Rock Mass In-Situ Stress - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 221

Also, increased horizontal stresses existing in overconsolidated clays such as the Lau-
enburg Clay appearing in Northern Germany can be similarly explained by means of
pre-loading due to a glacial ice load and subsequent unloading as a result of melting
and erosion (Wittke & Kiehl 1995).

9.5 Residual Stresses

Residual stresses exist in a finite rock body when its interior is subjected to stresses in the
absence of applied surface and body forces. In general, residual stresses may be related
to physical or chemical processes occurring in restricted volumes of rock. For example,
non-uniform cooling of a rock mass of different lithological units with different coeffi-
cients of thermal expansion may produce residual stresses. Processes other than cooling
such as recrystallization, changes in the water content, changes in mineral aggregates
or generation of heat during metamorphism can also result in local strains and thus in
residual stresses (Voight 1966b, Jaeger & Cook 1979, Brady & Brown 2006).

9.6 Effect of Rock Mass Inhomogeneity

Rock mass in-situ stress can vary significantly within the same site. A factor causing this
variability, apart from topography, is rock mass inhomogeneity caused by faults, joints
and inclusions.
At regional scale, the in-situ stress remains unaffected by the rock mass inhomogeneity.
However, at the scale of engineering structures major faults and shear zones can con-
siderably alter the regional in-situ stress state as a result of stress redistributions and the
corresponding displacements which have taken place along such discontinuities.

The results of stress measurement can be affected by stress variations due to a rock
mass inhomogeneity of smaller scale, such as a joint.
Inclusions are extrusive features such as dykes and veins of minerals such as quartz.
If the inclusion was emplaced under pressure, the stress in the inclusion would be con-
siderably higher than in the surrounding rock which depends also on the difference of
deformation moduli of the inclusion and the host rock. As a result, high stress gradients
may exist in the host rock in the vicinity of the inclusion, while the state of stress within
the inclusion will be relatively homogeneous (Brady & Brown 2006).