Environmental Engineering Reference
In-Depth Information
Precipitation
Hydraulic conductivity limitation
Precipitation = 0.0
Precipitation = k sat
Precipitation = 50% k sat
Soil hydraulic conductivity curve
Range of measurement
Figure 7.31 Field illustration of a steady-state test procedure that
can be used in the laboratory.
Soil suction (
ψ
), kPa
Figure 7.30 Design considerations when using direct measure-
ment of unsaturated coefficient of permeability.
in Fig. 7.31. As long as the applied rainfall is well below
the saturated coefficient of permeability, infiltration will take
place under a gradient close to 1.0.
Second, the measured permeability of the soil must be smaller
than the saturated permeability of the high-air-entry ceramic
disk. Consequently, the steady-state method is limited to a
narrow measurement zone, as illustrated in Fig. 7.30. The
above limitations also reveal that the permeability measure-
ments made with low applied matric suctions are likely to be
inaccurate. Therefore, it is more acceptable practice to inde-
pendently measure the saturated coefficient of permeability
of any soil for which the permeability function is required.
7.6.3 Instantaneous Profile Method
The instantaneous profile method has been described
by Richards and Weeks (1953), Hamilton et al., (1981),
Meerdink et al., (1996), Fujimaki and Inoue (2003), Cui
et al., (2008), and others. The instantaneous profile method
is an unsteady-state method that can be used either in the
laboratory or in situ. The method uses a cylindrical specimen
of soil that is subjected to a continuous water flow from one
end of the specimen. The test method has several variations.
The methodologies differ mainly in the flow process used
and in the measurement of hydraulic head gradient and flow
rate. The flow process can be a wetting process where water
flows into the specimen or a drying process where water
flows out of the specimen under gravity.
The hydraulic head gradient and flow rate at various points
along the specimen can be obtained using one of several pro-
cedures (Klute, 1972). For one of the procedures, the water
content and pore-water pressure head distributions can be
measured independently. The water content distribution can
be used to compute flow rates. The pore-water pressure head
gradient can be calculated from the measured pore-water
pressure head distribution. The gravitational head gradient
is obtained from the elevation difference.
Using another procedure, the water content distribution
is measured while the pore-water pressure head is inferred
from the SWCC. There is still another possible procedure
where the pore-water pressure head distribution is measured,
and the water content is inferred from the SWCC. Ten-
siometers and psychrometers have been used to measure the
pore-water pressure distribution along the permeameter. Of
the above procedures, the first one appears to be most satis-
factory. All variations of the instantaneous profile procedure
are based on the same theoretical principles. The hydraulic
7.6.2.5 Simplified Steady-State Laboratory Measurement
of Permeability
There is a rather simple procedure that can be used to mea-
sure the unsaturated coefficient of permeability of a soil. Let
us suppose that a permeameter is filled with dry sand and
then a constant water flux is applied at the top of the per-
meameter. Let us also start with water fluxes that are well
below the saturated coefficient of permeability of the soil. If
a tensiometer were placed near the top and bottom of the per-
meameter, it would be found that both tensiometers would
read essentially the same value under steady-state seepage
conditions. The two tensiometers would read the same value
because the infiltration is occurring under a gradient of 1.0.
The applied flux becomes the unsaturated coefficient of
permeability for the soil. The tensiometer reading is the
matric suction in the soil corresponding to the applied flux.
The above experiment can be repeated for different applied
water fluxes, and each time the applied water flux becomes
the unsaturated coefficient of permeability of the soil. This
is essentially a steady-state permeability test where there is
no need for a high-air-entry disk.
As long as the matric suction in the soil is quite small,
there is no need for a high-air-entry disk. The theory related
to this simple permeability test can be understood by con-
sidering the infiltration of water at ground surface, as shown
 
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