Effects of coring directions on the mechanical properties of Chinese shale

This paper presents an experimental study of the effects of bedding planes on the mechanical properties of shale samples. Two specimens, sourced from Sichuan Basin in China, cored perpendicular and parallel to the beddings were tested using a uniaxial compressive strength (UCS) machine, and the corresponding crack propagation patterns were analysed using acoustic emission (AE) data. In addition, a digital camera system called ARAMIS was used to acquire the lateral and axial strains during the loading. It was observed that there was significant influence on the peak strength of shale when tested for various bedding planes. As expected, the sample with perpendicular load had nearly twice as high maximum axial strength than the sample with parallel load, and the peak cumulative energy release of the parallel load on the specimen was 1.5 times higher than that of the perpendicular load. The axial strain was higher than the lateral strain in perpendicular load, while under parallel load, the lateral strain was higher than the axial strain. When the axial stress was small, for the shale sample with perpendicular load, the axial strain increased gradually with the increase of stress. In contrast, the lateral strain remained near to zero during the loading period for the sample with parallel load, and both the axial strain and lateral strain were not affected by the increasing load and remained zero. The volumetric strain of the two samples indicated that shale with a perpendicular load shows a compaction behaviour, while parallel load leads to a dilatancy characteristic. The AE data showed that the compression period consists of three stages on both of the two tests and energy release occurs mainly in the third stage. From the ARAMIS results, no perpendicular cracking was observed in the normal stress test, while under parallel loading, the cracks were mostly parallel to the load direction.