Mechanical and failure characteristics of layered sandstone under true triaxial unloading condition
FENG Fan1, LI Chenglin1, CHEN Shaojie1*, ZHANG Chengguo2, LI Diyuan3, LIU Yangshuo1
(1. College of Energy and Mining Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China;
2. School of Minerals and Energy Resources Engineering, University of New South Wales, Sydney, NSW 2052, Australia;
3. School of Resources and Safety Engineering, Central South University, Changsha, Hunan 410083, China)
Abstract：To investigate the mechanical behavior and failure characteristics of layered rock masses under complex three-dimensional stress states, true triaxial unloading tests were conducted on layered sandstone using a QKX-YB200 true triaxial testing apparatus, considering varying bedding angles ( ), lateral confinement, and unloading directions (unloading the intermediate principal stress ( ) and the minimum principal stress ( )). The results show that: (1) larger lateral confinement will increase the elastic modulus of layered sandstone under different unloading directions. The peak strength is minimum at = 60° and maximum at = 40 MPa under unloading . increases monotonically with the increase of under unloading . The spatial relationship between lateral confinement and bedding plane will affect the peak strength of the specimen. In addition, the dependence of on decreases with the increase of . (2) The failure modes of layered sandstone under true triaxial unloading can be divided into dominant failure along layer, dominant failure across layer longitudinally, dominant failure across layer transversely, dominant failure along layer - across layer longitudinally. The higher is more likely to induce dominant failure along layer-across layer longitudinally under unloading (except = 0°). The failure mode is dominant failure across layer transversely under unloading . (3) The proportion of tensile cracks decreases first and then increases with the increase of under unloading . Regardless of the unloading direction, the lateral confinement has an inhibitory effect on the development of shear cracks. The degree of fragmentation of the sample is positively correlated with and lateral confinement when the lateral confinement surface is perpendicular to the bedding plane, while it is negatively correlated with the lateral confinement when the free surface is perpendicular to the bedding plane. (4) The relationship between the strength parameters of rock mass (cohesion and internal friction angle) and bedding angle is established. A modified Mogi-Coulomb strength criterion considering is proposed. It is shown that the criterion has great advantages in predicting the true triaxial unloading strength of layered sandstone by comparing with the experimental results. (5) Combined with the stress state of the bedding plane, the failure mode of the specimen and the fractal characteristics, the influence of the bedding angle, the lateral confinement and the unloading direction on the macroscopic crack propagation degree of the layered sandstone under the true triaxial unloading condition was discussed.
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2026, Vol. 45 
