Experimental investigation on the spallation characteristics of mudstone under biaxial compressive stress

Mudrock spallation has critical implications for tunneling rock control. Spallation experiments of mudstone under biaxial compression were conducted with mitigated end friction. The results showed that the stress–strain curve of mudstone under biaxial compression displayed a significant strain rebound. With the layered spalling rock structure, the rocks load carrying performance exhibited stronger elastic characteristics, and the overall deformation increased significantly. The rock in the extreme zone (where the tensile strain gradient was approximately zero) in the lateral tensile strain field owing to the significant effect of Poisson’s ratio reached the tensile strain limit first, and tensile fracture occurred. The tensile strain extreme zone (TSEZ) of rock subjected to biaxial compressive stress was continuously distributed in a plane owing to the influence of the free surface, and the shape of the tensile fracture region was similar to the ideal plane. The tensile fracture flaws interconnected to form a flat spalling plane. Influenced by the end lateral constraint, the TSEZ appeared in the rock near the excavation boundary where spallation occurred. Spalling disturbed the stable equilibrium of the load-carrying structure of the rock mass. While the rock mass readjusted its internal stress field distribution during deformation, spalling occurred repeatedly in the new TSEZ. With repeated spalling, progressive spalling destruction developed toward the high-stress area of the rocks. With an increase in the end lateral constraint, the aperture and length of the spalling fractures decreased. There may be a threshold for the contribution of the intermediate principal stress to the rock load carrying capacity. Article highlights Strain rebound, planar tensile failure, large deformation are seen in the mudstone spalling under biaxial compression. The evolution of lateral tensile strain field well explained the planar spalling morphology and mechanism. The impact of end friction on the spalling direction and the dynamic spalling process in field were revealed.