42 In situ rock stress.
Figure 4.2 Perturbation of in situ rock stress state caused by a fracture.effect adjacent to many fractures at all scales in a rock mass leads to the
expectation that local values of in situ stress, and associated site investig-
ation values, are likely to be variable. Also, the activities of the engineer
when excavating the rock mass will perturb the stress state adjacent to
excavations.4.2 Questions and answers: in situ rock stress
44.1 There is no internationally agreed terminology for words de-
scribing the state of stress in a rock mass. However, describe in one
sentence what you think is meant by each of the following terms.
Natural stress Thermal stress
Induced stress Pa laeostress
Gravitational stress Near-field stress
Tectonic stress Far-field stress
Residual stress local stressA4.1
Natural stress: the in situ stress which exists prior to engineering.
Induced stress: the natural stress state as perturbed by engineering2.
Gravitational stress: the stress state caused by the weight of the rock
above.
Tectonic stress: the stress state caused by tectonic plate movement.
Residual stress: the stress state caused by previous tectonic activity.
Themzal stress: the stress state caused by temperature change.
Palaeostress: a previous natural stress that is no longer acting.
Near-fieldstress: the stress state in the region of an engineering per-
turbation.
Far-feld stress: the stress state beyond the near-field.
Local stress: the stress state in a region of interest.
Some writers use 'induced stress' to mean the actual stress after engineering; others
use the term to mean the stress changes caused by engineering. It is important to be sure
which definition is being used in any particular context.