1111
Technical Committee 106 /
Comité technique 106
effect of volume change on the interpretation of the correct air-
entry value for the soil.
Shrinkage curves and soil-water characteristic curves were
measured on Regina clay. Slurry Regina clay was prepared at a
gravimetric water content slightly above its liquid limit. The
shrinkage curve results are presented in Figure 2. The void ratio
of Regina clay decreases as water evaporates from the soil
surface. The clay begins to desaturate near its plastic limit. The
best-fit parameters for the shrinkage curve are
a
sh
= 0.48,
b
sh
=
0.17, and
c
sh
= 3.30. The specific gravity of the soil was 2.73.
Figure 3 shows the gravimetric water content,
w
, plotted versus
soil suction for Regina clay was preloaded at 196 kPa. Its initial
water content was 53.5%. The high water content specimen
showed that a gradual break or change in curvature around 50
kPa. The curvature is not distinct and does not represent the true
air-entry value of the material. The gravimetric water content
SWCC was best-fit with the FX (1994) equation and yielded the
following parameters; that is,
a
f
= 140 kPa,
n
f
= 0.87, and
m
f
=
0.72. Residual suction was estimated to be around 200,000 kPa.
It is necessary to use the shrinkage curve to calculate other
volume-mass soil properties and properly interpret the SWCC
results for the true AEV.
Figure 2. Shrinkage curve for several samples of Regina clay.
The best-fit shrinkage curve equation can be combined with
the equation for the FX (1994) equation for the SWCC. The
resulting plot of degree of saturation,
S
, versus soil suction is
shown in Figure 4. The results show that there is a distinct air-
entry value for Regina clay is about 2,500 kPa. The true air-
entry value was also found to be similar for all preconsolidated
Regina clay samples. The degree of saturation SWCCs must be
used to estimate the AEV of the soil and subsequently the
calculation of the unsaturated hydraulic conductivity function.
The degree of saturation also indicates that residual condition
can be more clearly identified as being at a suction of about
200,000 kPa (i.e., residual degree of saturation of 20 percent).
Several other SWCC tests were performed on the Regina
clay; each test starting with soil that had been preconsolidated
from slurry to differing applied pressures. Figure 5 shows the
gravimetric water content versus soil suction plot for a soil
preconsolidated to 6.125 kPa. The FX (1994) fitting parameters
are
a
f
= 18.0 kPa,
n
f
= 0.88,
m
f
= 0.76 and
h
r
= 800 kPa.
Figure 6 shows the gravimetric water content versus soil
suction plot for a soil preconsolidated to 49.0 kPa. The FX
(1994) fitting parameters are
a
f
= 90.0 kPa,
n
f
= 1.10,
m
f
= 0.70
and
h
r
= 2000 kPa. Figure 7 shows the gravimetric water
content versus soil suction plot for Regina clay preconsolidated
to the highest pressure of 392 kPa. The FX (1994) fitting
parameters are
a
f
= 120.0 kPa,
n
f
= 0.84,
m
f
= 0.70 and
h
r
=
2000 kPa.
Figure 3. Gravimetric water content versus soil suction for Regina clay
preconsolidated to 196 kPa.
Figure 4. Degree of saturation versus soil suction for Regina clay
preconsolidated to 196 kPa.
The measured SWCCs for Regina clay show that the
measurement of the gravimetric water content SWCC and the
shrinkage curve for a soil are all that is required to obtain an
approximation of the volume-mass versus soil suction
relationships when the applied net normal stress is zero.
6 INTERPRETATION OF THE REGINA CLAY RESULTS
The difference between the break in the gravimetric water
content SWCC and the true AEV for Regina clay is expressed
as [AEV/(Break in curvature on
w
SWCC)]. The volume change
of the soil is once again expressed as the change in void ratio,
e
, divided by (1 +
e
) and all void ratio values are determined
from the shrinkage curve.
The horizontal axis of Figure 8 shows that the Regina clay
soil specimens changed in volume by 65% to 150% as soil
suction was increased to residual suction conditions. At 70%
volume change, the true AEV is 60 times larger than the break
in curvature indicated by the gravimetric water content SWCC.
Also at 120% volume change, the true AEV is 129 times larger
than the break in curvature indicated by the gravimetric water
content SWCC. The laboratory test results clearly indicate the
significant influence that volume change as soil suction
increases has on the interpretation of the data.
