Effect of thickness ratio on uniaxial mechanical behavior of soft-hard composite rock masses: experimental analysis and modeling

The properties of laminated soft-hard composite rock masses are significantly influenced by the thickness ratio of the soft and hard rock layers, due to their inherent heterogeneity and anisotropy. This study investigates the mechanical properties and mesoscopic damage of soft-hard composite rock mass with varying layer thickness ratios. Soft-hard composite rock samples with different thickness ratios were prepared, and uniaxial compression experiments were conducted using an acoustic emission (AE) system to examine the mechanical behaviour and failure characteristics of these composite rock masses. The key findings are as follows: (1) Mechanical parameters exhibit a linear decline with increasing soft rock thickness. When the soft rock thickness ratio increased from 10% to 90%, the peak strength of the composite rock mass decreased by 62.8%, from 45.12 MPa to 16.8 MPa; the elastic modulus decreased by 36.8%, from 9.77 GPa to 6.17 GPa. (2) The number of surface cracks gradually decreases as the proportion of soft rock thickness increases, while oblique cracks progressively increase. The failure mode transitions from splitting failure to shear failure. When soft rock thickness is low, splitting failure dominated by hard rock predominates. As soft rock thickness increases, the failure mode shifts to predominantly shear failure, with the duration of the plastic stage extending from 200 s to 600 s. (3) Based on the law of conservation of energy, the energy evolution of the rock under unidirectional loading was analysed. The relationship between the energy evolution of soft-hard composite rock mass and the layer thickness ratio was constructed by considering the elastic constant and peak strength of the soft-hard composite rock mass. (4) A damage constitutive model was proposed based on Weibull distribution theory. This model accounts for the influence of the layer thickness ratio on macroscopic deformation localization, considering microscopic rupture damage. It provides a better fit for the stress-strain curve of soft-hard composite rock masses under unidirectional loading, offering an effective characterization of their mechanical behaviour.