Page 402 - Rock Mechanics For Underground Mining

PILLAR SUPPORTED MINING METHODS

Lunder and Pakalnis (1997) and empirical strength formulae such as that introduced
Hedley and Grant (1972) for factors of safety of 1.0 and 1.4 and width to height ratios
of up to 1.5. The empirical data and the computed results were presented as plots of
the ratio of pillar stress to unconfined compressive strength of the pillar rock material
against the pillar width to height ratio for constant values of factor of safety.

13.4 Design of a stope-and-pillar layout

Design of a supported mining layout should seek to achieve the highest possible ex-
traction ratio of mineral, while assuring locally stable stope spans and general control
of near-field rock. In typical design practice, involving irregular stope-and-pillar ge-
ometry, it is usually preferable to apply one of the computational methods described
in Chapter 6. These may be used to determine stress and displacement distributions
associated with various extraction strategies, stope-and-pillar geometries, and stope
mining sequences. However, it is useful lo explore some general aspects of stope-and-
pillar design, and mine layout, using the tributary area method. This is appropriate
since there should be a convergence between the outputs of the independent methods
of design analysis, for simple geometric conditions in a mine structure. Some broad
geomechanical principles of mine layout may then be proposed from these generic
studies.
When the tributary area method of stress analysis is used in the design of a mining
layout in a flat-lying, stratiform orebody, five parameters are involved in the design
analysis. The field stress component, p zz , acting perpendicular to the plane of the
orebody is determined by the geomechanical setting. The four variables to be estab-
lished in the design process are the working or pillar height h, the room or stope span
w o , pillar width w p , and the factor of safety, F, against pillar failure. Although the
following discussion considers square pillars, of side length w p , it applies equally to
long, rib pillars.
As has been noted previously, the stope span which will ensure the local stability of
the stope walls can be determined using the design procedures appropriate for isolated
excavations. That is, stope span may be established independently of the other design
variables.
The selection of an appropriate factor of safety against pillar failure is based upon
engineering experience. In his retrospective analysis of the in situ performance of
South African coal pillars, Salamon produced the data shown in Figure 13.14. The
histograms illustrate the frequency distributions of pillar collapse and intact, elastic
pillar performance as a function of factor of safety. In particular, the distribution
of intact pillar performance is concentrated in the range of F from 1.3 to 1.9. A
reasonable design value of F in this case is suggested to be 1.6. In any other min-
ing setting, a similar approach could be used to establish an appropriate factor of
safety.
These observations indicate that the remaining parameters to be determined in the
design process are the pillar dimensions, w p , and the working height, h. At first sight,
it may appear that a degree of arbitrariness is involved in the solution to the layout
design problem. Consider the following example. A 2.5 m thick horizontal orebody is
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located at a depth of 80 m, with the rock cover having a unit weight of 25 kN m .An
initial mining layout is based on 6.0 m room spans and 5.0 m square pillars, with the
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