Theory of Water Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 359

head gradient and the flow rate are determined concurrently
and instantaneously at various elapsed times, after the flow
of water has commenced.

using a hypodermic needle at the right-end plate. Filter paper
is also placed across the right-end surface of the permeame-
ter. Several ports are provided along the permeameter wall
for the installation of tensiometers or psychrometers.
Tensiometers are used for a relatively moist soil having
matric suctions less than approximately 90 kPa. Thermo-
couple psychrometers can be used for measuring suctions
ranging from approximately 100 to 8000 kPa. The tensiome-
ters or psychrometers are inserted through the ports in the
permeameter and extended into small holes drilled into the
soil specimen. The entire apparatus should be placed in a
temperature-controlled chamber with a high relative humid-
ity when psychrometers are used.
The test commences with an unsaturated specimen and
proceeds toward a saturated condition. The initial suction is
first measured under equilibrium conditions. The equilibrium
condition is then altered by slowly injecting water into the
specimen using the hypodermic needle. The water inflow rate
should be selected such that the pore-water pressure is always
negative across the length of the specimen. Flow rates in the
range of 0 . 2-5 cm 3 / day are commonly satisfactory (Daniel,
1983). Soil suctions are measured at various time intervals
(e.g., every 24 h). Psychrometers can replace the tensiometers
when soil suction increases above 90 kPa.
The test is terminated when the pore-water pressure at
the entrance of the permeameter (i.e., the left end) becomes
positive. Positive pore-water pressures may produce flow
along the inside wall of the permeameter or around the
tensiometers. Upon completion of the test, the soil specimen
is divided into several sections for final water content
measurements. The final volumetric water contents along
the specimen are plotted against the corresponding suctions
to produce a SWCC. The entire test may take as long as 2-3
weeks. Results from independent measurements of the water
content versus soil suction relationship (i.e., the wetting
curve) can also be used in the determination of the SWCC.

7.6.3.1 Laboratory Instantaneous Profile Method
The apparatus and procedure for the instantaneous profile
method proposed by Hamilton et al. (1981) is shown in
Fig. 7.32. A water flux is controlled at one end of the soil
specimen while the other end is vented to the atmosphere.
Water flows in the horizontal direction as a result of a
gradient in the pore-water pressure head. Gravitational head
gradient effects are thereby negligible. Hamilton et al. (1981)
elected to measure the pore-water pressure head distribution
during unsteady-state water flow and obtained water contents
from the soil-water characteristic curve. The hydraulic head
gradient and the flow rate vary with time during the test.
The pore-water pressures, and therefore the hydraulic head
gradients, are measured at several points along the soil
specimen.
The change in water content is related to a change in neg-
ative pore-water pressure (or matric suction) through use of
the SWCC. Flow rates are then computed from the change in
volumetric water content. The ratio between the flow rate and
the hydraulic head gradient gives the coefficient of permeabil-
ity. Measurements at different locations along the specimen
at different times during the unsteady-flow process produce
a series of coefficients of permeability measurements. Each
measured coefficient of permeability corresponds to a partic-
ular matric suction (or water content). The method does not
require an assumption of uniform hydraulic properties in the
soil, as is the case in the steady-state method.
The following procedure is used to illustrate a permeabil-
ity test performed using the wetting process. A compacted
or undisturbed soil specimen is inserted into a cylindrical
permeameter (Fig. 7.32). Both ends of the permeameter are
covered by end plates with O-ring seals. Water is supplied to
the left-end plate using a hypodermic needle and is distributed
across the soil surface through the use of several sheets of fil-
ter paper. Air in the specimen is vented to the atmosphere

7.6.3.2 Computations for Instantaneous Profile Method
The coefficient-of-permeability calculations are performed
by plotting the pore-water pressure head versus the volumet-
ric water content from various points along the specimen, as
illustrated in Figs. 7.33a and 7.33b, respectively. The pore-
water pressures are obtained from either the tensiometer or
psychrometer readings. The total suction profile obtained
from psychrometers can be taken as the negative pore-water
pressure profile when the osmotic suction gradient is neg-
ligible and the air pressure is atmospheric. The pore-water
pressure can be divided by the unit weight of water (i.e.,
ρ w g ) to give the pressure head. The hydraulic head gradi-
ent (i.e., pore-water pressure head gradient) at a point in
the specimen for a specific time is equal to the slope of the
hydraulic head profile:

Figure 7.32 Apparatus for measuring the coefficient of water per-
meability using the instantaneous profile method (after Hamilton
et al., 1981).

dh w
dx

i w = (7.69)