7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 1/13
1. INTRODUCTION
Volumetric changes (usually swell) of expansive soils in
presence of water are undesirable from stability reasons.
The consequence of swell leads to an increase in volume
till suction pressure comes to equilibrium as determined by
the environment. The amount of swell to satisfy the suctionpressure depends on the magnitude of the vertical loading
and soil properties that include soil composition, natural
water content and density, and soil structure. The rate of
swell depends on the coefficient of permeability (hydraulic
conductivity), thickness, and soil properties. If a structure
is founded on such expansive soils, then its presence along
with the foundation prevents this volume increase (swell)
and as a consequence, leads to swelling pressure. This swell-
ing pressure has serious consequences in the form of cracks
and distress on the structures founded on expansive soils.
Lightweight structures are severely affected due to high
swelling pressure exerted by these soils. These aspects ofswelling and their consequences on building have been well
documented in the literature (e.g., Gibbs and Holtz, 1956;
Seed et al., 1962; Brackley, 1973; Chen, 1975; Katti et al.,
1975; Sridharan et al., 1986).
The first and essential step before any construction activ-
ity on expansive soils is to assess the degree of expansiveness
and the likely swelling pressure on the structure, so, as to
adopt preventive measure to overcome the same. Hence, thestudy of swelling behavior and the associated parameters of
expansive soils assume importance. To assess the degree of
swell, many procedures, both simple and elaborate includ-
ing laboratory methods of determining swell pressure have
been developed by geotechnical researchers and engineers
(e.g., Gibbs and Holtz, 1956; Ladd, 1960; Seed et al., 1962;
Komornik and David, 1969; Ranganatham and Satyanarayan
1969; Brackley, 1973; Chen, 1975; Sridharan et al., 1986).
A number of factors influencing swelling behavior have
been reported in the past (Ranganatham and Satyanarayan
1969; Komorik and David, 1969; EI-Sohby and EI-Sayed,
1981, 1983; AL-Mhaidib, 1999; Azam and Abduljauwad2000). Among the identified factors that influence the swell-
ing behavior, type and amount of clay, initial placement
conditions, stress history and stress path, nature of pore
fluid, size and thickness of the sample are found to be more
important.
The swelling pressure of an expansive soil is primarily
dependent on the initial dry unit weight or void ratio and
also on the moisture content. The methods to determine
swelling pressure influences the ultimate value of the same.
H. B. Nagaraj,1* and M. Mohammed Munnas2 and A. Sridharan3
Swelling behavior of expansive soils
ABSTRACT: Volumetric changes (usually swell) of expansive soils in presence of water are undesirable from stability reasons
Swell and Swelling pressure of an expansive soil is primarily dependent on the initial dry unit weight or void ratio and also
on the moisture content. In this study, attempts have been made to study the effect of introducing varying number of vertical
drains into the compacted soil at varying initial dry densities on the swell and swelling pressure behavior. Both percent swell
and the swelling pressure are significantly influenced by the presence of vertical drains (facility of water availability). The
percent swell versus time relationship could be identified as a rectangular hyperbola, facilitating the prediction of ultimate
percent swell. This is advantageous to predict the swell from the data obtained from the initial stages of the swell test and the
experiment could be terminated without continuing till equilibrium conditions are reached. This concept was also extended
to predict swelling pressure determined by constant volume method. Irrespective of the presence or absence of drains, the
density effect on rate of secondary swelling and consolidation behavior of swollen sample was found to be almost the same for
all testing conditions used in this study.
KEYWORDS: Clays, compaction, expansive soils, swell, swelling pressure, vertical drains
*Corresponding Author
1Assistant Professor, Department of Civil Engineering, BMS College of
Engineering, Bangalore 560 019, India. email: [email protected]
2Post Graduate student, Department of Civil Engineering, BMS College of
Engineering, Bangalore 560 019, India. email: [email protected]
3Professor Emeritus, Department of Civil Engineering, Indian Institute of
Science, Bangalore 560 012, India. email: [email protected]
99
International Journal of Geotechnical Engineering (2010) 4: (99-111)
DOI 10.3328/IJGE.2010.04.11.99-111
J. Ross Publishing, Inc. © 2010
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 2/13
In this study, attempts have been made to study the effect
of introducing vertical drains into the compacted soil on the
swell and swelling pressure behavior. The effect of varying
initial dry density without and with varying number of verti-
cal sand drains on time versus swell and also on time versus
swelling pressure determined by constant volume method is
also presented. The results of experiments that compare thetime required to attain the initial swell and primary swell
and also the rate of secondary swelling obtained without
and with introduction of vertical drains is presented in this
paper. The study also includes the comparison of predicted
ultimate swell and swelling pressure determined by constant
volume method to the actual value obtained experimentally.
The consolidation behavior of swollen sample has also been
presented. Some useful conclusions have been drawn from
this study.
2. MATERIAL AND METHODS
One natural expansive soil, namely black cotton soil, which is
extensively involved in construction, with high value of liq-
uid limit obtained from Belgaum, Karnataka state, India was
selected and used in this study. The soil was characterized
for their physical properties according to ASTM Standards
and the results are summarized in Table 1. It may be seen
that it has a free swell ratio of 2 (Prakash, K. and Sridharan,
A., (2004)), and the primary clay mineral being montmoril-
lonite. (Free swell ratio is defined as the sediment volume in
water per gram of dry soil to the sediment volume in kero-
sene/carbon tetra chloride per gram of dry soil)
While other properties viz., liquid limit, plastic limit and
shrinkage limit and activity are presented in Table 1, they
no way reflect the swelling nature of the soil (Sridharan, A.
(2005)).
3. EXPERIMENTAL PROGRAM
The apparatus used in this study of swell and swelling pres-
sure is essentially the same as that of a laboratory one-dimen-
sional consolidation test with fixed type ring (oedometer).
The rings were of 60 mm in diameter and 20 mm in height.Smoothly ground porous stones have been used in the
oedometer to minimize seating displacements. Filter papers
are used to avoid intrusion of soil into the porous stones.
Porous stones used before placing inside the oedometer
are air dried and it fits close to the oedometer ring to avoid
extrusion or punching at high vertical pressures.
In this study an attempt has been made to introduce
varying number of vertical sand drains to ensure complete
saturation with uniform moisture contents across the thick-
ness of the specimen in the swollen state. Also an attempt has
also been made to study the effect of initial dry density on the
swelling behavior and its associated parameters like, swell-
time behavior and swelling pressure-time behavior without
and with five and nine vertical drains.
3.1. Preparation of Specimens
It was decided to maintain the height of the specimen
approximately two thirds the height of the ring to facilitateswelling of the soil on imbibing water. Hence, a height of 14
mm was used for all the tests in this study. Oven dried soil <
425 µm was used to prepare the soil specimens for the swell
test at three desired densities using static compaction. All
swell tests in this study were conducted at three selected dry
densities of 1.2 Mg/m3 (loose), 1.35 Mg/m3 (medium) and 1.5
(high) Mg/m3.
At each of the selected dry densities the requisite amount
of dry soil < 425 µm was calculated and placed in the oedom-
eter rings and statically compacted by placing a metal spacer
specially fabricated for the purpose to achieve a height of
14 mm on compaction. A plain paper (template) havingdiameter of 60 mm with the number and pattern of holes
to be drilled for making vertical drains was placed on top
of the compacted soil specimen. Using a manually operated
drill fitted with a drill bit of diameter 2.8 mm vertical holes
were made over the thickness of the statically compacted soil
specimen in the oedometer rings. Care was taken to drill
holes for vertical drains without disturbing the sides. The
required numbers of vertical holes were drilled in the similar
manner. At this stage, an air-dried, smoothly ground porous
100 International Journal of Geotechnical Engineering
Table1. Physical properties of soil used in the present study
Sl. No. Item Values/Description
1 Soil Black Cotton Soil from
Belgaum
2 Specific Gravity, G 2.7
3 Liquid Limit (%) 87.9
4 Plastic Limit (%) 36.3
5 Plasticity Index (%) 51.6
6 Shrinkage Limit (%) 11.1
7Grain Size
Distribution
Gravel (%) 1
Sand (%) 3
Silt (Size) (%) 27
Clay (Size) (%) 69
8 Unified Soil Classification CH
9 Activity 0.75
10 Free Swell Index 100
11 Free Swell Ratio 2
12 Principal Clay Mineral Montmorillonite
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 3/13
Swelling behavior of expansive soils 101
stone was positioned into the base of a dry oedometer. A
standard filter paper was placed on top of the porous stone.
The ring with the specimen having requisite number of verti-
cal holes was placed on top of the bottom porous stone and
filter paper. The vertical holes were carefully filled with sand
of particle size less than 425 µm by using a paper cone with
a small opening at the bottom. Sufficient care was taken tofill the sand into the vertical drains without spilling around
the top of the specimen. An air-dried filter paper was placed
on the top of the specimen with vertical sand drains, on top
of which a porous stone in dry condition and the loading
pad were placed. The oedometer was secured to the base by
means of screws. Thus the test specimen in the oedometer
were mounted and positioned on the loading frame with a
vertical deflection dial gauge properly adjusted and fixed in
position to give proper dial reading. A seating load of 6.25
kPa was applied on the hanger. The initial dial gauge reading
was adjusted, so as to facilitate swell and later load the speci-
men to find the swelling pressure.
Figs. 1(a) and (b) are typical views of the compacted
specimen with five and nine vertical drains.
3.2. Swell and Swelling pressure test
The swell and swelling pressure are generally determined in
the laboratory with the one-dimensional oedometer. Swell is
determined by subjecting the laterally confined soil specimen
to a seating pressure of 6.25 kPa and by giving both the top
and bottom of the prepared specimen access to free water
(usually distilled) to cause swell. The time-swell observations
were recorded.The swelling pressures were determined at various
densities by constant volume method/zero swell method.
Continuous loading was done in this method, allowing water
to be imbibed by the specimen and keeping the volume
change nearly zero.
After the completion of determination of swelling pres-
sure, unloading was done in stages until the seating load
was reached. Under the seating load the specimen was left
to reach equilibrium for sufficient time. Then the specimen
was completely unloaded and removed for determining the
equilibrium water content over the depth of the specimen.
3.3. Water content Determination
In the routine practice of conducting swell test, it is not sure
that the specimen has reached uniform water content over
the thickness of the sample. To verify this it was planned
to determine the water content over the thickness of the
sample. Thus, after the completion of the swell pressure test,
the sample was removed from the oedometer ring, and sliced
horizontally into three portions namely top, middle and bot-
(a)
(b)
Figure 1. Fig. 1(a) Solid 3-D view showing 5 vertical drains in specimen;
(b) Solid 3-D view showing 9 vertical drains in specimen; (c) See through
view of 9 vertical holes for vertical drains; (d) Final view of specimen
filled up with sand to make 9 vertical drains
(c)
(d)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 4/13
102 International Journal of Geotechnical Engineering
tom. The sliced portion of the samples were put into separate
containers for water content determinations taking care to
place the sliced sample from each portion into two contain-
ers, to obtain the average water content of each portion.
Water content was determined by oven drying method. Care
was taken to remove the sand being used for vertical drains
before slicing into three portions.
4. RESULTS AND DISCUSSION
4.1. Results of Swell Tests
In this study, three dry densities i.e., 1.2 Mg/m3; 1.35 Mg/m3;
and 1.5 Mg/m3 were selected to study the influence of initial
dry density on swell behavior of expansive soils with a spe-
cific interest of studying the influence of introducing vertical
drains in combination with varied initial dry densities.
Fig. 2 (a) shows the swell-time behavior of the selectedsoil at three dry densities without vertical drains. It can be
seen from the figure that the amount of swell has increased
with an increase in the dry density. In order to have a check
on the reproducibility of the test results, swell tests were
repeated in the entire experimental program. One such
reproducibility test is shown in the figure at a dry density of
1.20 Mg/m3. It can be observed that there is good reproduc-
ibility on the swell-time behavior as well on the amount of
swell.
Since drilling holes to make vertical drains at lower
density of 1.2 Mg/m3 was not possible, further swell tests
at that density was not considered. Figs. 2 (b) and (c) showthe swell-time behavior having five and nine vertical drains
respectively at two dry densites namely 1.35 Mg/m3 and 1.5
Mg/m3. The amount of swell has shown to increase with
increase in density for the same number of vertical drains.
Also the tests have been shown to be reproducible.
Figs. 3 (a) and (b) show the influence of introducing
vertical sand drains on the swell-time behavior at particular
densities of 1.35 Mg/m3 and 1.5 Mg/m3 respectively. It can
be observed from Fig. 3 (a) that both the swell-time behavior
and the amount of swell have increased with the introduc-
tion of vertical drains as compared to that of without vertical
drains; with very marginal variation in the behavior betweenfive and nine vertical drains. Similar swell-time behavior and
increased ultimate swell can be observed at higher density of
1.5 Mg/m3 with the introduction of vertical drains as shown
in Fig. 3 (b). However, there is marked influence both on the
swell and swell-time behavior between five and nine vertical
drains at this higher dry density of 1.5 Mg/m3. Table 2 sum-
marizes the amount of swell in divisions without and with
vertical drains at varied dry densities.
4.2 Characterization of swell versus time plot
It could be seen from Figs. 2 and 3 that the shape of swell
versus log time plot is a mirror image of the conventional
compression versus log time plot. One can indentify three
clear phases in swelling viz., the initial swelling, the primary
swelling and the secondary swelling (Sridharan and Gurtug,2004). Fig. 4 shows an idealized curve of swell versus log
time, which identifies three different phases of swelling.
The time taken for completion of initial (t i) and primary
swelling (t p) for different testing conditions adopted in this
study has been read out from the Figs. 2 and 3, and the same
is been tabulated in Table 3. From Table 3 we can see that
as the density increases, the time taken to attain initial and
primary swelling has increased. At any density, introduction
of vertical drains has reduced the time required to attain
initial and primary swelling by nearly 50 % as compared to
that without vertical drains. However, the values are almost
similar for five and nine vertical drains.
4.3. Prediction of Ultimate Swell
Fig. 5(a) is a typical plot of swell versus time on a natural
scale at a dry density of 1.5 Mg/m3 without and with five
and nine vertical drains. The shape of the curves resembles
the shape of a rectangular hyperbola. Similar observation
was found at various placement conditions i.e., various dry
densities; without and with vertical drains. It was earlier sug-
gested by Kondner (1963) that non linear stress-strain curves
of soils could be represented by a rectangular hyperbolic
equation. Later using this concept of linking non-linear rela-tionships to that of an hyperbolic equation, researchers have
made attempts to linearise the same by modifying the way of
the plotting the same results (Sridharan and Rao 1986). If one
can assume that the time versus swell relationship represents
a rectangular hyperbola, then the time versus time/swell
relationship is a straight line. Using this concept, attempts
have been made to predict the ultimate swelling using rect-
angular hyperbola concept (Dakshinamurthy, 1978; Rao
and Kondandaramaswamy, 1981; Sridharan et al., 1986,
Sridharan and Gurtug, 2004) from the reciprocal of the slope
of the straight line portion of (time/swell) versus time plot.
In the present study attempts have been made to seewhether prediction of the ultimate swelling for expansive soil
with vertical drains also holds good. Fig. 5(b) show the (time/
swell) versus time relationship for the plot presented in Fig
5(a), which is a straight line. The reciprocal of the slope of
the straight line portion have been calculated to predict the
ultimate swell and the same have been tabulated in Table 4
along with the actual experimental swell values. It can be seen
that the predicted values of swell matches well with experi-
mentally obtained values of swell, even for cases with vertical
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 5/13
Swelling behavior of expansive soils 103
Figure 2. Swell versus log time relationship for varying density with zero vertical drains; (b) Swell versus log time relationship for varying density
with five vertical drains; (c) Swell versus log time relationship for varying density with nine vertical drains
(a)
(b)
(c)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 6/13
104 International Journal of Geotechnical Engineering
Figure 3. (a) Swell versus log time relationship for same density (=1.35 Mg/m3 ) with varying vertical drains;
(b) Swell versus log time relationship for same density (=1.50 Mg/m3 ) with varying vertical drain.
(a)
(b)
Table 2. Experimental values of Swell
Number
of VerticalDrains
Swell Values (mm)
Dry density 1.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
zero2.015 (Trial - 1)
2.65 4.361.88 (Trial - 2)
five ...3.10 (Trial - 1)
4.823.14 (Trial - 2)
nine ...3.16 (Trial - 1)
5.083.16 (Trial - 2)
(Note: All tests also include Top and Bottom Drains)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 7/13
Swelling behavior of expansive soils 105
Table 3. Time taken for initial and primary swelling
Number
of
Vertical
Drains
Time taken for initial and primary swelling (min)
Dry density
1.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
Initial Primary Initial Primary Initial Primary
zero1.5 (Trial - 1) 17 (Trial - 1)
4 32 4 451.5 (Trial - 2) 18 (Trial - 2)
five ... ...18 (Trial - 1) 16 (Trial - 1)
2.6 301.8 (Trial - 2) 1.7 (Trial - 2)
nine ... ...1.6 (Trial - 1) 16 (Trial - 1)
2.6 301.8 (Trial - 2) 18 (Trial - 2)
(Note: All tests also include Top and Bottom Drains)
Figure 5. (a) Swell versus Time relationship for same density (=1.50
Mg/m3 ) with varying vertical drains; (b) Time/swell versus Time relation-
ship for same density (=1.50 Mg/m3 ) with varying vertical drains.
(a)
(b)
Figure 4. Schematic diagram showing the separation of initial, primary
and secondary swelling.
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 8/13
106 International Journal of Geotechnical Engineering
drains also. From Fig.6, which is a plot of the experimentally
obtained swell values versus the predicted ultimate swell, it
can be observed that there is very good correlation between
the two as indicated by a good regression coefficient (r = 1).The predicted percent swell is 1.002 times the experimental
percent swell. The predicted value ought to be more because
the predicted swell is the asymptotic value of the swell versus
time plot. Therefore the asymptotic value is more than the
value obtained from the experimental results taken up to a
finite time. Theoretically, it takes infinite time to attain the
asymptotic value. Thus it is advantageous to predict the swell
from the data obtained from the initial stages of the swell test
and the experiment could be terminated without continuing
till equilibrium conditions are reached.
From Table 5, which summarizes the water content over
the thickness of the specimen for various placement condi-tions, it can be observed that for tests conducted without ver-
tical drains, the moisture content varied over the depth. The
variation of water content is observed to be about 2% and
3% at 1.35 Mg/m3 and 1.5 Mg/m3 respectively. Introduction
of vertical drains has reduced the variations in moisture con-
tent over the thickness significantly. This variation in water
content over the thickness was observed to be almost the
same with five and nine vertical drains. Herein it is evident
that introducing vertical drains has improved the access for
the sample to imbibe water needed for swelling and in turn
has resulted in increase in the value of swell. Conventional
method determines reduced value of swell possibly resultingin unsafe values.
4.4. Prediction of Swelling Pressure
Fig. 7 is a typical plot of void ratio versus swelling pressure by
constant volume method. The experimental values of swell-
ing pressure have been tabulated in Table 6. Similar to pre-
diction of swell, Sridharan et al. (1986) successfully extended
rectangular hyperbola concept to predict swelling pressureFigure 7. Log pressure versus void ratio (Constant Volume Method)
relationship for density (=1.50 Mg/m3 ) with five vertical drains.
Figure 6. Predicted swell versus Actual swell for the entire test results.
Table 4. Ultimate Percent Values – Predicted and Actual
Number
of
Vertical
Drains
Ultimate Percent Swell Values (%)
Dry density
1.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
Predicted Actual Predicted Actual Predicted Actual
zero
14.44 (Trial - 1) 14.39 (Trial - 1)
19.02 18.93 31.27 31.1413.46 (Trial - 2) 13.43 (Trial - 2)
five ... ...22.19 (Trial - 1) 22.14 (Trial - 1)
34.50 34.4322.48 (Trial - 2) 22.43 (Trial - 2)
nine ... ...22.62 (Trial - 1) 22.57 (Trial - 1)
36.39 36.2922.62 (Trial - 2) 22.57 (Trial - 2)
(Note: All tests also include Top and Bottom Drains)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 9/13
Swelling behavior of expansive soils 107
in a constant volume method. In this study attempts have
been made to predict swelling pressure from the pressure-
time plots for the case of a soil sample compacted to a higher
initial dry density, without and with the introduction of both
five and nine vertical drains. Fig. 8(a) shows the pressure ver-
sus time relationship for samples compacted to an initial dry
density of 1.5 Mg/m3 without and with vertical drains. The
shape of curves for various placement conditions appears
to resemble the shape of a rectangular hyperbola. Fig. 8(b)
shows the transformed curves of (time/pressure) versus time.
The reciprocal of the slope of the straight line portions gives
the predicted swelling pressure, and the same has been tabu-
lated in Table 6 along with the experimentally obtained val-
ues of swelling pressure. From the good agreement between
the predicted and actual values of swelling pressure, it can
be advantageous to predict the swelling pressure with a care-
fully recorded time versus pressure data without continuing
the experiment until the equilibrium conditions are reached
From the data during the initial stages of the experiment,
once the straight line portion of the (time/pressure) versus
time is reached to obtain its slope, the swelling pressure could
be predicted from the reciprocal of the slope of the straight
line. It can be observed that the predicted values of swelling
pressures are slightly on the higher side which is safer. This is
acceptable because the predicted values of swelling pressure
ought to be more than the experimentally obtained value asthe predicted swelling pressure is the asymptotic value of the
swelling pressure versus time plot.
From Table 7, which summarizes the water content
over the depth of the specimen for various placement con-
ditions, it can be observed that for tests conducted without
vertical drains, the moisture content varied over the depth
by the constant volume method and observed to be 11%. The
variations are observed to be reduced significantly with the
introduction of the vertical drains, being 3% for five vertical
Figure 8. (a) Pressure versus Time (Method – 2) relationship for density
(=1.50 Mg/m3 ) with varying vertical drains; (b) Time/Pressure versus
Time (Method – 2) relationship for density (=1.50 Mg/m3 ) with varying
vertical drains].
(a)
(b)
Table 5. Values of Moisture Content (%) determined over
the depth of the specimen
Number
of
Vertical
Drains
Moisture Content (%) determined after consolidation
of swollen sample
Dry density
Specimen
Portion 1.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
zero
Top 47.10 45.36 36.44
Middle 46.92 43.54 33.47
Bottom 47.25 45.74 36.20
five
Top … 43.79 34.68
Middle … 43.18 33.32
Bottom … 43.42 34.56
nine
Top … 43.40 31.20
Middle … 43.18 31.08
Bottom … 43.37 31.27
(Note: All tests also include Top and Bottom Drains)
Table 6. Actual and Predicted Values of Swelling Pressure
for Density = 1.50 Mg/m3 (Constant volume method)
Number
of Vertical
Drains
Swelling Pressure (kPa) by constant volume
method for Dry density = 1.50 Mg/m3
Actual Predicted
Predicted/
Actual
zero 635.54 692.76 1.09
five 826.98 894.26 1.08
nine 970.85 1027.10 1.06
(Note: All tests also include Top and Bottom Drains)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 10/13
108 International Journal of Geotechnical Engineering
drains and about 2 % for nine vertical drains. Also, it can
be seen from the Table 6 that the ratio of (predicted/actual)
swelling pressure has reduced with the introduction of ver-
tical drains. It could be reasoned that vertical drains has
improved the access for the sample to imbibe water needed
for swelling as seen in the water content variation and in turn
has reflected in the value of swelling pressure.
Since, there is relatively better access for the sample to
imbibe water; the rate of swelling was more with the intro-
duction of vertical drains. Thus, in this method of swell test,
in order to keep the volume change nearly zero, frequent and
continuous loading was done to the specimen with vertical
drains compared the specimen without vertical drains where
the rate of swell was less. By frequently loading the specimen,
one gets more points on the curve during the initial portion
of the test. This can be advantageously used to predict the
swelling pressure by quickly terminating the test once the
straight line relationship is obtained (Fig. 8(b)). As observed
from Fig. 8(b), more the number of vertical drains, more
quickly, the straight line is obtained and hence reduced time
required for the test to predict the swelling pressure.
4.5. Rate of Secondary SwellingThe three distinct phases of swelling i.e., the initial swelling,
the primary swelling and the secondary swelling as observed
through Figs 2 and 3, has been idealized as presented in Fig. 4.
Initial swelling is that portion of the swell time curve which is
almost parallel to the abscissa. The end of initial swell and the
beginning of primary swell is arbitrarily defined as the point
of intersection of the tangent to the initial portion of the
curve with the straight line portion of the curve representing
the primary swell. The completion of primary swell is arbi-
trarily defined as the intersection of the tangent to the curve
at the point of inflection, with the tangent to the straight line
portion representing a secondary time effect. Knowledge of
rate of secondary swelling will facilitate in long-term predic-
tion of swell, which may be a useful data for taking precau-
tionary measures for long term swell behavior when dealingwith expansive soils. The secondary swelling almost bears a
linear relationship in swell versus log of time plot. Similar
to what is known as rate of secondary compression, one can
call this as rate of secondary swelling (Sridharan and Gurtug,
2004). Similar to the definition of rate of secondary compres-
sion, one can define the rate of secondary swelling as:
Rate of secondary swelling =Δ δH
s
H ∙Δlog
10t
Table 8 shows the values of rate of secondary swelling
which can be used in predicting the long term swell. It can
be observed that the rate of secondary swelling is found to
be almost the same i.e., in the range of 0.022 for all the test-
ing conditions used in this study. Thus it can be seen that,
irrespective of the presence or absence of drains, the density
effect on rate of secondary swelling is around 0.022 for all
dry densities used in this study. Introduction of vertical
drains has not much influence on the rate of secondary swell.
Further, these values are comparable with the values reported
by Sridharan and Gurtug (2004).
4.6. Consolidation behavior of swollensample
For the various testing conditions of dry density and verti-
cal drains, after the sample reached equilibrium, they were
loaded in stages keeping the load increment ratio (LIR) of
unity to study the consolidation behavior of swollen samples.
Next load increment was carried out when primary consoli-
dation was complete, indicated by the near constancy of the
dial gauge readings between two time intervals. Each load
Table 7. Values of Moisture Content (%) determined over
the depth of the specimen
Number
of Vertical
Drains
Specimen
Portion
Moisture Content (%) determined for
the sample used to determine swelling
pressure for dry density = 1.50 Mg/
m3 by constant volume method / zero
swell method
zero
Top 31.40
Middle 20.60
Bottom 31.56
five
Top 30.36
Middle 25.44
Bottom 30.66
nine
Top 29.30
Middle 27.29
Bottom 29.44
(Note: All tests also include Top and Bottom Drains)
Table 8. Rate of secondary Swell
Number
of Vertical
Drains
Swell Values (mm)
Dry density
1.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
zero0.020 (Trial - 1)
0.022 0.0230.019 (Trial – 2)
five ...0.024 (Trial - 1)
0.0230.022 (Trial - 2)
nine ...0.025 (Trial - 1)
0.0260.023 (Trial - 2)
(Note: All tests also include Top and Bottom Drains)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 11/13
Swelling behavior of expansive soils 109
increment lasted not more than 2 hours. Loading was con-
tinued till the sample compressed, so as to obtain the straight
line portion of e – log σ ʹυ curve. Fig. 9 shows the plot of the
void ratio versus effective vertical consolidation pressure
for the swollen samples. From the figure, it can be seen that
the limb of the straight line portion of the e – log σ ʹυ curves
obtained from consolidating the swollen samples for vari-
ous testing conditions are almost parallel. The slope of thestraight line portion of e – log σ ʹυ curve (C
c) is obtained and
tabulated in Table 9. These slopes which are similar to the
compression index of regular consolidation test appear to be
almost the same, being around 0.44. Thus it can be seen that,
irrespective of the presence or absence of drains, and also
with varied densities, it has no influence on the consolidation
behavior of swollen samples.
Sridharan et al., (1991) have proposed an improved
technique for estimation of pre-consolidation pressure (σ ʹc).
They suggested that pre-consolidation pressure could be
easily and accurately estimated from the plot of log (1 + e)
against log pressure. Fig. 10 (a) shows a typical plot of log (1+ e) versus log pressure without VDs for test conducted at
varied densities. Fig. 10 (b) shows a typical plot of log (1 + e)
versus log pressure at 1.50 Mg/m3 for zero VD, five VDs and
nine VDs respectively. The estimated σ ʹc have been summa-
rized in Table 10. It can be observed from the table that σ΄c
increases with the increase in the initial dry density for any
particular VDs. Further, at any particular dry density, σ ʹc has
decreased with the increase in the number of VDs. Also it can
be seen that the ratio of σ ʹc for ρ
d = 1.5 Mg/m3 to 1.35 Mg/m3
Figure 9. Void ratio versus log pressure relationship for all testing conditions
Table 9. Compression index, Cc obtained from consolidation
of swollen sample
Number
of Vertical
Drains
Compression index, Cc obtained from consolidation
of swollen sample
Dry density
1.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
zero 0.44 0.43 0.44
five … 0.45 0.44
nine … 0.43 0.43
(Note: All tests also include Top and Bottom Drains)
Table 10. Pre-consolidation Pressure, σ΄ c obtained from con-
solidation of swollen sample
Number
of
Vertical
Drains
Pre-consolidation Pressure, σ΄ c (kPa)
Dry density Ratio of σ΄ c for
ρd = 1.50 to
1.35 Mg/m31.20 Mg/m3 1.35 Mg/m3 1.50 Mg/m3
zero 60 67 75 1.12
five … 50 55 1.10
nine … 45 47 1.04
(Note: All tests also include Top and Bottom Drains)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 12/13
110 International Journal of Geotechnical Engineering
has decreased with the increase in the number of VDs. This
may be because of better water accessibility with the increase
in VDs, the net attractive force has reduced and hence led
to the reduction in σ ʹc. The above observations indicate that
even for normal consolidation tests, vertical drains may
help to obtain better results. Conventional method without
any vertical drains determines higher σ ʹc. This needs furtherinvestigations.
CONCLUSIONS
The swelling behavior of expansive soil has been studied
without and with vertical drains. Attempts have been made
to study the effect of introducing five and nine vertical drains
with varied initial dry densities. The following conclusions
have been drawn.
Introducing vertical drains into the compacted soil is
found to have a marked influence on the swell and swelling
pressure behavior. The amount of swell has increased with
the introduction of vertical drains as compared to that of
without vertical drains. Introduction of vertical drains hasreduced the variations in moisture content over the thick-
ness significantly. The moisture content variation with the
thickness of sample is minimal with 5 or 9 vertical drains.
The conventional test results show the water content at the
middle portion of the sample to be much lower than the
one at the top and bottom of the samples. This results in
much lower value of percent swell as well as swelling pres-
sure. Herein, it is evident that introducing vertical drains has
improved the access for the sample to imbibe water needed
for swelling and in turn has reflected in the value of swell.
Percent swell versus log time relationship has essentially
three phases viz., initial, primary, and secondary portion. At
any density, introduction of vertical drains has reduced the
time required to attain initial and primary swelling by nearly
50 % as compared to that without vertical drains.
Prediction of both ultimate swell and also swelling pres-
sure with vertical drains using the concept of rectangular
hyperbola has found to agree well with the experimental
values. The advantage of the prediction method is that it
could be done with only the data from the initial stages of
the swell test and the experiment could be terminated with-
out continuing till equilibrium conditions are reached. The
ratio of predicted / actual swelling pressure has reduced withthe introduction of vertical drains. It could be reasoned
that vertical drains has improved the access for the sample
to imbibe water needed for swelling as seen in the water
content variation and in turn has reflected in the value of
swelling pressure. More the number of vertical drains, more
quickly the linear relationship of time/pressure versus time
is obtained and hence reduced time of the test to predict the
swelling pressure.
Introduction of vertical drains has not much influ-
ence on the rate of secondary swell. Knowledge of rate of
secondary swelling will facilitate in long-term prediction of
swell, which may be a useful data for taking precautionarymeasures for long term swell behavior when dealing with
expansive soils. Preconsolidation (σ ʹc) pressure increases with
the increase in the initial dry density for any particular VDs.
Further, at any particular dry density, pc has decreased with
the increase in the number of VDs. The precompression
pressure, σ ʹc determined without vertical drains can be more
because of lesser accessibility of water at the centre.
Figure 10. (a) 1+e versus log pressure relationship for varying density
with zero vertical drains; (b) 1 + e versus log pressure relationship for
same density (=1.50 Mg/m3 ) with varying vertical drain
(a)
(b)
7/21/2019 Swelling Behavior of Expansive Soils
http://slidepdf.com/reader/full/swelling-behavior-of-expansive-soils 13/13
Swelling behavior of expansive soils 111
REFERENCES
Al-Mhaidib, A., (1999). “Swelling behavior of expan-
sive shales from the middle region of Saudi Arabia,”
Geotechnical and Geological Engineering , 16, 291-307.
Azam, S. and Abduljauwad, S. N., (2000). “Influence of gyp-
sification on engineering behavior of expansive clays,” J.of Geotech. and Geoenvi. Eng , 126 (6), 538-542.
Brackley. J. J. A., (1973). “Swell Pressure and Free Swell in a
Compact Clay,” Proc. of the 3rd Int. Conf. on Expansive
Clays, Israel Institute of Technology, Haifa, 1, 169 – 176.
Chen, F. H. (1975). Foundations on Expansive Soils, Elsevier,
Amsterdam.74-80.
Dakshinamurthy, V., (1978). “A new method to predict
swelling using hyperbola equation”, Geotechnical
Engineering, Journal of South East Asian Society of Soil
Engineering , 9, 29 – 38.
EI – Sohby, M. A., and Rabba, E. A., (1981). “Some fac-
tors affecting swelling of clayey soils,” Geotechnical
Engineering , 12, 19 – 39.
Holtz, W. G., and Gibbs, H. J., (1956). “Engineering proper-
ties of expansive clays,” Transactions, ASCE, 121, 641
– 663.
Katti, R. K. (1975). “Regional soil deposits of India,” Proc. 5th
Asian Regional Conf., Bangalore, 2, 35-52.
Komornik, A. and David, D., (1969). “Prediction of swelling
pressure of clays”, J. SM&FD, ASCE, 95, 209 – 225.
Kondner, R. L., (1963). “Hyperbolic stress – strain response
of cohesive soils,” J. SM&FD, ASCE, 89, 115 – 143.
Ladd, C. C. (1960). “Mechanism of swelling by compactedclay,” Bull. Highway Res. Board , (245), 10-26.
Prakash, K. and Sridharan, A., (2004). “Free swell ratio and
clay mineralogy of fine grained soils,” Geotechnical
Testing Journal , 27 (2).
Rao, N. S. and Kondandaswamy, K., (1981). “The pre-
diction of settlements and heave in clays,” Canadian
Geotechnical Journal , 17, 623 – 631.
Sathyanarayana, B. and Raganatham, B. V., (1969).
“Interaction of primary factors on swell and swell pres-
sure,” Journal of the Indian National Society of Soil
Mechanics and Foundation Engineering , 8, 23 – 40.
Seed, H. B., Woodward, R. J. and Lundgren, R. (1962).“Prediction of swelling potential for compacted clays,”
Journal of the Geotechnical Engineering Division,
Proceeding of ASCE, 88 (3), 53 – 87.
Sridharan, A., Rao, A. S. and Sivapullaiah, P. V. (1986a).
“Swelling pressure of clays,” Geotechnical Testing
Journal, ASTM , 9 (1), 24 – 33.
Sridharan. A., Abraham. B. M., Jose. B. T., (1991). “An
improved technique for estimation of preconsolidation
pressure,” Géotechnique, 41 (2), 263-268
Sridharan, A. and Gurtug, Y., (2004). “Swelling behavior of
compacted fine – grained soils,” Engineering Geology
72, 9 – 18.
Sridharan, A. (2005). “On swelling Behavior of Clays.”
Keynote Lecture, Proc Intl Conf on Problematic soils
Eastern Mediterranean University North Cyprus, 499-
516.
Top Related