Geology Reference
In-Depth Information
Figure 3.27
Miocene sandstone
with very few
visible joint traces,
Capela da Sra da
Algarve, Portugal.
ISRM (1978) and by Priest (1993). The standard guidelines for practi-
tioners, however, treat joints largely as statistical entities rather than
geological features, and this is an area where geology has the potential
to offer great insight and time-saving in geotechnical engineering; an
opportunity that has been rather disregarded to date. This is partly
because joint origin is still a dif
cult, rather poorly understood and
highly debated subject (Pollard & Aydin, 1988).
The following discussion refers to the stress conditions that initiate
fracturing, relative to the strength of the soil or rock at the time of joint
formation. This is explained, through reference to Mohr
is stress cir-
cles, in some detail in
Chapter 5
in introducing triaxial testing, and the
reader is recommended to go through that section in order to under-
stand the following discussion. Mohr
'
s circles are also well explained
in most soil mechanics textbooks (Craig, 1992), rock mechanics text-
books (Hudson & Harrison, 1997) and structural geology textbooks
(Davis & Reynolds, 1996), which demonstrates the importance of
these concepts to different scienti
'
c disciplines.
Most joints are thought to develop as extensional fractures (in ten-
sion), parallel to a compressive major principle stress,
1
. In a cooling
igneous body, the extensional stresses might be due to contraction.
Alternatively, the tensile stress
σ
3
might be tectonic due to pulling apart
of plates, as at the Mid-Atlantic Ridge or along the East African Rift
Valley, or the result of bending and relaxation during uplift or exhu-
mation (Price, 1959; Price &Cosgrove, 1990; Rives et al., 1994). It can
σ
