Civil Engineering Reference
In-Depth Information
is defined as
head, as follows:
KA(h
1
−
h
2
)
Q
=
(5.2)
Ql
A(h
1
−
h
2
)
=
Vl
(h
1
−
h
2
)
l
K
=
(5.1)
In most rock types flow through intact rock
is negligible (defined by
K
primary
), and essen-
tially all flow occurs along the discontinuities
(defined by
K
secondary
). For example, the primary
hydraulic conductivity for intact granite and
basalt is about 10
−
10
cm/sec, while for some
coarse grained, poorly indurated sandstones the
primary hydraulic conductivity may be as high
as 10
−
4
cm/sec. The term secondary hydraulic
conductivity refers to flow in the rock mass and
encompasses flow in both the intact rock and
any discontinuities that are present. These condi-
tions result in secondary hydraulic conductivities
having a wide range of values depending on the
persistence, width and infilling characteristics of
the discontinuities. For example, granite that has
a very low primary hydraulic conductivity usually
contains tight, clean, low persistence joints, so the
secondary hydraulic conductivity is also low. In
contrast, sandstone may have some primary con-
ductivity, and the presence of persistent bedding
planes may result in high secondary conductiv-
ity in the direction parallel to the bedding. For
further discussion on flow in fractured rock see
Section 5.4.
where
V
is the discharge velocity. Substitution of
dimensions for the terms in equation (5.1) shows
that the hydraulic conductivity
K
has the same
dimensions as the discharge velocity
V
, that is
length per unit time. The units most commonly
used in ground water studies is centimeters per
second, and a number of hydraulic conductivity
conversion factors are given in Table 5.1.
Equation (5.1) can be rearranged to show
the volume of water,
Q
flowing through the
sample shown in Figure 5.3 under a specified
Table 5.1
Hydraulic
conductivity conversion table
To convert cm/s to Multiply by
10
−
2
m/s
1.00
×
10
−
2
ft/s
3.28
×
US gal/day/ft
2
10
4
2.12
×
10
6
ft/year
1.03
×
10
5
m/year
3.14
×
Borehole
Water table
Borehole
P
1
w
P
2
Area
A
h
1
w
h
2
z
2
z
1
l
Figure 5.3
Illustration of Darcy's
Law for definition of hydraulic
conductivity.
Reference
datum
