Civil Engineering Reference
In-Depth Information
Almost constant rain
Frequent rain
Infrequent rain
Temperate
Tropical
Semi-desert
Water
table
Variable recharge
Almost no
recharge
Abundant
recharge
Fluctuating
water table
Water table
Figure 5.2
Relationship between water table level and precipitation (modified from Davis and de Wiest (1966)).
Granisle Copper and Island Copper in Canada)
that successfully operated below, and close to,
substantial bodies of water. However, in such
operations significant seepage may develop into
the pit, as well as instability resulting from high
water pressures.
Another important factor influencing ground
water within a slope is distribution of rock types,
and details of the structural geology such as fault
infillings, persistence of joint sets and the pres-
ence of solution cavities. These features can result
in regions of low and high hydraulic conduct-
ivity within the slope that are termed aquitards
and aquifers, respectively. These matters are dis-
cussed in more detail in Section 5.4 later in this
chapter.
Because of the important influence of water pres-
sure on slope stability, it is essential that the best
possible estimates of the likely range of pressures
should be available before a detailed stability ana-
lysis is attempted. There are a large number of
factors that control ground water flow in jointed
rock masses, and it is only possible in this topic
to highlight the general principles that may apply.
If detailed studies of ground water conditions are
required, it is advisable to obtain additional data
from such sources as Freeze and Cherry (1979)
and Cedergren (1989) on ground water flow ana-
lysis, and Dunnicliff (1993) on instrumentation.
5.3.1 Hydraulic conductivity
The basic parameter defining the flow of ground
water, and the distribution of water pressure,
in geologic media is hydraulic conductivity. This
parameter relates the flow rate of water through
the material to the pressure gradient applied
across
5.3 Hydraulic conductivity and flow nets
Where ground water effects are to be included in
slope design, there are two possible approaches
to obtaining data on distributions of the water
pressures within a rock mass:
it
(Scheidegger,
1960;
Morgenstern,
1971).
Consider a cylindrical sample of soil or rock
beneath the water table in a slope as illustrated
in Figure 5.3. The sample has a cross-sectional
area of
A
and length
l
. Water levels in boreholes
at either end of this sample are at heights
h
1
and
h
2
above a reference datum and the quantity of
water flowing through the sample in a unit of
time is
Q
. According to Darcy's law, the coef-
ficient of hydraulic conductivity
K
of this sample
(a)
Deduction of the ground water flow pattern
from consideration of the hydraulic conduct-
ivity of the rock mass and sources of ground
water.
(b)
Direct measurement of water levels in bore-
holes or wells, or of water pressure by means
of piezometers installed in boreholes.
