Geology Reference
In-Depth Information
from faults. All fractures are the consequence of overstressing the rock
or soil material. The nature of joints
-
their orientations, roughness
and persistence
is controlled by the local stress conditions that caused
them to develop, together with the strength of the rock, other condi-
tions, including temperature and water pressure, and subsequent his-
tory. Many joints occur as sets of fractures, pervasive through large
volumes of rock, and owe their origins to processes such as cooling,
burial or orogenic events (e.g. Hancock, 1985; Mandl, 2005). A set
comprises a roughly parallel series of joints. Sets that are apparently
related in terms of origin are called systematic. Joints can also be non-
systematic or random. Joints can be regarded as essentially:
-
-
primary
-
associated with the geological formation of rock
-
secondary
caused by tectonic and gravitational stress including
the result of uplift and bending or
-
-
tertiary
-
due to local geomorphological or weathering in
uences.
-
Many of these begin as proto-joints that develop with time
they begin
as general planes of weakness, which only become visible traces and,
later, mechanical discontinuities, on uplift and exposure. Rock masses
that have few or no joints (as visible traces or mechanical fractures)
include deep-seated, unweathered igneous plutons (Martin, 1994), as
illustrated in
Figure 3.26.
The water-lain sandstone in
Figure 3.27
also
has very few visible joints
ciently
over-stressed during burial for the formation of hydraulic joints, as
discussed later. Furthermore, the later uplift that must have occurred
did not involve tension, bending or relaxation to cause differential
stresses suf
-
presumably the rock was not suf
cient to induce fracturing.
Much effort is made to try to characterise joint networks in rock
masses in geotechnical engineering
orientation, spacing, persistence
and aperture, especially. Guidance is given in BS5930 (BSI, 1999),
-
