Assessing collapse risk for any near surface mine opening or for a civil tunnel is challenging, while defining an appropriate minimum rock cover thickness that should be left above the crown is a particularly difficult design task. The difficulties of predicting behaviour and of correctly dimensioning crowns is borne out by the fact that in recent years there have been a number of highly publicized near-surface mine working cave-ins and urban tunnel collapses, with breakthroughs to surface that have in several cases seriously damaged surface infrastructure. The fact that problems from collapses of near-surface workings have plagued mining operations for years led to the development nearly 20 years ago of the Scaled Span empirical design guidelines for crown pillar rock thickness dimensioning. This paper presents an updated methodology and a new chart developed through use of logistic regression techniques aimed at improving the ease of application of the Scaled Span method of Surface Crown Pillar dimensioning for assessing collapse risk. Updated aids to application of the guidelines are presented for applying the mining Scaled Span concept for determining both mine opening and civil tunnel cover thicknesses of variable quality.
Problems from collapses of near-surface workings have plagued mining operations for years, not just at remote mine sites but often also in urban settings, (Plate 1). Civil tunnel collapses to surface are also not uncommon in urban areas. Assessing the risk for whether or not any near surface mine opening or civil tunnel might collapse and break through to impact surface infrastructure is challenging; while defining an appropriate minimum rock crown cover thickness that should be left above an opening is a particularly difficult design task. An attempt to address such problems in a mining context, nearly 20 years ago lead to the development of the Scaled Span empirical design guidelines for crown pillar rock thickness dimensioning (Golder Associates, 1990, Carter, 1992, Carter and Miller, 1995). These initial guidelines, which were mainly developed looking at steep orebody geometries, were targeted at helping mining engineers define potential collapse risk levels for new or abandoned mined openings, and also for establishing critical crown thickness dimensions for new designs.
Plate 1: Two urban crown collapse examples(available in full paper)
Over the years these original mining assessment approaches have been extended to civil tunnelling situations and to shallow dipping mining situations (Carter et al., 2002). The problem increasingly though has been that economic, logistical or scheduling incentives oftentimes drive engineers to push the limits on opening dimensions and/or on crown thickness, or on skipping the necessary step of carrying out sufficient site investigation, such that excavations sometimes get planned much closer to the rock surface than is realized; thereby perhaps unknowingly unnecessarily increasing failure risk.
2. SCALED SPAN EVALUATION
Designing for stability of near surface crown pillars over excavated openings requires an understanding of many factors including the excavation geometry, the characteristics of the rock mass, data on stress conditions, overburden loads, and ultimately an understanding of the relative degree of risk (factor of safety) associated with the planned near surface excavation, (Hutchinson, 2000).