Altération et rupture d'échantillons d'argilite lors de sollicitations thermomécaniques en conditions humides contrôlées

Résumé

L'objectif de ce travail est de réaliser des essais de fluage sur des échantillons d'argilite en faisant une description microtexturale des mécanismes à l'origine de leur endommagement et de leur rupture sous sollicitations thermomécaniques et hygrométriques. Dans cet article on montre le déroulement d'un essai représentatif. Le dispositif expérimental appelé "microcellule CGI" qui est utilisé permet de suivre visuellement et à différentes échelles l'évolution dans le temps de la texture d'un échantillon taillé sous la forme d'une lame épaisse de dimensions 50×40×5 mm. Dans les conditions fixées pour les essais sur des argilites du Callovo-Oxfordien de l'Est de la France, les déformations observées sont des déformations cataclastiques (fissuration) et on met en évidence le rôle essentiel joué par la texture des argilites et par l'altération des pyrites qu'elles contiennent.

Abstract

The objective of this work was to undertake creep tests on shale and describe the microtextural phenomena which initiate internal damage/rupture under thermomechanical loading and relative humidity conditions. The CGI test cell is a system developed at the Centre de Geologie de l'Ingénieur specifically to allow observation of the fabric of a thick slide (50×40×5 mm) at different magnifications during the test. The CGI test cell imposes both an axial and a planar strain on the sample. The first step is to prepare a thick slide from a core and to describe its "initial" state. The second stage is to undertake a creep test to rupture, while the third step is to study the fragments of the ruptured material using XRD (X-ray diffractometry), SEM (scanning electron microscopy) and EDS (energy dispersive spectrometry). The samples studied are from the shales of the Callovo-Oxfordian of the Paris Basin, to the south of Bar-le-Duc in the east of France. These rocks consist of clay minerals (illite and kaolinite), quartz, calcite (22.8%), dolomite, feldspar and pyrite (1%). The pyrite is essentially found in the bioclasts. The creep test is carried out under thermomechanical loading with an axial stress of up to 12 MPa and a temperature rising to 80  °C. The relative humidity is maintained at more than 90%. After 33 days the axial strain reached 0.76%, with rupture of the thick slide occurring 10 days later. The deformation was related to the texture/heterogeneity of the rock and occurred mainly along fissuring. Rupture also occurred along bedding planes, particularly where oxidized pyritic bioclasts were present. The chemical phenomena noted were in part related to the formation of iron hydroxides and sulphuric acid. This reacts with the calcite to form gypsum, which has a lower density than the original pyrite and calcite. The XRD indicated that the iron hydroxides observed on the discontinuities are dominantly limmonite. Examination of the SEM and EDS suggested an increase in porosity, even at some distance from the macroscopic fissures.