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Evolution des escarpements calcaires instables. Quantification des mouvements actuels par G.P.S. et inclinométrie dans les Alpes du nord (France)

Abstract : The escarpments of alpine calcareous massifs are particularly sensitive to instabilities,because of their stratified structure witb beds of inequal hardness and of numerous factors which accelerate their erosion, such as water and ice in the rock cracks. The instability of calcareous escarpments is expressed as rockfalls or progressive movements. Progressive movements of two escarpments with comparable lithology located in Haute-Savoie have been quantified using two techniques. G. P. S. (Global Positionning System) has been employed to measure significative movements on the slope of Allèves between 1996 and 1998, using a network of 18 markers. Distancemetric measurements have been made using lines of sight between markers of the G. P. S. network and prisms installed on the top of the Valanginian escarpment named Tours Saint Jacques. The combination of the G. P. S. and the distancemetric techniques has allowed to determine the movements of the escarpment. Detailed treatment of the G. P. S. data realized with tbe BERNESE G. P. S. Software has allowed to correct the biases related to mask effects and tropospheric delay. The horizontal coordinates of the markers has thus been obtained with a precision of a 5 millimeters. On the Passy escarpment, silica inclinometers measure the tilting of the slope every hour. Signal processing techniques have been employed to analyse a six years time series (1991-1996), with the aim of determining the influence of temperature and the periodicities hidden in the signal. The site of Allèves presents a horizontal shortening revealed by G. P. S. of 1 cm/year, to the SSE, with respect to the village of AIIèves, located at the foot of tbe slope. This displacement suggests a landslide on a structural surface with an inflexion along an axis dipping to the W. An acceleration of the movements has been observed during the 1997-98 measurement period with respect to 1996-97. This acceleration might show the delayed influence of the 1996 Epagny earthquake. The topmost escarpments show horizontal displacements suggesting the destabilization of the edge of the escarpments. The movements of the Tours Saint-Jacques show their present dislocation. The upper tower seems to be divided in two or more compartments, separated with vertical fractures oriented N140°. A dextral strike-slip fault can explain the relative displacements between the prisms. The movements of the prisms installed on the middle tower, on either side of a vertical fracture oriented N120° cutting the NW face of the tower, express the present-day behaviour of two compartments with independant evolution. The displacements of the prisms of the lower tower may represent the present-day opening of vertical fractures oriented N135. On the site of Passy, no correlation has been observed between rainfall and the inclinometric signal, but strong correlation exist between temperature and inclinometric data in the low frequency field. In spite of these correlations, the drift of the signal shows that the escarpment undergoes uphill oriented tilting. A horizontal axis of rotation oriented N60° may explain the observed tilting. It suggests a landslide of rotational type.
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Submitted on : Monday, January 16, 2012 - 12:05:41 PM
Last modification on : Friday, November 6, 2020 - 3:37:51 AM
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  • HAL Id : tel-00660253, version 1

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Jean François Marconnet. Evolution des escarpements calcaires instables. Quantification des mouvements actuels par G.P.S. et inclinométrie dans les Alpes du nord (France). Géologie appliquée. Université de Savoie, 1999. Français. ⟨tel-00660253⟩

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