Page 367 - Rock Mechanics For Underground Mining

ROCK MASS RESPONSE TO STOPING ACTIVITY

mining in the immediate vicinity of the orebody. They can be estimated using one of
the analytical or computational methods described in Chapter 6.
In previous chapters, attention has been devoted to the design of mine excavations
in different types of rock masses. The preceding discussion indicates that the location
and design of all elements of a mine structure are related to the strategy adopted
for excavating the orebody. This implies that the formulation of the complete layout
for a mine must evolve from consideration of the geomechanical consequences of
the selected method for recovering the ore from the orebody. The need is apparent
for an elaboration of the principles and scope for application of the various mining
methods, and this is now presented. It is intended as a prelude to discussion of the
procedures applied in the design of elements of the mining layouts generated by the
various mining methods, which are the concern of some subsequent chapters.

12.2 Rock mass response to stoping activity

The dimensions of orebodies of industrial significance typically exceed hundreds of
metres in at least two dimensions. During excavation of an orebody, the spans of the
individual stope excavations may be of the same order of magnitude as the orebody
dimensions. It is convenient to describe the performance of the host rock mass during
mining activity in terms of the displacements of orebody peripheral rock induced by
mining, expressed relative to the minimum dimension d m of excavations created in
the orebody. It is also useful to consider the rock mass around an orebody in terms
of near-field and far-field domains. In a manner analogous to definition of the zone
of influence of an excavation, the near field of an orebody may be taken as the rock
contained within the surface distance 3d m from the orebody boundaries.
Different mining methods are designed to produce different types and magnitudes
of displacements, in the near-field and far-field domains of an orebody. For example,
the mining method illustrated schematically in Figure 12.2 is designed to restrict rock
displacements in both the near field and the far field of the orebody to elastic orders
of magnitude. Following the usual notions of engineering mechanics, prevention of
displacements is accompanied by increase in the state of stress in and around the sup-
port units preserved to control near-field rock deformation. The result is to increase
the average state of stress in the orebody near field. The orebody peripheral rock is
fully supported, by pillar remnants in the orebody, against large-scale displacements
during stoping activity. Such a fully supported mining method represents one con-
ceptual extreme of the range of geomechanical strategies which may be pursued in
the extraction of an orebody.
A fundamentally different geomechanical strategy is implemented in the mining
method illustrated in Figure 12.3. In this case, mining is initiated by generating
pseudo-rigid body displacements of rock above an excavation in the orebody. In this
region, the initial displacements are of the same order of magnitude as the vertical
dimension of the excavation. As extraction proceeds, the displacement field propa-
gates through the orebody, to the near and far fields. The success of this operation
relies on spatially continuous and progressive displacement of near-field and far-field
rock during ore extraction. In this caving method of mining, rock performance is
the geomechanical antithesis of that generated in the supported method described
earlier.
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