Civil Engineering Reference
In-Depth Information
in the sandstone and exits the slope at the
sandstone/shale contact. This seepage line on the
valley wall would be an indication of the location
of the contact. Second, flow in the lower, confined
aquifer is recharged from a source up-dip from the
valley that develops artesian pressure in the sand-
stone. This condition could be demonstrated by
completing a piezometer in the lower sandstone,
in which the water would rise above the ground
surface to the level of the equipotential in which
it is sealed. Third, flow in the confined aquifer is
refracted at the boundary and flows upwards in
the shale to exit in the valley floor.
(a)
Isotropic
1
1
(b)
1
10
5.4.4 Anisotropic rock
In formations such as that shown in Figure 5.7, in
which the conductivity of one set or sets of discon-
tinuities is higher than another set, the rock mass
will exhibit
anisotropic
hydraulic conductivity.
For the rock shown in Figure 5.7, the vertical
hydraulic conductivity will be considerably more
than that in the horizontal direction. On the
flow net, anisotropic hydraulic conductivity is
depicted by squares formed by the flow lines and
equipotentials being elongated in the direction of
the higher hydraulic conductivity. In general, the
aspect ratio of the flow line/equipotential squares
is equal to
(K
1
/K
2
)
1
/
2
.
Examples of flow nets in isotropic and aniso-
tropic rock are shown in Figure 5.10. The sig-
nificance of these conditions to slope stability is
as follows. First, in rock with high hydraulic
conductivity in the horizontal direction, such as
horizontally bedded sandstone, the ground water
can readily drain from the slope (Figure 5.10(b)).
For these conditions there will be relatively low
water pressures on potential sliding surfaces com-
pared to the isotropic case. Second, in rock with
high hydraulic conductivity parallel to the face
such as a slope cut parallel to bedding, flow to
the face will be inhibited and high water pressures
will develop in the slope. For the slope shown
in Figure 5.10(c), the use of horizontal drains
that connect the high conductivity bedding planes
to the face would be effective in lowering water
pressures within the slope.
(c)
Figure 5.10
Flow nets in slopes with isotropic and
anisotropic hydraulic conductivity: (a) isotropic rock;
(b)
K
horizontal
=
×
K
vertical
;
(c)
K
parallel to slope
=
10
10
×
K
perpendicular to slope
(plots by W. Zawadzki).
5.4.5 Ground water in rock slopes
The discussion on ground water flow in rock
masses shows that details of the geology can have
a significant effect on water pressures and seep-
age quantities in rock slopes. In addition to the
conditions shown in Figures 5.9 and 5.10 that
relate to heterogeneous and anisotropic rock, a
variety of other possible ground water conditions
