Principal Stress Rotation of Surrounding Rock Induced by Soft Rock Roadway Excavation and Its Engineering Applications

Roadway excavation induces principal stress concentration and directional rotation in the surrounding rock, leading to its instability and seriously threatening the safety of soft rock roadways. By adopting the transformation method of Mohr's stress circles in the stress coordinate system, this study proposes a soft rock critical failure criterion accounting for principal stress directions. It investigates the dual evolution mechanism of the magnitude and direction of the surrounding rock’s principal stresses during excavation, explores the spatial characteristics of principal stress rotation, and reveals the excavation-induced surrounding rock instability mechanism. Results show that during roadway excavation, the magnitude and direction of the surrounding rock’s principal stresses follow a three-stage pattern: "initial stability, severe disturbance, and new equilibrium". Principal stress concentration is most significant in the two sidewalls, reaching 152% of the initial stress; after severe disturbance, their direction restores to the initial state at the new equilibrium stage. Principal stresses in the roof and floor follow, accounting for 132% and 121% of the initial stress respectively; severe disturbance causes the maximum and minimum principal stresses to swap directions. Based on the principal stress rotation trajectory during roadway working face advancement, this study presents optimal principles for determining roadway advancement direction under single weak plane, multiple weak plane sets, and random weak plane conditions. These principles can guide the advancement direction of soft rock roadway working faces and provide new insights for surrounding rock deformation control in soft rock roadways.