3126
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
the validity of mellowing technique, an attempt to look for
alternative treatments was made.
2 MATERIALS AND EXPERIMENTAL PROGRAM
2.1 Test Soils and Basic Tests
Criteria for selection of the test soils is as follows: soils have to
belong to different geological formation and classification.
Based on these criteria Sherman and Childress, Texas soils were
sampled for the testing program. As per USCS classification,
Sherman soil is classified as ‘CH’ whereas Childress is
classified as ‘MH’ soil. Gypsum is the sulfate source for both
the soils. Soluble sulfate contents were determined using the
TxDOT method (Tex-145-E, Colorimetric method). A 1:20
initial soil/water dilution ratio is used in this method. Turbidity
caused by the presence of sulfate is determined using
“Colorimeter” and converted in to ppm. Soluble sulfate content
of Childress and Sherman soils are 24,000 ppm and 44,000 ppm
respectively. Based on the classifications mentioned above these
two soils are termed as “high sulfate soils”.
Hydrated lime is used as the stabilizer for the soils
understudy. Lime dosage is determined as per “Eades and
Grim” test. Test soils were treated with various percentages of
lime and pH test was conducted. The dosage at which soil pH
reaches a value of 12.4 is considered as the optimum lime
dosage. The optimum dosage of lime for both soils is 6% by dry
weight. Optimum moisture content and dry density of natural
and treated soils were obtained by conducting standard proctor
tests as per ASTM standard procedures (ASTM D-698). It was
observed that maximum dry density decreased and optimum
moisture content increased up on lime treatment. Classification
and standard Proctor test results are summarized in Table 1.
Table 1. Classification and Proctor Test Results
Atterberg
Limits
Untreated Soil
6% Lime
Treated Soil
Soil
LL PL PI
OMC
(%)
MDD,
psf
OMC
(%)
MDD,
psf
Sherman
72 30 42 27
89
28
87
Childress 71 35 36 21
103
22
96
Note: LL – Liquid Limit; PL – Plastic Limit; PI – Plasticity
Index; OMC – Optimum Moisture Content; MDD- Maximum
Dry Density
A series of chemical tests were conducted on test soils to
determine the cation-exchange capacity (CEC), specific surface
area (SSA) and total potassium (TP). Based on the
mineralogical test results, clay mineralogy of the test soils was
assessed as per the procedure outlined by Chittoori and Puppala
(2011). Clay mineralogy indicated the dominance of
Montmorillonite mineral in Sherman soil and Kaolinite mineral
in Childress soil. Both soils exhibited swell potential upon
hydration.
2.2 Engineering Tests and Mineralogical Tests
Engineering tests were performed on untreated test soils to
assess the swell, shrinkage and strength characteristics. These
tests include three dimensional (3-D) volumetric swell,
shrinkage and unconfined compressive strength (UCS) tests.
As alumina and silica constitute the chemical composition of
Ettringite and Thaumasite, measurement of alumina and silica is
essential. Alumina and silica that participate in the sulfate
reactions are called “reactive alumina and silica”. Reactive
alumina and silica measurements were conducted as part of the
mineralogical tests. All the engineering tests were conducted at
optimum moisture content (OMC) and wet of optimum
moisture content (WOMC) corresponding to 95% of maximum
dry density. Wet of optimum moisture content is 2% and 3%
higher than the optimum moisture content for Childress and
Sherman soils.
Mellowing technique has been successful in stabilizing soils
with sulfate concentration up to 7,000 ppm (Harris et al., 2004).
Applicability of mellowing for high sulfate soils has not been
studied so far. To assess the validity of mellowing technique in
high sulfate soils both the soils are treated with 6% lime and
corresponding moisture from the proctor curve. Lime treated
soils were mellowed for periods of 0 and 3 days in a moisture
controlled environment. Since lime treatment makes the soil
dry, to compensate for the moisture loss during the mellowing
process, additional 3% moisture is provided for 3 day mellowed
soils. Another reason for provision of additional moisture is to
increase the solubility of gypsum and early depletion as the
sulfate reactions occur during the mellowing period. After the
elapsed mellowing periods, soil samples were recompacted and
engineering tests were conducted. Data from the engineering
tests is compared with untreated soils to witness the sulfate
reactions occurring in high sulfate soils. Reactive alumina and
silica measurements were performed on the samples subjected
to swelling and loss of aluminates and silicates were calculated.
A brief description of the tests conducted is given below:
2.2.1 Three Dimensional Volumetric Swell (3-D Swell) Test
3-D swell tests were conducted on natural and treated soils to
determine the maximum possible volumetric swell which is a
combination of vertical and radial swell. These tests are
conducted on 4 in. (101.6 mm) diameter and 4.6 in. (116.8 mm)
height samples. The samples are prepared using a gyratory
compactor machine. Porous stones are placed at the top and
bottom of the sample and a rubber membrane is placed around
the sample. The samples are double inundated and a dial gauge
is placed on the top of the sample to record the vertical swell
with time. Vertical swell readings are collected with time until
there is no further swell for 24 hours. Radial swell of the sample
is measured after the completion of the test using a pi-tape.
Double inundation provides the worst possible scenario in field
where the soil is 100% saturated and maximum swell is
expected in this case. Researchers across United States and UK
have successfully used double inundation technique for
measuring the volumetric swell.
2.2.2 Three Dimensional Volumetric Shrinkage Test
3-D shrinkage tests were conducted as per the procedure
developed by Puppala et al., 2004 to measure the decrease in the
total volume of soil specimens due to the loss of moisture
content in field samples during a dry spell. In order to replicate
the worst possible conditions, drying from a compacted state to
completely dry state is considered in this test. Soil samples were
compacted to 2.26 in. (57 mm) diameter and 5 in. (127 mm)
height using a static compaction machine. Initial sample height
and diameter are measured at three locations and averaged.
Samples are prepared at optimum and wet of optimum moisture
contents and dried on bench top for 12 hours followed by oven
drying for 24 hours. Steps used in sample preparation and
extraction are similar to the UCS sample except the sample
sizes are different. After 24 hours, samples are removed from
the oven and sample dimensions are taken. Volumetric
shrinkage is calculated as the difference of initial and final
volume divided by the initial volume expressed in percentage.
2.2.3 Unconfined Compressive Strength (UCS) Test
Unconfined Compressive Strength (UCS) tests were conducted
as per ASTM D 2166 method. The main intention of these tests
is to determine the strength changes during the mellowing
process. Soil samples are treated with lime and allowed to
mellow for 0 and 3 days. After the mellowing period soil
samples were compacted to 2.8 in diameter (70 mm) and 5.6 in.
(140 mm) height and moist cured in a 100% relative humidity
