Microscopic deformation and failure mechanism of interbedded rock masses under freeze-thaw action

In the frost-thaw region, prolonged freeze-thaw weathering can induce fracture and weaken rock masses, threatening engineering stability. While interbedded rock masses are common in such projects, their failure mechanisms remain insufficiently investigated in freezing and thawing environments. Therefore, this research establishes a particle flow code (PFC2D) model of interlayered rock masses with particular emphasis on the role of thickness variation. The analysis focuses on displacement, crack evolution, contact forces, and uniaxial compressive strength. The findings indicate that: (i) Completing 8 freeze-thaw cycles significantly increases displacement and contact forces, with crack growth accelerating markedly after 16 cycles. As the soft rock layer thickness ratio (H_s/H) increases, the peak contact force decreases by 18.3%, while the number of cracks rises by 48%. Once H_s/H exceeded 0.5, the rate of crack development decelerates. This reflects progressive bond degradation and damage accumulation: microscopic bonds weaken and rupture to form microcracks. Increased soft rock thickness promotes micro-damage accumulation, altering contact forces and intensifying degradation. (ii) Compressive cracks predominantly initiate in soft rock (limestone). After 20 cycles, cracking extends into the hard rock regions. As the H_s/H rises, compressive cracks first increase and then decline, with an overall reduction of 10.8%, while the compressive contact force exhibits a consistent downward trend. This trend indicates that freeze-thaw cycles cause severe microscopic degradation in soft rock, weakening its macroscopic strength and influencing compressive crack development. Increased soft rock thickness alters the stress state, thereby modifying crack propagation. (iii) Uniaxial compressive strength experiences a marked deterioration after 15 freeze-thaw cycles. It follows an exponential decay with increasing H_s/H, culminating in a total strength reduction of 76.9%. This demonstrates that freeze-thaw-induced microscopic damage deteriorates interparticle cohesion, reducing rock mass strength. A higher H_s/H ratio accelerates microscopic damage in the soft rock, causing cohesion to decay nonlinearly and macroscopic strength to drop exponentially. These results provide a theoretical basis for assessing the deformation and failure behaviors of rock masses under cyclic freeze-thaw action.