Impacts of bedding angle and cementation type of bedding planes on mechanical behavior of thin-layer structured bedded rocks under uniaxial compression

Abstract

To investigate the mechanical behavior and crack evolution properties of the anisotropic bedded rocks, a series of uniaxial compression tests were conducted using bedded limestone, phyllite, and shale specimens, and the bedding angle α of specimens was designed as 0°, 15°, 30°, 45°, 60°, and 90°. The acoustic emission (AE) and digital image correlation (DIC) techniques were applied to monitor the crack evolution mechanism. The cementation types of bedding planes were also different, which greatly affected the mechanical properties of bedded rocks. In this study, the cementation types of bedding planes for limestone, phyllite, and shale were defined as embedded cementation, oriented cementation, and cracked cementation, respectively. The experimental results showed that with the increase of the α, the peak strength σucs, elastic modulus E, AE hit rate, cumulative AE hit and cumulative AE energy all exhibited a decreasing firstly and then increasing trend, and all showed obvious anisotropy. A novel trigonometric strength criterion for bedded rocks was developed. The fluctuation of the b-value indicated that the propagation of the internal micro-cracks was unstable, and showed an overall downward trend. Meanwhile, a tensile and shear crack classification criterion combining the AE parameters, clustering idea and genetic algorithm was proposed, and then the rational dividing line of tensile and shear cracks was determined in AF-RA maps. The α and cementation types both affected the failure mode of bedded rocks. For limestone with α = 0°– 90°, the longitudinal splitting, forming approximate vertical macro tensile cracks, was the primary failure mode, and local shear slipping along the bedding plane occurred in the specimens with α = 30°– 60°. For phyllite and shale, when α≤ 30°, the inclined macro-cracks were not along the bedding planes but showed a shear-dominated failure mode. When 45° ≤α≤90°, the main macro-cracks were along the bedding planes, and some macro-sub cracks penetrated the bedding planes in shale, showing shear-dominated failure for specimens with 45°≤α<90° and tensile-dominated failure for specimens with α=90°. The method of rating scores corresponding to the uniaxial compressive strength in RMR has been improved and an optimization method of the revised rating scores of bedded rocks in different situations considering α and σucs has also been proposed.