Fabric control on strain and rupture of heterogeneous shale samples by using a non-conventional mechanical test

Abstract

This work was a part of the preliminary stage of the ANDRA project on a deep-seated research laboratory for the feasibility study of long-term radioactive waste storage in the Callovo–Oxfordian argillites of the Eastern part of the Paris Basin. These rocks are bedded, their density is about 2.4, and they contain an average of 30% carbonates and 25% quartz. Thick (5 mm) sections from cores were put through a non-conventional device called CGI testcell (CGI: Centre de Géologie de l'Ingénieur, i.e. Engineering Geology Centre). The aim of this versatile device is to achieve plane deformation tests under compressive uniaxial loading together with controlled temperature and humidity conditions, and to make, simultaneously, visual inspection of the sample's fabric. We also used X-ray diffractometry, scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Our methodology was applied to samples from two horizons with contrasted lithology: one from near the roof of the formation (445 m deep), occurring in a black shale and nodular micritic limestone unit, and the other from the middle of the formation (480 m deep), occurring in a silty calcareous shale unit with abundant pyritic burrows. These tests have shown the major role of texture, structure and iron sulphides content in the deformation and the rupture of these argillites. The thick slides are brittle and in the conditions of the test, the finite axial strain before the rupture is less than 1%. The clay content controls the deformability. The bedding is responsible for weakness surfaces in the sample. The strength of the carbonate and pyrite aggregates is higher than that of the matrix, and these heterogeneities in the texture control the opening of fissures. Two types of cracks are distinguished: tension fractures parallel to the direction of the compression and oblique shear fractures, guided by the bedding. The principal mechanism at work for the evolution of the sample's texture is the weathering of pyrite glomerocrysts, which results in limonite and gypsum neogenesis. This in turn induces swelling and increasing porosity. The coupled action of weathering and mechanical stress leads to the rupture. In order to take into consideration the respective contribution of mechanical stress and weathering, we used numerical modelling with a finite difference code. We were then able to: (1) reproduce the essential observations on the deformation of a thick section of argillite modelled with a softening elasto-plastic behaviour; (2) quantify the influence of a local heterogeneity; and (3) explain the differences between the real test and the numerical test by the role of the rock structure and texture during its deformation. The image given by the studied samples is representative of the horizons from which they were extracted: the content in pyrite is 1–2% in the Callovo–Oxfordian argillite layer of interest. Consequently, we are brought to consider that, in the purpose of evaluating the area of the possibly damaged zones around a gallery, one should not only take into account the damage from mechanical origin and the damage due to drying effects. Indeed, our results lead us to regard the weathering of pyrite as a supplementary cause of damage that will develop around the facility along with time.