1110
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3 USE OF A SHRINKAGE CURVE
The entire shrinkage curve, (i.e., the plot of total volume (or
void ratio) versus gravimetric water content), from an initially
saturated soil condition to completely oven-dry conditions is of
value for the interpretation of SWCC data. As saturated clay
soil dries, a point is reached where the soil starts to desaturate.
Upon further drying, another point is reached where the soil
dries without significant further change in overall volume. The
corresponding gravimetric water content appears to be close to
residual soil suction.
The shrinkage curve can be experimentally measured from
initial high water content conditions to completely dry
conditions. A digital micrometer can be used for the
measurement of the volume at various stages of drying as
shown in Figure 1. Brass rings can be used to contain the soil
specimens (i.e., the rings have no bottom). The rings with the
soil are placed onto wax paper and dried through evaporation.
The dimensions of the soil specimens are appropriately selected
such that cracking of the soil is unlikely to occur during the
drying process. The initial dimensions selected for the shrinkage
curve specimens used in this study were a diameter of 3.7 cm
and a thickness of 1.2 cm.
The mass and volume of each soil specimen can be
measured once or twice per day. Four to six measurements of
the diameter and thickness of the specimen were made at
differing locations on the specimens. It has been observed that
as the specimen diameter begins to decrease, with the specimen
pulling away from the brass ring and the rate of evaporation
increases.
The “shrinkage curve” can be best-fit using the hyperbolic
curve proposed by Fredlund et al., (1996, 2002). The equation
has parameters with physical meaning and is of the following
form:
e
(
w
)
a
sh
c
sh
w b
sh
1
1
c
sh
(2)
where:
a
sh
= the minimum void ratio (
e
min
),
b
sh
= slope of the
line of tangency, (e.g., =
e / w
when drying from saturated
conditions),
c
sh
= curvature of the shrink-age curve,
w
=
gravimetric water content,
G
s
= specific gravity and
S
= degree
of saturation.
Once the minimum void ratio of the soil is known, it is
possible to estimate the remaining parameters required for the
designation of the shrinkage curve. The minimum void ratio the
soil can attain is defined by the variable,
a
sh
. The
b
sh
parameter
provides the remaining shape of the shrinkage curve. The
curvature of the shrinkage curve commences around the point of
desaturation is controlled by the
c
sh
parameter.
4 DEGREE OF SATURATION
The degree of saturation of the soil can be written as a function
of gravimetric water content (as a function of suction) and void
ratio (as a function of gravimetric water content).
S
(
w
)
w
(
)
G
s
e
(
w
)
(3)
The degree of saturation can be further written as a function of
gravimetric water content and the equation for the shrinkage
curve, both which are functions of soil suction.
Figure 1. Digital micrometer used for the measurement of the diameter
and thickness of shrinkage specimens.
S
(
w
)
wG
s
a
sh
c
sh
w b
sh
1
1
c
sh
(4)
The degree of saturation SWCC can also be written as a
function of soil suction and the fitting parameters for the
gravimetric water content SWCC and the shrinkage curve.
S
(
)
w
s
C
(
)
G
s
a
sh
D
ln exp(1)
a
f
n
f
m
f
(5)
where:
D
c
sh
w
s
C
(
)
b
sh
ln exp(1)
a
f
n
f
m
f
1
1
c
sh
5 RESULTS ON REGINA CLAY
The effect of volume change on the interpretation of SWCCs
was studied for Regina clay. The laboratory test results are
presented and show significance of overall volume change on
the interpretation of the SWCC.
The air-entry value, AEV, for Regina clay was determined
from the degree of saturation SWCC. The AEV remained
constant around 2500 kPa. An empirical construction procedure
involving the intersection of two straight lines on a semi-log
plot was used to determine a single number associated with the
break in curvature.
Regina clay had a liquid limit of 75%, a plastic limit of 25%
and contained 50% clay size particles. The material was
prepared as slurry and then subjected to various consolidation
pressures under one-dimensional loading. After the applied load
was removed, the soil specimens were subjected to various
applied matric suction values. High suction values were applied
through equalization in a constant relative humidity
environment. The study then assumed that the air-entry value
determined from the degree of saturation SWCC remained a
constant value. (This was confirmed by the experimental
results). The “
w Break
” on the gravimetric water content
SWCCs were then compared to the air-entry value for the soil.
The ratio of AEV to
w Break
was used as a measure of the
