Page 409 - Rock Mechanics For Underground Mining

THE ELLIOT LAKE ROOM-AND-PILLAR MINES

Equation 13.22 describes the bearing capacity developed under a long rib pillar.
For pillars of length l p , the expression for bearing capacity is modified to reflect the
changed pillar plan shape; i.e.

1
q b = w p N S + c cot N q S q − c cot (13.23)
2
where S and S q are shape factors defined by

S = 1.0 − 0.4(w p /l p )
S q = 1.0 + sin (w p /l p )

The factor of safety against bearing capacity failure is given by

FofS = q b / p

i.e. it is assumed that the average axial pillar stress is equivalently applied as a uni-
formly distributed normal load to the adjacent country rock. The coarseness of this
assumption justifies the practical choice of a factor of safety greater than 2.0.

13.6 The Elliot Lake room-and-pillar mines

The history of mining the uranium-bearing orebodies of the Elliot Lake district of
western Ontario, Canada, is interesting because of the evolution of the mining layout
and rock response as mining progressed down dip. Rock mechanics aspects of mine
performance have been described by Hedley and Grant (1972) and Hedley et al.
(1984). More than 30 years’ observations of roof and pillar performance are recorded
for the orebodies.
As described by Hedley et al. (1984), the conglomerate stratiform orebodies at
Elliot Lake are set on the north and south limbs of a broad syncline. Figure 13.20 is a
north–south cross section, looking east, showing the Quirke and Denison mines on the
north limb. The orebodies are from3mto8m thick and dip south at about 15 –20 ,
◦
◦
persisting to a depth of 1050 m. They are separated from the basement rock by a
quartzite bed, and overlain successively by beds of quartzite, argillite, a massive 250
m bed of quartzite, and conglomerate and limestone formations. Although the orebody
rock is unbedded, the hangingwall contact is commonly a prominent bedding plane
with an argillaceous parting. Diabase dykes and numerous normal faults transgress
the orebodies, and several thrust faults are prominent features. The rock material
strengths of the orebody, footwall and hangingwall rocks are generally greater than
200 MPa. The pre-mining state of stress is defined by a vertical principal stress, v ,
equal to the depth stress, an east–west horizontal stress about 1.5 v , and a north–south
component about equal to v .
At the Quirke Mine, the mining method resembled that shown in Figure 12.2, with
transport drifts developed along strike, at 47 m vertical intervals. This resulted in
stopes with a down-dip dimension of about 76 m. Crown and sill pillars protected
the rail haulages. Rib pillars, instead of the scattered, irregularly shaped panel pillars
shown in Figure 12.2, separated the stopes which were mined up dip from the haulage
level.
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