane slide. Geomechanical characterization of the rock mass has been done through the GSI. Finally a two dimensions model has been realized with UDEC. The simulation illustrates that the more deteriorated the mechanical properties are, the less the water level needs to be high to collapse the cliff. Failure expands from the bottom to the top of the cliff. It also seems to show that the failure of the top beds is indicating a more extensive phenomenon.

INTRODUCTION

The Viviers-Basques' rock slides (South West coast of France) are one of the most significant examples of the movements accelerating the Basques' rocky coastline retreat.

The studied cliff, within the city of Urrugne, is edged by the Atlantic Ocean at North-West and close to the "Corniche" road, threatening the tourist road. It is 350 m long and up to 40 m height (fig. I). The slides affect slopes formed by flysch (sedimentary jointed rock), dipping about 25° to the north-west.

Understanding collapse process requires knowing cliff geomorphological, geological, and hydrogeological features, subject of the first part of this paper. These studies allow the kinematics analysis, topic of the second part, with geomechanical characterization of rock mass. At last, with the purpose to understand to the failure mechanism, a numerical modelling has been realized.

(Figure in full paper)

CLIFF GEOLOGICAL CHARACTERISTICS
Regional geological context

Owing to the Viviers-Basques localization, North- West of Pyrenees Range, Viviers-Basques' rock mass structure is linked to the Pyrenean orogenesis Which can be divided into two major phases: distension then convergence.

During the Cretaceous, a big furrow takes place in the Pyrenees. The subsidence intensity implies very thick deposits (up to 5 km). Viviers-Basques' flyschs were deposited than, exactly during the Campanienlate Coniacien (Second Era; Razin, 1989).

In this region, the major compressive step began at Middle Eocene; Viviers-Basques' rock mass belongs to a unit called Urrugne's unit which was then individualized forming a monocline unit cut by discontinuities born of tectonic stresses (Razin, 1989).

As a result, the Viviers-Basques' jointed rock mass is formed by flysch dipping uniformly - at regional scale - to the ocean. The flysch rock mass is covered by various thicknesses of a clayey formation, the alterites, residue of flysch weathering.

Lithologic features

The flysch, sedimentary rock generated from deposit of old submarine slides, is composed by successive beds of different nature called Bouma's sequence by sedimentologists. In the basque flysch case, slides have then mobilized calcareous sediments.

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