Engineering Geology

Chapter 2
Figure 2.14
Geometric orientation of longitudinal, cross and diagonal joints relative to fold axis and to principal axes of stress.
diagonal or oblique joints run at an angle to both the ac and bc joints. Diagonal joints are
classified as shear joints, whereas ac and bc joints are regarded as tension joints.
Joints are formed through failure of rock masses in tension, in shear or through some combination
of both. Rupture surfaces formed by extension tend to be clean and rough with little detritus.
They tend to follow minor lithological variations. Simple surfaces of shearing are generally
smooth and contain considerable detritus. They are unaffected by local lithological changes.
Price (1966) contended that the majority of joints are post-compressional structures, formed
as a result of the dissipation of residual stress after folding has occurred. Some spatially
restricted small joints associated with folds, such as radial tension joints, are probably initiated
during folding. Such dissipation of the residual stresses occurs in the immediate neighbourhood
of a joint plane so that a very large number of joints need to form in order to dissipate
the stresses throughout a large volume.
Joints also are formed in other ways. For example, joints develop within igneous rocks when
they cool down, and in wet sediments when they dry out. The most familiar of these are the
columnar joints in lava flows, sills and some dykes. The cross joints, longitudinal joints, diagonal
joints and flat-lying joints associated with large granitic intrusions have been referred to
in Chapter 1. Sheet or mural joints have a similar orientation to flat-lying joints. When they are
closely spaced and well developed, they impart a pseudostratification to the host rock. It has
been noted that the frequency of sheet jointing is related to the depth of overburden, in other
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