Page 424 - Rock Mechanics For Underground Mining

PILLAR SUPPORTED MINING METHODS

Panel pillars in a yielding mine structure are designed to provide locally resilient
support for rock in the periphery of mine excavations. Thus, room and stope spans can
be designed on the same principles as applied in a conventional design. The potential
for pillar yield can be assessed directly from the rock mass properties which control
stable post-peak behaviour. These have been described in section 10.7. The design
of the panel pillars must take account of the global stability of the structure, using
the techniques discussed previously. Finally, since the ultimate load-bearing capacity
of a yielding structure resides in the barrier pillars, these must be designed to be of
virtually infinite strength. This implies that the barrier pillars each have a sufficient
width/height ratio to create a central core of confined rock capable of sustaining the
load shed by the yielding panel pillars. The design of such a pillar is best accomplished
using a finite element or finite difference code, so that local yield in the barrier pillar
can be incorporated explicitly in the design analysis.

Problems

1 A horizontal stratiform orebody at a depth of 150 m below ground surface is
planned for extraction using 6.0 m room spans and pillars 7.0 m square in plan. The
full stratigraphic thickness of 3 m is to be mined. The unit weight of the overburden
−3
rock is 22.5 kN m . Analysis of pillar failures in the orebody indicates that pillar
strength is defined by

S = 10.44h −0.7 w 0.5
p
where S is in MPa, and pillar height h and width w p are in m.
Determine the factor of safety against compressive failure of pillars in the planned
layout. If the factor of safety is inadequate, propose a mining layout which will achieve
a maximum volume extraction ratio, for a selected factor of safety of 1.6. State the
assumption made in this calculation.

2 A flat-lying coal seam 3 m thick and 75 m below ground surface has been mined
with 5.0 m rooms and 7.0 m square pillars, over the lower 2.2 m of the seam. Determine
the factor of safety of the pillars, and assess the feasibility of stripping an extra 0.6 m
of coal from the roof. The strength of the square pillars, of width w p and height h,is
given by
S = 7.5h −0.66 w 0.46
p
where S is in MPa, and h and w p are in m.
−3
The unit weight of the overburden rock is 25 kN m .
3 The orebody described in Problem 1 above is underlain by a clay shale, for which
−3
◦
c = 1.2MPa, = 28 , and = 22 kN m . If the mining layout is based on 6.0 m
room spans and pillars 10.0 m square in plan, determine the factor of safety against
bearing capacity failure of the floor rock.
4 A pillar with a width/height ratio of 1.2 is to be subjected to stress levels exceeding
the peak rock mass strength. For the elastic range, it is calculated that the pillar
−1
stiffness, ,is20GNm . The ratio / varies with the width/height ratio of the

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