Structure of Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 15

Folds may be large structures (Fig. 2.11) or may be of a smaller local scale (Fig. 2.12).
They appear predominantly in metamorphic and sedimentary rocks.

Figure 2.12 Anticline, Devonian sand-, silt- and claystones, Meschede (Germany)

2.6 Discontinuities

The term “discontinuity” in rock mechanics is used as a collective term for all planes of
weakness along which the coherence of intact rock is interrupted. In recent literature
the term discontinuity is often replaced by the term “fracture”.
A classification of discontinuities can be based on the magnitude of shear displacement
that the surfaces of the discontinuity have suffered. Discontinuities are called “joints” if
the shear displacement is zero or too small to be visible. Faults are discontinuities on which
larger shear displacements have taken place. Another classification of discontinuities with
regard to their extent was introduced by Müller (1963). Many further attempts to classify
discontinuities, particularly joints, according to certain attributes can be found in literature.

The terms “tension joint” or “extension joint” and “shear joint” are used to qualify the
origin of joints (Stini 1929). Joints may be formed by different processes such as the crack-
ing of initially latent bedding planes of already consolidated sediments due to shrinkage
during drying (tension joints) and by tectonic processes (tension or shear joints). The
term “contraction joint” is used for joints that are a result of tensile stresses initiated in
igneous rocks by shrinkage due to the cooling of magma. A “master joint”, also known
as a “main joint” or “major joint”, is a persistent joint plane of great extent, generally
constituting the dominant jointing of an area.
The walls of a discontinuity are frequently slickensided and may be coated with min-
erals of low shear resistance, such as graphite and chlorite. Slickensides consist of thin