Page 413 - Rock Mechanics For Underground Mining

THE ELLIOT LAKE ROOM-AND-PILLAR MINES

Table 13.2 Doe Run pillar condition rating system (after Roberts et al., 1998).

in orebody thickness and pillar height, causing a marked local reduction in pillar
strength. Thus, when orebody thickness is variable, stope and pillar design based on
maintaining a constant area extraction ratio will not provide assurance of intact pillar
performance.
A number of other hard rock mines, most notably in North America, have used or
adapted the Elliott Lake approach and the pillar strength formula developed initially
by Hedley and Grant (1972). In these cases, it has been common practice to represent
the constant in equation 13.24 as a constant times the uniaxial compressive strength
of the pillar rock material. Roberts et al. (1998) and Lane et al. (2001), for example,
discuss the approach to pillar design and stability evaluation developed at the Doe Run
Company’s lead mines in Missouri, USA, for room-and-pillar mining and subsequent
pillar extraction and backfilling. Pillar loads were estimated using the displacement
discontinuity code NFOLD and pillar strengths were represented satisfactorily by
Hedley and Grant’s formula for pillars of limited height. Subsequently, the pillar
condition rating system shown in Table 13.2 was developed and used with NFOLD
pillar load calculations to produce the stability curves shown in Figure 13.25 for
standard pillar widths (9 m by9matDoe Run with larger 11.5 m by 23 m panel
pillars introduced from 1993). The pillar condition rating and the calibration curves
have been applied to hundreds of pillars at four of Doe Run’s mines. They are used
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