Spatial Distribution of Rock Disturbance in Assessment of Roof Stability in Flat-Ceiling Cavities

A common misuse of the disturbance factor in the Hoek–Brown failure criterion is in assigning one value of it to the entire rock mass involved in stability analyses. Such a choice will typically underestimate the strength of the rock mass in much of the region considered in the analyses. In this study, the influence of a spatially varied rock disturbance on roof stability of underground cavities is assessed, based on the kinematic approach of limit analysis. Rather than dividing the rock mass into layers with a progressively changing disturbance factor, a continuous function is introduced, describing a decaying disturbance factor with increasing distance from the excavation. Stability numbers, factors of safety, and the supporting pressure required for stability are computed for flat-ceiling cavities in the disturbed rock at depths preventing the roof failure from propagating to the ground surface. The computational outcome for the cavities in rock with spatially varying disturbance factors yields stability measures that are more favorable than those from analyses with uniform disturbance; the failure mechanisms are quantitatively different for cases with varied and uniform disturbance. In a weak rock mass characterized by a low Geological Strength Index, the disturbance effect is more drastic compared to high-quality rock. The dimensions of the roof collapse mechanism expand with an increase in the magnitude of the disturbance factor, and the size of the failing block was found to be dependent on the spatial disturbance distribution. The value of the disturbance factor, disturbance decaying pattern, and the size of the disturbed zone all play important roles in the roof stability of underground cavities.