Civil Engineering Reference
In-Depth Information
10
1.0
10
-1
10
-2
10
-3
10
-4
10
-5
e
b
10
-6
10
-7
Figure 5.8
Influence of joint aperture
e
and spacing
b
on hydraulic conductivity
K
in the direction of a set of smooth
parallel joints in a rock mass.
10
-8
0.01
0.05
0.1
0.5
1.0
Joint aperture,
e
(mm)
the coefficient of kinematic viscosity (1.01
×
the construction of the ship locks at the Three
Gorges Project in China, which involved mak-
ing parallel excavations with depths up to 170 m
in strong, jointed granite (Zhang
et al.
, 1999).
The excavations caused relaxation of the rock
in the walls of the locks and the opening of the
joints, which resulted in the hydraulic conductiv-
ity increasing by a factor of 18. The application
of a support pressure of 2 MPa on the vertical
walls of the excavation resulted in the hydraulic
conductivity only increasing by a factor of 6 from
the
in situ
condition.
10
−
6
m
2
/s for pure water at 20
◦
C).
The equivalent conductivity of a parallel array
of discontinuities in relation to aperture and spa-
cing is shown in Figure 5.8. Since the hydraulic
conductivity is proportional to the third power
of the aperture, small changes in the aperture
due, for example, to increasing stress in the rock
will significantly decrease the conductivity. This
condition could develop at the toe of a steep
slope where high stresses decrease the aperture
and result in a build up of water pressure in the
slope.
Figures 5.4 and 5.8 demonstrate the applica-
tion of equation (5.5) to the actual hydraulic
conductivities of rock masses. For example, the
conductivity of sandstone is about 10
−
6
cm/s,
while that of fractured and jointed basalt is about
10
−
2
cm/s. This difference in conductivity of four
orders of magnitude can be attributed to the joint
spacing decreasing from 1 to 0.1 m, and the aper-
ture increasing by a small amount from 0.02
to 0.2 mm.
The relationship between discontinuity aper-
ture and hydraulic conductivity was studied for
5.4.2 Flow in filled discontinuities
Equation (5.5) applies only to laminar flow in
planar, smooth, parallel discontinuities and rep-
resents the highest equivalent hydraulic conduct-
ivity for fracture systems. The lowest equivalent
hydraulic conductivity occurs for infilled discon-
tinuities, and is given by
eK
f
b
K
=
+
K
r
(5.6)
