Rock Mass In-Situ Stress - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 215

Figure 9.6 Vertical stress at great depth due to gravity in a rock mass with irregular topography
(Wittke 1990)

9.3 Tectonic Stresses

Tectonic stresses are the result of the movement of tectonic plates, and they appear
at a regional scale as stress fields inside the plates that are related to seismic events
such as earthquakes. Information about their direction and magnitude may be obtained
from the study of structural features such as folds, faults, shear zones or joint systems
(Sections 2.4 to 2.6). In addition, the online database “World Stress Map” (WSM) pro-
vides information on the orientation of global and regional stress fields (Heidbach et
al. 2008).
As an example, the formation of a fold may be associated with horizontal normal stress-
es. The maximum principal normal compressive stress should be directed perpendicular
to the fold's axis and the intermediate or minimum principal normal stress parallel to
it. This assumption is supported by the discontinuities associated with folding that are
typically oriented parallel (longitudinal joints), perpendicular (transverse joints) and
diagonal (diagonal joints) to the fold's axis (Fig. 2.10). Examples of discontinuities as-
sociated with folds are described, for example, in Norris (1967), Stearns (1968) and
Handin et al. (1972).
The orientation of discontinuities in some cases allows us to draw conclusions with re-
gard to the directions of the principal normal in-situ stresses, since they were caused by
tectonic stresses. As an example, vertical joints in an uplifted rock mass such as a ridge
can be regarded as associated with small horizontal stresses acting normal to the joints
(Brady & Brown 2006). The same applies for extension joints, which are usually formed
normal to the minimum principal normal stress (Griggs & Handin 1960).

Efforts have been made by geophysicists to provide reliable knowledge of regional
stress fields and to relate these to the observed tectonic structures (Illies & Greiner