Page 391 - Rock Mechanics For Underground Mining

FIELD OBSERVATIONS OF PILLAR PERFORMANCE

Figure 13.4 Redistribution of stress
in the axial direction of a pillar accom-
panying stope development.

The structural response of a pillar to mining-induced load is determined by the rock
material properties, the geological structure, the absolute and relative dimensions of
the pillar and the nature of surface constraints applied to the pillar by the country
rock. Three main modes of pillar behaviour under stresses approaching the rock mass
strength have been recognised, which may be reproduced qualitatively by laboratory
tests on model pillars in a displacement controlled testing machine. These failure
modes are illustrated in Figure 13.5.
In relatively massive rock, the most obvious sign of pillar stressing involves spalling
from the pillar surfaces, as illustrated in Figure 13.5a. Fretting or necking of the pillar
occurs. In a detailed study, Lunder and Pakalnis (1997) described the progressive
stages of degradation of a pillar in terms of the modes of deformation represented
in Figure 13.6. Although the initial signs of rock stress may be local shear failure,
associated with the re-entrant geometry represented in Figure 13.6a, the formation
of surface spalls illustrated in Figure 13.6b is a more extensive failure indicative of
states of stress satisfying the conditions for fracture initiation and rock damage in
a significant volume of the pillar. In this condition, the pillar is partially failed, but
the core of the pillar is intact, in terms of the model of rock fracture and failure
discussed in Chapter 4. Higher states of stress lead to damage accumulation through
internal crack initiation and extension, and interaction of the network of cracks, as
shown in Figure 13.6c. When friction between the fully developed crack population
is fully mobilized, the pillar is at peak strength, and mechanically is at a state of
failure, illustrated in Figure 13.6d. This model of the progressive evolution of pillar
failure is consistent with the micromechanical modelling of pillar loading reported
by Diederichs (2002), in which progressive crack formation and localisation of shear
strain was observed.
The effect of pillar relative dimensions on failure mode is illustrated in Figure 13.5b.
For regularly jointed orebody rock, a high pillar height/width ratio may favour the
formation of inclined shear fractures transecting the pillar. There are clearly kinematic
factors promoting the development of penetrative, localised shear zones of this type.
Their occurrence has been reproduced in model tests by Brown (1970), under the
geometric conditions prescribed above.
The third major mode of pillar response is expressed in an orebody with highly
deformable planes of weakness forming the interfaces between the pillar and the
373