Influences of Mechanical Contrast on Failure Characteristics of Layered Composite Rocks Under True-Triaxial Stresses

Layered composite rocks (LCRs) are often encountered in underground excavation in which extensive deformation, buckling and local collapse near the layer interface frequently occur. However, there is limited advance knowledge of the failure mechanisms of LCRs to interpret the field observations due to the relative scarcity of polyaxial compression testing. To that end, this study conducts the true-triaxial experiments with the acoustic emission (AE) monitoring on the LCR specimens, with the emphases being on the effects of mechanical contrast (MC) between the layers on the fracture behaviors under true-triaxial stresses. First, the strength of LCRs is controlled by the weaker layer. Second, at the unstable fracturing phase, high AE event rate is faithfully recorded but intermittently interrupted by macrofracturing in granite but not in sandstone. Third, main failure planes of LCRs appear sub-orthogonally to σ₃ (minimum principal stress), taken the form of delamination fracture and exhibited the mode of σ₃-transverse-symmetric fracturing. As the distance from the unloaded surface increases, the fracturing transitions from σ₃-transverse-symmetric spalling to σ₃-transverse-symmetric shearing. Depending on the MC, both the fracture propagation through the interface and the fracture containment within sandstone are registered. Moreover, the interface effect is numerically revealed through a 3D finite-element modeling. This study gets insights into the failure mechanisms of LCRs with a changing mechanical property in individual layers under true-triaxial stresses.