200
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The geotechnical index properties were determined
according to the ASTM test methods as follows: (i) water
content (
w
) by the Standard Test Methods for Laboratory
Determination of Water (Moisture) Content of Soil and Rock by
Mass (D2216-05); (ii) dry unit weight (
γ
d
) by the Standard Test
Method for Density of Soil in Place by the Drive-Cylinder
Method (D2937-10) for the in situ sample and by the above-
mentioned method for the compacted sample; (iii) specific
gravity (
G
s
) by the Standard Test Methods for Specific Gravity
of Soil Solids by Water Pycnometer (D854-10); (iv) liquid limit
(
w
l
), plastic limit (
w
p
) and plasticity index (
I
p
) by the Standard
Test Methods for Liquid Limit, Plastic Limit, and Plasticity
Index of Soils (D4318-10); and (v) grain size distribution
(GSD) by the Standard Test Method for Particle-Size Analysis
of Soils (D422-63(2007)). The entire GSD data is not given in
this paper.
The SWCC was determined according to the ASTM
Standard Test Methods for Determination of the Soil Water
Characteristic Curve for Desorption Using a Hanging Column,
Pressure Extractor, Chilled Mirror Hygrometer, and/or
Centrifuge (D6836-02(2008)e2) on 10 mm thick samples
obtained from both the undisturbed core and the compacted
sample. Predetermined values of matric suction were applied
using pressure plate and pressure membrane extractors
manufactured by Soil Moisture Equipment Inc. These
equipment included the following: (i) a 5 bar pressure plate
extractor (Model 1600) for up to 200 kPa suction; (ii) a 15 bar
pressure plate extractor (Model 1500F1) for suction values
ranging from 300 kPa to 500 kPa; and (iii) a 100 bar pressure
membrane extractor (Model 1020) for suction values between
2000 kPa and 7000 kPa. The porous plates and the cellulose
membranes were submerged in distilled and de-aired water for
24 hours to expel air bubbles. Thereafter, the specimens along
with the retaining ring were placed on their respective porous
plate or cellulose membrane and allowed to saturate. Next, the
excess water was removed and each plate or membrane was
placed in the designated extractor. For each suction value, the
expelled water from the samples was monitored in a graduated
burette. When two consecutive readings nearly matched over a
24 hour period, the test was terminated and the sample water
content was determined.
The dew point potentiameter (WP4-T) was used for suction
measurement at low water content corresponding to total
suction values greater than 7000 kPa. The sampling cup was
half filled with soil to ensure accurate suction measurement
(Leong et al. 2003) by using about 5 mg of material with a
known water quantity. The unsaturated sample was forwarded
to the head space of the sealed measurement chamber, set at
25°C temperature, through a sample drawer and was allowed to
equilibrate with the surrounding air. Equilibration was usually
achieved within 10 min to 20 min, as detected by condensation
on a mirror and measured by a photoelectric cell. From
knowledge of the universal gas constant,
R
(8.3145 J/mol°K),
sample temperature,
T
(°K), water molecular mass,
X
(18.01
kg/kmol), and the chamber relative humidity,
p
/
p
o
, soil suction
was calculated (
ψ
=
RT
/
X
ln (
p
/
p
o
)) and displayed on the
potentiameter screen. The water content of the soil was
measured as described earlier.
The shrinkage curve was determined in accordance with the
ASTM Standard Test Method for Shrinkage Factors of Soils by
the Wax Method (D4943-08). To obtain the void ratio, the
volume of soil specimens was determined using the water
displacement method. Each specimen was coated with molten
microcrystalline wax (
G
s
= 0.9) and allowed to cool down at
room temperature. After wax solidification, the sample was
submerged in a 250 mL graduated cylinder that was filled with
distilled water. The water height in the cylinder was carefully
recorded using a Vernier caliper before and after sample
submersion in the cylinder. A graduated syringe was used to
remove the increased amount of water displaced by the sample
thereby bringing the water height back to the initial reading.
The displaced water mass was determined by weighing the
graduated syringe before and after water filling and recording
the difference. This quantity was readily converted to water
volume representing the volume of the wax-coated soil. The
volume of soil was obtained from the difference of volume of
the wax coated sample and the volume of wax (mass/0.9). A
7.4% correction was applied to account for the underestimation
due to air entrapment at the soil-wax interface, as suggested by
Prakash et al. (2008). The sample mass was also determined to
estimate the bulk unit weight of the soil that, in turn, was
converted to the void ratio using basic phase relationships.
3 RESULTS AND DISCUSSION
Table 1 summarizes the geotechnical index properties of the
investigated soil. The water content and the dry unit weight of
the in situ sample were found to be 31% and 1.34 g/cm
3
,
respectively. In early Fall when the sample was collected, the
soil generally experiences a net water deficit given the semi-arid
climate prevalent in the region. This was evident from the
unsaturated (
S
= 82%) state of the sample in the field: the field
void ratio was calculated to be 1.05. Similar initial conditions
(
w
= 38% and
d
= 1.29 g/cm3) were chosen for the compacted
sample to obtain comparable data: the corresponding saturation
and void ratio were found to be 86% and 1.18, respectively. The
high liquid limit and plastic limit indicate the high water
adsorption capability of the clay. These values are attributed to
the presence of expansive clay minerals such as smectite,
hydrous mica, and chlorite (Ito and Azam 2009). Likewise, the
clay size fraction (material finer than 0.002 mm) was found to
be around 65%. The fine grained nature of the soil suggests a
high water retention capacity. The calculated soil activity (
A
=
Ip
/
C
) of about 0.8 is associated with moderate swelling.
Overall, the soil was classified as CH (clay with high plasticity)
according to the Unified Soil Classification System (USCS).
Table 1. Summary of geotechnical index properties
Property
In situ
Compacted
Water Content,
w
(%)
31
38
Dry Unit Weight,
d
(g/cm
3
)
1.34
1.29
Specific Gravity,
G
s
2.75
2.74
Void Ratio,
e
*
1.05
1.18
Degree of Saturation,
S
(%)†
81
86
Liquid Limit,
w
l
(%)
83
77
Plastic Limit,
w
p
(%)
30
27
Plasticity Index,
I
p
(%)
53
50
Clay Size Fraction,
C
(%)
66
64
USCS Symbol
CH
CH
*
e
= (
G
s
w
/
d
) - 1
†
S
=
w G
s
/
e
Figures 1 shows SWCC with gravimetric water content. The
samples were put in a water tub for one week and the water
content measured 38% for the in situ sample and 46% for the
compacted sample. Irrespective of the initial water content, the
SWCC data fitted well to bimodal distributions with two air
entry values: a lower value (10 kPa) corresponding to drainage
through fissures followed by a higher value (300 kPa and 100
kPa for the two samples, respectively) associated with seepage
through the soil matrix. When the samples were gradually
desaturated, air first entered into the fissures at low suction.
