Page 311 - Rock Mechanics For Underground Mining

MINE STABILITY AND ROCKBURSTS

10.6 Mine stability and rockbursts

In considering mine global stability, the concern is comprehensive control of rock
mass displacement throughout the mine near-field domain. Assurance of mine global
stability must be based on the principles of stability of equilibrium well known in
basic engineering mechanics. They are discussed in detail in texts by Croll and Walker
(1973) and Thompson and Hunt (1973). Essentially, the requirement is to make sure
that any small change in the equilibrium state of loading in a structure cannot pro-
voke a sudden release of energy or large change in the geometry of the structure.
In a mine structure, small perturbations might be caused by a small increase in the
mined volume, transient displacements caused by blasting, or an episodic local fail-
ure. Increasing depth of mining, resulting in increased states of stress relative to rock
strength, or the need for increased extraction ratios from near-surface orebodies, both
promote the possibility of mine global instability. Under these circumstances, ana-
lytical techniques to identify the potential for mine instability and design concepts
which will prevent the development of instability become important components of
mining rock mechanics practice.
A general procedure for determining the state of equilibrium in a system is de-
scribed by Schofield and Wroth (1968). The concepts are indicated schematically in
Figure 10.16, where a body is in equilibrium under a set of applied forces P i . Suppose
a set of small, probing loads, P j , is applied at various parts of the structure, resulting
in a set of displacements, U j . The work done by the small probing forces acting
through the incremental displacements is given by
¨ 1
W = P j U j (10.73)
2
¨
In this expression, W represents the second order variation of the total potential energy
of the system. The following states of equilibrium are identified by the algebraic value
¨
of W:
¨
(a) W > 0 stable equilibrium
¨
(b) W = 0 neutral equilibrium (10.74)
¨
(c) W < 0 unstable equilibrium

Figure 10.16 Probing the state of
equilibrium of a body under load.

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