An Instability Line for Spalling Around Circular Openings and the Limiting Effects of Scale

Depending on the type of failure mechanism, the strength of intact rock may be susceptible to the effects of scale. The influence of scale is apparent in the spalling of rock around underground openings. To reconcile field-scale observations, an understanding of the unconfined rock strength has to be appreciated from the standpoint of energy instability, particularly for instances in which the mechanism of failure involves propagation of fractures. Fracture energy principles were used to create a link between laboratory-measured uniaxial compressive strength (UCS) and rock failure as observed in spalling around circular openings. An Instability Line (IL) is proposed to predict the stresses above which spalling could occur with different opening diameters. Good comparisons were obtained for a range of dimensions from small boreholes to large tunnels. Based on field observations and analytical study, the results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the IL. The findings suggest that the effects of scale are limited by the crack initiation stress. For large-diameter openings, instability occurs immediately at crack initiation stress, whereas for smaller-diameter openings instability occurs at higher stresses post-crack initiation stress. Highlights This paper aims to explain the discrepancies in the rock mechanics literature to predict spalling for small-diameter boreholes as against large-diameter tunnels. The failure mechanism of spalling is studied based on the framework of energy stability. An Instability Line is proposed based on the analytical study. The results of the analytical study compared well with published laboratory and field observations. The results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the Instability Line . The findings suggest that the effects of scale are limited by the crack initiation stress.