Mechanical response and energy evolution of interbedded shales subjected to multilevel constant/increasing-amplitude cyclic loading

The established research hotspots of shales, which play a significant roles in the stability of slopes, mainly focuses on its bedding plane effect under conventional stress paths, although landslides frequently suffer from complex stress disturbances. This study aims to investigate the effects of cyclic loading paths and interbed structure on the mechanical properties and energy evolution characteristics of shale. In the experiments, shale specimens with five types bedded angles (0°, 30°, 45°, 60°, and 90°) were prepared, and multilevel constant-amplitude cyclic loading (stress path Ⅰ) and multilevel increasing-amplitude cyclic loading (stress path Ⅱ) were performed. The results indicated that the mechanical behaviors including strength, deformation and failure modes, energy dissipation, and internal damping of shale samples were all highly affected by the bedding plane angles and the cyclic loading paths. Specifically, with the increasing of bedding angle, the peak strength, total dissipated energy, and total input energy of the shale specimens showed a “U” trend, the ultimate macro-failure mode changed from mixed tension-shear failure to failure along the bedding planes, and the damping ratio firstly increased and then decreased. In addition, test schemes of stepwise increase of the lower stress limit or keeping it constant were the differences between stress path Ⅰ and stress path Ⅱ, which significantly influenced the evolution of irreversible deformation of shale specimens. Peak strength of shale samples under stress path Ⅰ ranged from 4.3% to 23.9% lower than under stress path Ⅱ. Compared with stress path Ⅱ, elastic modulus and damping ratios of shales had the greater variations, under stress path Ⅰ, and dissipated energy and elastic strain energy were relatively low by an order of magnitude.