Civil Engineering Reference
In-Depth Information
Karstic limestone
Fractured/jointed basalt
Fractured igneous and metamorphic rocks
Carbonate rocks (limestone and dolomite)
Sandstone
Rocks
Unfractured igneous and metamorphic rocks
Shale
Marine clay
Glacial till
Silt
Silty sand
Clean sand
Unconsolidated
deposits
Gravel
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
1 0
2
K
(cm/s)
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
1 0
2
10
3
10
4
10
5
K
(m/d)
Figure 5.4
Hydraulic conductivity of various geologic materials (Atkinson, 2000).
Typical ranges of secondary hydraulic con-
ductivity for a variety rock types, as well as
unconsolidated deposits, are shown in Figure 5.4.
The range of hydraulic conductivities for geolo-
gical materials covers 13 orders of magnitude,
and for any single rock type the range can be four
orders of magnitude. This shows the difficulty in
predicting water inflow quantities and pressures
within slopes.
Figure 5.3 also shows that the total head
h
at
any point can be expressed in terms of the pres-
sure
P
and the height
z
above a reference datum.
The relationship between these parameters is
5.3.2 Porosity
The total volume
V
T
of a rock or soil is made up
of the volume of solid portion
V
s
and the volume
of the voids
V
v
. The porosity,
n
, of a geologic
material is defined as the ratio:
V
v
V
T
n
=
(5.4)
In general, rocks have lower porosities than
soil. For example, the porosities of sand and clay
are in the range of 25-50% respectively. In com-
parison, fractured basalt and karstic limestone
may have porosities in the range of 5-50%, while
the porosity of dense crystalline rock is usually in
the range of 0-5%.
The significance of the porosity to rock slopes
is in the design of drainage systems. For example,
drains installed in a low porosity granite need
only discharge a small amount of water to lower
the water pressures, while drains in karstic lime-
stone may discharge large volumes of water with
little effect on the ground water table.
P
γ
w
+
=
h
z
(5.3)
where
γ
w
is the density of water. The total head
h
represents the level to which water will rise in a
borehole standpipe.
Darcy's law is applicable to porous media and
so can be used to study ground water flow in both
intact rock, and rock masses on a macroscopic
scale. However, it is required that the flow be lam-
inar, so Darcy's law is not applicable in the event
of non-linear or turbulent flow in an individual
fracture.
5.3.3 Flow nets
The graphical representation of ground water
flow in a rock or soil mass is known as a flow net
