To analyse the fracture coalescence behaviour of rock, rectangular prismatic sandstone specimens (80 9 160 9 30 mm in size) containing three fissures were tested under uniaxial compression. The strength and deformation behaviours of the specimens are first analysed by investigating the effects of the ligament angle b 2 on the peak strength, peak strain and crack initiation stress of the specimens. To confirm the sequence of crack coalescence, a photographic monitoring technique is used throughout the entire period of deformation. Based on the results, the relationship between the real-time crack coalescence process and the axial stress-strain curve of brittle sandstone specimens is also developed, and this relationship can be used to evaluate the macroscopic deformation characteristics of pre-cracked rock. The equivalent strain evolution fields of the specimen, with a = b 1 = 45°and b 2 = 90°, are obtained using the digital image correlation technique and show good agreement with the experimental results of pre-cracked brittle sandstone. These experimental results are expected to improve the understanding of fracture mechanisms and be used in rock engineering with intermittent structures, such as deep underground excavated tunnels.

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Shale swelling can cause a series of problems during the exploitation of shale gas. However, because of the complex fabric of shale, a thorough understanding of shale swelling has not been gained. In this work, an experimental multiscale approach is employed to characterize the swelling properties of a shale sample from Sichuan basin of China. Wetting/drying tests are performed by environmental scanning electron microscopy (ESEM), and deformation of the material is measured by analyzing the ESEM micrographs using digital image correlation (DIC) techniques. Investigations are conducted at three distinct scales, enabling the macroscopic behavior to be linked to the microscopic mechanisms (e.g., the role of inclusions, the behavior of clay particles). The macroscopic swelling of the tested shale is less than 2%. This limited swelling is mainly due to the presence of nonswelling inclusions as well as the low swelling capacity of the clay inside the shale. Microcracking is evidenced along both the wetting and the drying paths. It is demonstrated that the clay particles swell rather undirectionally perpendicular to their orientation. Moreover, a preferred orientation of the clay particles is found parallel to the bedding plane, which finally leads to a moderate anisotropy of the macroscopic swelling.

International audienceAn experimental study of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock under hydration is presented. The investigation, which combines environmental scanning electron microscopy (ESEM) and digital image correlation techniques, has been carried out at the micrometric scale of the composite microstructure of the rock. Specimens were hydrated in the ESEM over a wide range of relative humidity and observations conducted on two planes: plane 1 parallel to the bedding plane, and plane 2 perpendicular to it. The observations reveal that the local swelling (which can be quantified at a local gauge length of about 5 µm) is strongly anisotropic in both planes. The global swelling, measured over areas of about 500 µm in width, is also clearly anisotropic in plane 2 (with major swelling direction normal to the bedding plane), but not in plane 1. The global isotropy in plane 1 arises from the uniform distribution of the orientation of anisotropic local strains, while the anisotropic swelling in plane 2 is due to a preferred local orientation. © 2013 Mineralogical Society

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