Engineering Rock Mechanics

The nature of in situ rock stress 41

Compressive stress Tensile stress

in the surface lavers in the core

Section through car windscreen Section through rock mass

Figure 4.1 The concept of locked-in stresses, in a car windscreen and in a rock mass.

(b) Current tectonicforces. Due to the movement of tectonic plates, the
earth's crust is subject to tectonic forces, as it has been during geological
history. The rock mass may be strong enough to sustain these forces
and the associated stresses. However, if the stresses are high enough,
the rock will fracture and crumple causing the joints, faults (generically
termed fractures) and folds, which are formed during high stress and
orogenic events. In many regions of the world, the tectonic plates are in
a state of limiting equilibrium and there are high horizontal stresses in
the rock mass. In fact, it is the rule rather than the exception that the
maximum principal stress in a rock mass will be acting sub-horizontally,
not vertically.
(c) Previous tectonicforces. There may also be residual stresses from
earlier tectonic events: when a fractured rock mass is compressed and
then unloaded, stresses can be left locked in the rock mass. An analogue
example of this effect is a pre-stressed car windscreen.
The section through the windscreen in Fig. 4.1 indicates how a layer
on each surface in a compressive stress condition is balanced by the
tensile stress in the central region. The purpose of this pre-stressing
is to increase the windscreen's resistance to breakage by reducing the
tensile bending stresses that can develop on either side during an
impact. Similarly, and as a result of loading and unloading, the fractured
rock mass may well contain significant residual stress. The subject
of residual stress is a controversial one, but it is possible that many
high horizontal stress components observed in rock masses are residual
stresses.
All three of the main causes described above can contribute to the
stress state at a point in a rock mass. In addition, other factors can alter
the stress state, such as erosion which will reduce the vertical stress
component more than the horizontal components. Fractures at all scales
will perturb the stress field.
To understand how a fracture pattern perturbs the stress field, as-
sume that in a horizontal section through a rock mass, as shown in
Fig. 4.2, the stress state 'A' indicates the two horizontal components
of the pervasive stress state in the rock. Nearer the fracture, states 'B'
and 'C', the principal stress directions are rotated and the magnitudes
of the principal stresses change. In the case of an open fracture, no
normal or shear stress can be sustained perpendicular and parallel to
the fracture surface, so the fracture surface becomes a principal stress
plane with a principal stress value of zero (cf. A3.5). Imagining this