Civil Engineering Reference
In-Depth Information
discusses methods of collecting structural geolo-
gical data, including mapping and drilling.
causing the rock to fracture and form joints and
faults. In sedimentary rock, bedding planes that
are coincident with breaks in the continuity of
sedimentation will also form.
Figure 2.2(a) shows how the stresses in rock
increase with burial, assuming that there are no
water, thermal or tectonic pressures acting in the
rock (Davis and Reynolds, 1996). The vertical
stress, which is the major principal stress
σ
1
,is
equal to the weight of the overlying rock and is
given by
2.2 Mechanism of joint formation
All the rocks observed in outcrops and excav-
ations have undergone a long history of modi-
fication over a time of hundreds of millions, or
even billions of years. The sequence of modi-
fication that a sedimentary rock, for example,
typically undergoes is deposition at the surface,
gradual burial to depths of up to several kilo-
meters with imposition of heat and pressure, and
then uplift to the surface (Figure 2.2). Through-
out this sequence, the rock may also be subject
to deformation comprising folding and faulting.
These processes usually result in the stresses in
the rock exceeding its strength a number of times,
σ
1
=
γ
r
H
(2.1)
where
γ
r
is the unit weight of the rock and
H
is
the depth of burial. The horizontal stress, which
is the minor principal stress
σ
3
, also increases
with the depth of burial due to the effect of the
(a)
Tension
Stress in rock (MPa)
Compression
-20
0
20
40
60
80
100
120
140
1
Major principal
stress,
1
2
Depth of
burial,
H
(km)
Minor principal
stress,
3
3
4
5
(b)
(c)
Rock
strength
80
1
Figure 2.2
Development of jointing
due to burial and uplift of rock:
(a) stress changes in rock during
burial; (b) Mohr diagram showing
conditions for rock fracture;
(c) inclination of joints with respect
to stress direction (adapted from
Davis and Reynolds (1996)).
Shear
stress,
60
H
=5km
3
3
40
(MPa)
H
=2km
20
2
1
3
20
1
-
+
40
60
80
100 120
Normal stress,
(MPa)
