Page 164 - Rock Mechanics For Underground Mining

PRE-MINING STATE OF STRESS

surrounding rock. Any loading of the system, for example by change of effective
stress in the host rock mass or imposed displacements in the medium by tectonic
activity, will generate relatively high or low stresses in the inclusion, compared with
those in the host rock mass. A relatively stiff inclusion will be subject to relatively
high states of stress, and conversely. An associated consequence of the difference in
elastic moduli of host rock and inclusion is the existence of high-stress gradients in
the host rock in the vicinity of the inclusion. In contrast, the inclusion itself will be
subject to a relatively homogeneous state of stress (Savin, 1961).
An example of the effect of an inclusion on the ambient state of stress is provided
by studies of conditions in and adjacent to dykes in the Witwatersrand Quartzite. The
high elastic modulus of dolerite, compared with that of the host quartzite, should lead
to a relatively high state of stress in the dyke, and a locally high stress gradient in the
dyke margins. These effects appear to be confirmed in practice (Gay, 1975).

5.2.5 Tectonic stress
The state of stress in a rock mass may be derived from a pervasive force field imposed
by tectonic activity. Stresses associated with this form of loading operate on a regional
scale, and may be correlated with such structural features as thrust faulting and folding
in the domain. Active tectonism need not imply that an area be seismically active, since
elements of the rock mass may respond viscoplastically to the imposed state of stress.
However, the stronger units of a tectonically stressed mass should be characterised
by the occurrence of one subhorizontal stress component significantly greater than
both the overburden stress and the other horizontal component. It is probable also that
this effect should persist at depth. The latter factor may therefore allow distinction
between near-surface effects, related to erosion, and latent tectonic activity in the
medium.

5.2.6 Fracture sets and discontinuities
The existence of fractures in a rock mass, either as sets of joints of limited continuity,
or as major, persistent features transgressing the formation, constrains the equilibrium
state of stress in the medium. Thus vertical fractures in an uplifted or elevated rock
mass, such as a ridge, can be taken to be associated with low horizontal stress com-
ponents. Sets of fractures whose orientations, conformation and surface features are
compatible with compressive failure in the rock mass, can be related to the properties
of the stress field inducing fracture development (Price, 1966). In particular, a set
of conjugate faults is taken to indicate that the direction of the major principal field
stress prior to faulting coincides with the acute bisector of the faults’ dihedral angle,
the minor principal stress axis with the obtuse bisector, and the intermediate principal
stress axis with the line of intersection of the faults (Figure 5.3). This assertion is
based on a simple analogy with the behaviour of a rock specimen in true triaxial
compression. Such an interpretation of the orientation of the field principal stresses
does not apply to the state of stress prevailing following the episode of fracture. In
fact, the process of rock mass fracture is intrinsically an energy dissipative and stress
redistributive event.
The implication of the stress redistribution during any clastic episode is that the
ambient state of stress may be determined by the need to maintain equilibrium con-
ditions on the fracture surfaces. It may bear little relation to the pre-fracture state of
stress. A further conclusion, from considerations of the properties of fractured rock,
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