Failure Characteristics of Layered Composite Sandstones Under Varying Mechanical Contrasts: A True Triaxial Experimental Study

The deformation, strength, and failure characteristics of layered composite sandstones (LCSs) are critical issues in a stability evaluation in underground engineering. These issues were examined in this study using true triaxial experiments, with the main focus being on the influences of mechanical contrast (MC) between the layers in a three-dimensional (3D) restricted stress state. The overall strength of LCSs is mainly controlled by the strength of the weaker sandstone when MC exists between the layers. Failure under true triaxial stress results in σ₂-oriented fractures coexisting with σ₃-oriented fractures within LCSs. The overall failure of an LCS test specimen exhibits the σ₃-transverse symmetric fracture mode, i.e., σ₃-transverse symmetric tension near the σ₃-loading surface and σ₃-transverse symmetric shear in the middle of the specimen. A new strength criterion is proposed to predict the true triaxial strength of LCSs effectively. The study results provide insights into the cracking morphologies of LCSs with mechanical property changes in individual layers under true triaxial stress.