68 In situ stress
discontinuities at various scales. Naturally, the zone of influence of a
discontinuity will depend on its size. Through such considerations, we
again arrive at a concept similar to the representative elemental volume,
as shown in Fig. 4.12. It is not possible to provide a coherent diagram
covering all eventualities, but we show in Fig. 4.21 the type of relation that
could exist between stress state and scale.
In addition to the effects just described, we must expect that consider-
able deformation of rock masses has occurred during geological history.
This results not only in alterations to the existing far-field stress, but also
leads to the presence of residual stresses which are superimposed at
different scales. We have attempted to illustrate these combined effects in
Fig. 4.21, where the horizontal axis is domain size and the vertical axis
represents some upper value of a chosen stress component. Indicated on
the figure are stresses on the scales of a point, overcore strain gauge,
borehole, engineered structure and an intraplate block. The reader should
be aware that the curve shown in Fig. 4.21 is an envelope, not an explicit
curve. The figure clearly demonstrates that a spread in the results of stress
determination programmes must be expected, but this spread will reduce
as the size of the sampled volume increases.
4.1 0 Glossary of terms related to stress states
in rock masses
It should be clear by this stage that the determination of stress-and indeed
the description of a stress state in words-is fraught with complications.
As a result, a plethora of terms is used by many authors. Here, we provide
a glossary of terms which are consistent with our explanations and the
motivations for determining stress. The descriptive terms are defined for
their further use throughout this book and are in part adapted from Hyett
et al. (1986).
Natural stress. The stress state that exists in the rock prior to any artificial
disturbance. The stress state is the result of various events in the
geological history of the rock mass. Therefore, the natural stresses present
could be the result of the application of many earlier states of stress.
Synonyms include: virgin, primitive, field and active.
Induced stress. The natural stress state, as perturbed by engineering.
Residual stress. The stress state remaining in the rock mass, even after the
originating mechanisms have ceased to operate. The stresses can be
considered as within an isolated body that is free from external
tractions. A synonym is remanent stress.
Tectonic stress. The stress state due to the relative displacement of
lithospheric plates.
Gravitational stress. The stress state due to the weight of the super-
incumbent rock mass. A synonym is overburden stress.
Thermal stress. The stress state resulting from temperature change.
Physico-chemical stress. The stress state resulting from chemical and/or
physical changes in the rock, e.g. recrystallization, absorption of fluid.