Effects of Persistent Joints on the Strain Failure of the Roof and Floor around a Circular Opening

<title>Abstract</title> <p>To investigate localized damage concentration and coalescence-induced instability of surrounding rock where fault fracture zones coexist with roadway excavations, a circular opening was used to represent a roadway or underground cavern, while a through-going filled fissure represented a fault fracture zone or filled weak structural plane. Biaxial compression tests under low lateral confinement were conducted on intact sandstone, sandstone with a single circular opening, and opening–fissure specimens with different fissure inclinations and filling thicknesses. Global deformation, local deformation at the opening roof and floor, and acoustic emission (AE) responses were monitored simultaneously. At a fissure inclination of 15°, increasing the filling thickness from 2 mm to 5 mm and 10 mm increased the peak strength from 43.16 MPa to 48.96 MPa, followed by a slight decrease to 47.90 MPa, while the peak axial deformation generally increased. At a filling thickness of 2 mm, increasing the fissure inclination from 10° to 15° and 20° reduced the peak strength from 53.47 MPa to 43.16 MPa and then increased it to 55.61 MPa, whereas the peak axial deformation first decreased and then increased markedly. These results indicate non-monotonic effects of both fissure inclination and filling thickness. Increasing the filling thickness intensified AE activity during the middle and late loading stages and markedly increased the proportion of shear-type events, which reached 67.5% and 63.9% for the 5 mm and 10 mm specimens, respectively. At a fissure inclination of 20°, the shear-event proportion increased to 21.0%, indicating a transition from tensile-dominated failure to tensile–shear mixed or shear-dominated failure. Considering peak strength, local deformation around the opening, and local instability time, specimen s-15-2 exhibited the strongest deformation concentration and earliest instability, indicating the poorest overall stability, whereas s-15-5 showed relatively high strength, the latest roof instability, and limited floor deformation, indicating comparatively better stability. These findings provide an experimental basis for instability assessment, hazardous-zone delineation, and support optimization for roadways adjacent to fault fracture zones.</p>