Environmental Engineering Reference
In-Depth Information
0.50
10
−
4
0.40
10
−
6
q
1
10
−
8
0.30
0.20
a
f
= 10,
n
f
= 1,
m
f
= 1
a
f
= 10,
n
f
= 2,
m
f
= 1
a
f
= 10,
n
f
= 3,
m
f
= 1
10
−
10
0.10
10
−
12
0.00
0.01
0.1
10
Soil suction, kPa
1
100
1000
(
u
a
−
u
w
)
1
10
−
14
0.01
0.1
1
10
100
1000
10,000
Soil suction, kPa
10
−
4
Figure 8.82
Determination of matric suction values at horizontal
ground surface for soil subjected to steady-state rainfall flux
q
1
(after Kasim, et al., 1998).
10
−
6
10
−
8
a
f
= 10,
n
f
= 1,
m
f
= 1
a
f
= 10,
n
f
= 2,
m
f
= 1
a
f
= 10,
n
f
= 3,
m
f
= 1
Increasing air-entry value
or decreasing
q
/
k
sat
rat
i
o
10
−
10
Ground
surface
10
−
12
0.01
0.1
1
10
100
1000
Soil suction, kPa
Infinite depth
(b)
Figure 8.84
SWCCs and permeability functions with varying
n
values of soils used in parametric study (after Zhang et al., 2004).
Ground surface
Hydrostatic condition
Groundwater table
distinct. The matric suction decreases at shallow depths into
the slope but remains essentially constant at greater depths.
It takes about 1 day for the wetting front to move to a depth
of 1 m. After 3 days the infiltration depth of the wetting front
is about 4 m. The negative pore-water pressure essentially
disappears after 6 days of rainfall.
The pore-water pressure profiles in the soil when
a
f
=
10 kPa resembles the profiles of the soil with
a
f
=
5 kPa. However, the negative pore-water pressures almost
disappear after 4 days of rainfall infiltration. The transition
between the infiltration zone and the underlying zone
becomes less distinct for
a
f
>
10 kPa. The remaining
matric suctions in the soil decrease at a more rapid rate
when the
a
f
value is increased.
The time required to eliminate matric suction varies sig-
nificantly with the air-entry value of the soil. Figure 8.86
shows the matric suction profiles after 24 h for soils with
differing air-entry values. After 24 h of rainfall, soils with a
low air-entry value (i.e.,
a
f
<
5 kPa) have an average infil-
tration depth in the order of a few millimeters. However,
soils with a greater air-entry value (e.g.,
a
f
>
10 kPa) have
an infiltration depth of wetting that can extend a few meters.
When the
a
f
value is even greater (i.e.,
a
f
>
100 kPa), the
soil can be fully saturated after 24 h of rainfall when a
flux rate equal to the saturated coefficient of permeability is
applied.
Figure 8.83
Cutoff matric suction profiles in unsaturated soil with
horizontal ground surface (after Kasim et al., 1998).
8.4.4 Effect of Air-Entry Value on Wetting Front
during Transient Modeling
Figures 8.85a-8.85f show the pore-water pressure profiles
under transient condition with various
a
f
values for the
Fredlund and Xing (1994) SWCC. The rainfall flux is set
equal to the saturated coefficient of permeability, which
means that the ground surface is subjected to the maximum
moisture flux it can accept. The results illustrate that differ-
ent infiltration patterns than those calculated for steady-state
seepage are generated for the pore-water pressure profile.
When the
a
f
value is equal to 1 kPa, the wetting front
is sharp and distinct. The matric suction near the ground
surface decreases with time, but the rate of downward move-
ment of the wetting front is small. The depth of the wetting
front is only about 1 m below the ground surface after 50
days of rainfall with a flux equal to the saturated coefficient
of permeability.
When
a
f
5 kPa, the transition zone between the infil-
tration zone and the unaffected zone is still quite sharp and
=
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