Environmental Engineering Reference
In-Depth Information
40
Soil-water characteristic curve
30
Desorption curve
20
Adsorption curve
10
Selected suction
0
10
6
0.1
1
10
100
1000
10,000
100,000
Soil suction, kPa
10
−
5
Permeability function
10
−
6
10
−
7
Desorption permeability
10
−
8
10
−
9
Desorption
permeability function
Adsorption permeability
10
−
10
Adsorption
permeability function
10
−
11
10
6
0.1
1
10
100
1000
10,000 100,000
Soil suction, kPa
Figure 6.11
Effect of hysteresis upon drying and wetting of a soil.
ground surface boundary as precipitation or it is leaving the
ground surface through actual evaporation AE or transpira-
tion
T
. Water may also be shed from the ground surface
through runoff
R
. The components of moisture flux at the
ground surface and the net infiltration component,
I
, can be
written in the following form:
The calculations associated with determining AE have
proven to be the most challenging. PE occurs when there
is an ample supply of water at the ground surface. The AE
can be thought of as evaporation from a ground surface
where the soil is resisting evaporation because of soil suc-
tion or the affinity of the soil for water. The AE requires that
the effect of suction in the soil be taken into consideration
as part of the computational procedure. It is important that
the physics associated with the determination of PE be fully
understood prior to attempting to calculate AE.
Each of the components of net infiltration must be
assessed as part of the determination of moisture entering
the soil at ground surface. The components of net moisture
flux are discussed along with a brief description of the cal-
culations and main assumptions required when performing
the calculations.
The assessment of most factors influencing net infiltra-
tion is made using approximate soil properties and average
daily moisture fluxes. The average effects may not provide a
complete and accurate picture of long-term performance of
a cover system. Extreme events or high-intensity storms can
have a significant influence and may cause serious damage
to cover systems.
Net infiltration
(
I
)
=
precipitation
(
P
)
−
actual evaporation
(
AE
)
(6.6)
−
transpiration
(
T
)
−
runoff
(
R
)
In abbreviated form, the net infiltration at ground surface
can be written as
=
−
−
−
I
P
AE
T
R
(6.7)
There has not been a long history of calculating ground
surface moisture flux conditions in geotechnical engineer-
ing. There are complexities associated with the analysis and
numerous assumptions must be made as part of the compu-
tational procedures.
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