Geology Reference
In-Depth Information
and variable hydraulic conductivity (controlling effective stress).
Pre-existing bedding, schistosity and faults, will have a controlling
in
uence on the way joints develop (e.g. Rawnsley et al., 1992).
Where the rock mass has a long and complex geological history,
there may be several generations of fracturing, each in
uenced by the
former condition (see Rawnsley et al., 1990). Deciphering that history
is made more dif
cult once it is appreciated that all fractures that we
now see as obvious visible mechanical discontinuities at the Earth
s
surface, may have only been incipient or integral proto-joints at the
time of later joint formation and therefore might have had little in
'
u-
ence on the formation of the later joints (Hencher & Knipe, 2007).
The proto-joint network provides relatively easy directions for
breaking otherwise massive rock, such as the rift and grain directions
in granite quarrying (Fujii et al., 2007) or as preferential directions for
breakage in laboratory testing (Douglas &Voight, 1969). Proto-joints
develop as persistent mechanical fractures later, following the pre-
imposed geological blueprint (location, orientation and spacing),
through weathering processes and/or stress changes. The development
of each joint is progressive as microfractures merge and extend over
geological time (Hencher, 1987; Selby, 1993; Rogers & Engelder,
2004; Hencher, 2006; Hencher & Knipe, 2007). At any particular
moment, a joint may be made up of open sections, sections where a
trace is visible but where there is still considerable tensile strength and
sections where the rock is apparently intact (rock bridges). Only
microfractures mark the line of the future development of a mechanical
fracture. That this is so, is evident from the obvious tensile strength of
many rock joints, even though they are clearly visible as traces
(Figures 3.30
and
3.31)
.
Figure 3.30 Partly
developed joints
through granite,
north of Seoul,
South Korea.
