Page 407 - Rock Mechanics For Underground Mining

DESIGN OF A STOPE-AND-PILLAR LAYOUT

Figure 13.18 Maximum volumetric
extraction ratio for various depths of
orebody and orebody thickness (after
Salamon, 1967).

Consider the hypothetical case of a set of orebodies, of thickness M ranging from
1.5 m to 6.0 m, at varying depths below ground surface. Suppose the maximum
−3
stable room span w o is 6.0 m, the unit weight of the cover rock is 25 kN m ,
and the pillar strength formula is defined by equation 13.8. In each orebody, both
the full orebody thickness and the maximum stable room span can be used in the
determination of pillar plan dimensions which will yield a factor of safety for pillars of
1.6. (The procedure described for option (ii) of the initial design exercise is employed
to calculate w p .) Since any such stoping geometry will provide the maximum mineral
∗
yield, the maximum volumetric extraction ratio, R , for the particular case of orebody
depth and thickness can be calculated directly from equation 13.21. The results of a
set of such computations are presented graphically in Figure 13.18.
Two particular observations may be made from Figure 13.18. The obvious one is
that, for any orebody thickness, the maximum safe extraction from a pillar-supported
operation decreases significantly with increasing depth of the orebody below ground
surface. Thus, if one were dealing with a gently dipping orebody, an increasing pro-
portion of the ore reserve would be committed to pillar support as mining progressed
down dip. The other observation concerns the low maximum extraction ratio possible
with a thick seam or orebody using intact pillar support and a single phase mining
strategy. For the 6 m thick seam, at a depth of 244 m below ground surface, the yield
from single phase mining is less than 25% of the total mineral reserve.
Some general conclusions regarding supported methods of mining may be formu-
latedfromtheseexploratorystudiesbasedonthetributaryareamethodofpillardesign.
The first is that, if no pillar recovery operations are to be conducted, a pillar layout
must be based on the largest stable spans for stopes, to assure maximum recovery of
389