Soft rock deformation and failure modes under principal stress rotation from roadway excavation

Both the magnitude and direction of principal stress will change during roadway excavation, while the direction often ignored in traditional rock mechanics, leads to large deformation of surrounding rock. To investigate the deformation and failure modes under principal stress rotation during roadway excavation, a numerical calculation model was established to study the evolution law of the magnitude and direction of the principal stress in the surrounding rock during excavation process. The results reveal significant disturbances in principal stress within a range of three times the roadway diameter, with a 1.66-fold increase in the major principal stress and a 41.61° rotation angle change. Then, hollow torsional shear tests were conducted under the assumption of in-plane principal stress rotation. Key parameters such as spherical stress (p), generalized deviatoric stress (q_J), intermediate principal stress coefficient (b), and principal stress rotation angles (α, β, γ) were controlled in the tests. Experimental results indicate that the strong disturbance zone is primarily influenced by deviatoric stress, intermediate principal stress coefficient, and principal stress direction. The major failure modes observed include main shear and torsional shear cracks, with larger principal stress rotation angles leading to more significant torsional shear cracking. The roof exhibits the most severe cracking, while the floor shows the least. These findings underscore the importance of considering changes in principal stress during the design and construction of roadway excavations to prevent instability and cracking.