Materials Sciences and Applicatio ns, 2011, 2, 629-633
doi:10.4236/msa.2011.26085 Published Online June 2011 (http://www.SciRP.org/journal/msa)
629
Characterization of Dispersive Soils
T. S. Umesh1, S. V. Dinesh1, Puvvadi V. Sivapullaiah2
1Department of Civil Engineering, Siddaganga Institute of Technology, Tumkur, India; 2Department of Civil Engineering, Indian
Institute of Science, Bangalore, India.
Received December 27th, 2010; revised March 28th, 2011; accepted April 27th, 2011.
ABSTRACT
Dispersive soils which occur in many parts of the world are easily erodible and segregate in water pose serious prob-
lems of stability of earth and earth retaining structures. The mechanism of dispersivity of soils is reasonably well un-
derstood. However there is simple method to identify the dispersivity of the soils and even more difficult to quantify the
dispersivity. Visual classification, Atterberg’s limits and particle size analysis do not provide sufficient basis to differ-
entiate between dispersive clays and ordinary erosion resistant clays. Pinhole test and double hydrometer test are the
only two tests that are in vogue to identify the dispersive soils. This paper explores the possibility of using other stan-
dard tests such as shrinkag e limit and unconfined co mpressive stren g th tests to qua ntify th e dispersivity of th e so ils. The
rationale of using the methods and correlation between the dispersivity determined by various methods has been ex-
plained. It has been concluded that dispersivity ascertained from strength tests is more reliable.
Keywords: Atterberg’s Limit, Dispersive Soils, Particle Size Distribution, Unconfined Compression Strength
1. Introduction
The soils that are highly susceptible to erosion and con-
taining high percentage of exchangeable sodium ions are
called Dispersive soils. These soils are found exten sively
in United States, Australia, Greece, India, Latin America,
South Africa and Thailand. These soils are erodible in
nature and have tendency to segregate in presence of
water and erode under small seepage velocity leading to
problems of stability of earth and earth retaining struc-
tures. Soil dispersivity is mainly due to the presence of
exchangeable sodium present in the structure. The ero-
sion due to dispersion of soil depends on mineralogy and
clay chemistry and the dissolved salts in pore water. Un-
der saturated conditions, the attractive forces are less
than the repulsive forces, an d this will he lp the particle to
segregate and to move in suspension. The dispersive
mechanism have been reported by various researchers
such as Sherard et al., [1], Heinzen and Arulalandan [2],
Holmgren and Flanagan [3]. Many slope and earth dam
failures, foundation and pavement failures have been
observed in these types of soils [4]. Most of the failures
in embankments, earth dams and slopes were composed
of clays with low-to- medium plasticity (CL and CL-CH)
that contain montmorillon ite. In order to assess the ex tent
of damage and also to plan for suitable remedy, it is nec-
essary to characterize these soils. It is necessary to esti-
mate the extent of dispersiveness so that the damage po-
tential can be estimated. At present, dispersivity is ob-
tained from pin hole test, double hydrometer test, crumb
test etc. Though pin hole test is considered to be accurate
but care has to be taken to properly simulate field condi-
tion in terms of soil state and flowing condition [5]. The
crumb test indicates erodibility of clayey soil, but a dis-
persive soil some times give a non dispersive reaction. In
double hydrometer test number of tests should be per-
formed since soil dispersiveness can be altered over short
distance in a borrow area, canal alignment, embankment
etc (U.S.Department of the interior 1991).
The problems related to dispersive soils are common
through out the world. Earth dams constructed on disper-
sive soils have suffered internal and surface erosion. The
erosion features such as rill and gully marks, channels,
internal cavities and tunnels with in the soil mass have
been observed in natural slopes of dispersive soils. The
failure of slopes due to dispersion of clay particles by
seepage water along cracks, fissures and root holes are
initiated by erosion of soil. Thus the failure initiated by
piping makes the embankments constructed on dispersive
soil susceptible [4]. Dispersive piping in dams has oc-
curred either on the first reservoir filling or, less fre-
quently, after raising the reservoir to highest level. Tun-
neling failures commence at the upstream face when the
reservoir is filled for the first time, the settlement may

Characterization of Dispersive Soils 630
accompany saturation of the soil, particularly if the soil
was placed dry of optimum and not well compacted. Set-
tlement below the phreatic surface and arching above can
result in crack formation. Water moving through the
cracks picks up dispersive clay particles, with the rate of
removal increasing as the seepage velocity increases [5].
2. Dispersion Phenomenon
The clay fraction in dispersive soil that comes in contact
with water behaves like a single grained particle with less
electrochemical attraction and does not adhere with other
soil particle. The electrical surface force (inter particle
repulsive force) exceeds the Van der walls attraction and
the detached clay particles are carried away causing pip-
ing in earth dams. The dispersiveness of soils is mainly
due to the presence of sodium ions in the soil structure
and not due to the presence of sodium in the pore water
[6].
The erosion occurs when shearing stress induced by
fluid flow on a surface is large enough to cause particle
removal from the surface. The resistance to erosion is
offered by the submerged weight of the sediment, i.e.
gravity forces for n on-coh esive soils. But in cohesive so il
the structure of the soil and the interaction between pore
and eroding fluids at the surface is the phenomenon in-
volved in soil erosion. The amount and type of clay, pH,
organic matter, temperature, water content, thixotrophy
and type and concentration of ions in the pore and erod-
ing fluids are the factors that affect the critical shear
stress required to initiate erosion. The osmotic influences
and the structure of clay produce swelling of clay surface
because of the difference in the concentration between
the pore and eroding fluid. The interparticle bonding
forces are reduced by the swelling and this is a factor in
the erosion of cohesive soils by water. The swelling
caused by the concentration gradients existing at a
clay-water interface will be greater if the soil system is
more dispersed. The erosion is accelerated by a process
called slaking in partially saturated soils. The slaking is
due to excess air pressure in the capillaries because of
surface tension force in partially saturated soil. The
pressure exerted by the entrapped air in the pores break
loose small bits of soil on the surface. The slaking is
more for the highly flocculated and low plasticity soils
[2].
3. Determination of Dispersion of Soil
Visual classification, Atterberg’s limits and particle size
analysis do not provide a basis for differentiation be-
tween dispersive clays and ordinary erosion resistant
clays [5]. The conventional laboratory tests performed to
determine the dispersive clays includes pinhole test and
double hydrometer test.
In the pinhole test, distilled water is allowed to flow
through a 1.0 mm diameter hole drilled through a com-
pacted specimen. The water becomes muddy and the hole
rapidly erodes in dispersive clays. For nondispersive
clays the water is clear and there is no erosion. The pin-
hole test is considered most reliable but it is important
that the samples correctly simulate the soil state and the
water composition expected in the field [5].
The soil conservatio n service laboratory d ispersion test,
also known as the double hydrometer test is one of the
first methods developed to assess dispersion of clay soils.
The current test method was developed in 1937 from a
procedure proposed by Volk [7]. The particle size distri-
bution is first determined using the standard hydrometer
test in which the soil specimen is dispersed in distilled
water with a chemical dispersant. A parallel hydrometer
test is then made on a duplicate soil specimen, but with-
out a chemical dispersant. The percent dispersion is the
ratio of the dry mass of particles smaller than 0.005 mm
diameter of the test without dispersing agent to the test
with dispersing agent expressed as a percentage. Proce-
dures for performing the test are outlined in USBR 5405,
Determining Dispersibility of Clayey soils by the Doub le
Hydrometer Test Method. The criteria for evaluating de-
gree of dispersion using results from the double hy-
drometer test are shown in Ta b le 1 . Test results indicate
that a high percentage of soils with dispersive character-
istics, exhibited 30 percent or more dispersion when
tested by this method.
The use of other methods such as shrinkage limit and
unconfined compression strength test to characterize the
dispersivity of the soil has been explored in the present
study.
4. Soil Used
The soil used in present study locally called Suddha soil
is present in Southern parts of Karnataka. It is wide
spread below a d epth of 1.5 m from the ground level and
extends to depths greater than 10 m. It possesses good
strength in dry condition and upon increase in moisture
content looses strength. Many failures have been ob-
served along canal slopes, road bases, and foundations at
sites where Suddha soil is present. This soil is silty sand
with clay percent less than 20. This is considered as a
problematic soil in view of wide spread damage under
Table 1. Degree of dispersion from double hydrometer test.
Percent dispersion Degree of dispersion
<30 Non-dispersive
30 to 50 Intermediate
>50 Dispersive

Characterization of Dispersive Soils631
saturated conditions. To know the nature and type of
problem with the soil its properties are determined. Table
2 shows the Geotechnical properties of Suddha so il.
5. Effect of Water Content on the Strength
of Suddha Soil
The unconfined compressive strength of Suddha soil has
been determined for soil compacted with different water
contents with the same compactive effort. It is seen very
clearly from Figure 1 that the strength of soil decreases
steeply with increase in molding water content. Howev er,
initially the strength increases with increase in molding
water content due to increase in maximum dry density.
But beyond the optimum moisture content there is very
steep reduction in strength.
6. Double Hydrometer Tests for
Determination of Dispersion
The double hydrometer test was conducted on Suddha
soil. Figure 2 shows the results of double hydrometer
test conducted on Suddha soil. The results indicate that
Suddha soil has dispersion percent of about 35 percent.
Thus it indicates that this soil’s degree of dispersion is in
the intermediate range.
7. Shrinkage Limit Test as a Measure of
Dispersivity
The shrinkage limit is the water content where further
loss of moisture will not result in any more volume
reduction.The shrinkage limit of a natural soil is primar-
ily a result of the packing phenomenon, which in turn is
governed by the grain-size distribution of the soil and the
shrinkage limit of pure clays may be affected by the fab-
ric also. Even though clay-sized particles play an impor-
tant role in the shrinkage phenomenon, there is an opti-
mum clay content at which the shrinkage limit of a soil
can become minimum. It has b een further illustrated that
the shrinkage limit is not at all related to the plasticity
characteristics of the soil. The effect of dispersion on the
shrinkage limit of Suddha soil has been studied in this
section. Shrinkage limit test was carried out on lime
treated Suddha soil using 7.5 and 15 percent dispersing
agent. The results are shown in Table 3.
It can be seen that the shrinkage limit of Suddha soil
which is about 22 decreases to 20 percent with addition
of 7.5 percent dispersing agent solution which further
decreases to 16 percent when the dispersing agent con-
centration is increased to 15 percent. Addition of lime
which induces flocculation of particles increases the
shrinkage limit. The higher is the concentration of lime
the higher is the shrinkage limit. With any lime content
the shrinkage is more with lower dispersing agent. It is
thus clear that Suddha soil loses strength due to disper-
Table 2. Geotechnical properties of Suddha soil.
Sl. No. Properties Value
Particle size analysis
Gravel (%) 4
Sand (%) 57
Silt (%) 26
1
Clay (%) 13
2 Liquid limit (%) 41
3 Plastic limit (%) 24
4 Plasticity index 17
5 Shrinkage limit (%) 22
6 Specific gravity 2.6
Compaction Characteristics
Optimum Moisture Content (%) 14
7
Maximum dry density (kN/m3) 17.8
8 Soil Classification SM
Figure 1. Effect of water content on the strength of Suddha
soil.
sion of its particles.
8. Unconfined Compression Strength Test as
a Measure of Dispersivity
It is clear from the above that the dispersivity of the soil
particles can be reduced with the addition of lime which
induces flocculation. The effect of varying amounts of
lime on the unconfined strength of soil has been studied.
For this purpose the cylindrical specimens were com-
pacted at the optimum moisture content and maximum
dry density of Suddha soil with and without dispersing

Characterization of Dispersive Soils 632
Figure 2. Double hydrometer test for Suddha soil.
Table 3. Shrinkage limit and spec i fic gravity of Suddha soil using dispersing agent.
with 7.5% dispersing agent with 15% dispersing agent
Sl. No. % Lime added Shrinkage limit
(%) Specific gravity Shrinkage limit (%) Specific gravity
1 No lime and no
Dispersing Agent 22 2.6 22 2.6
2 0% 20 2.54 16 2.57
3 1% 32 2.58 31 2.54
4 2% 34 2.59 33 2.58
5 3% 36 2.59 35 2.53
6 6% 39 2.59 37 2.54
Table 4. Unconfined compressive strength of Suddha soil
with and without dispersing agent.
agent and they were subjected to unconfined compressive
strength test. The results are shown in Tab l e 4 . It is ob-
served that the unconfined compressive strength increases
with increase in lime content when Suddha soil was
compacted with water. Addition of dispersing agent alone
decreases the strength of soil greatly. Addition of any
amount of lime increases the strength of soil with or
without dispersing agent. But the increase is more when
there is no dispersing agent. The dispersiv ity of soil with
different lime content is calculated and summarized in
Table 4.
Sl. No.% Lime
added
Unconfined compressive
strength, (kN/m2) with
dispersing agent
Unconfined compressive
strength, (kN/m2)
without dispersing agent
1 0% 77 171
2 1% 121 218
3 2% 148 246
4 3% 189 288
5 6% 206 305
It is clear from Ta bl e 5 that the dispersivity from the
strength test is about 55 percent. It was earlier observed

Characterization of Dispersive Soils633
Table 5. Dispersivity of Suddha soil with different lime
contents.
Sl. No. Lime content, % Dispersivity (%)
1 0% 54.9
2 1% 44.4
3 2% 39.8
4 3% 34.3
5 6% 32.4
that the dispersivity of the soil is about 35 percent from
the particle size analysis by doub le hydrometer tests with
and without dispersing agent. The higher dispersivity
from the strength tests clearly indicates that this method
is more reliable than by particle size distribution method.
The dispersivity of the soil goes on decreasing with in-
creasing additions of lime. However, the dispersivity
does not reduced below 30 percent. The very gradual
decrease in dispersivity between 3 and 6 percent of lime
shows the optimum lime content is abou t 3 percent.
9. Conclusions
The paper brings out that the dispersivity of soil can be
assessed by comparing the values of results of various
tests carried out on soil with and without dispersing agent.
The dispersivity of the soil generally increases with the
amount of dispersing agent. The dispersivity of the soil
decreases after stabilizing the soil with lime. Among the
various methods used such as double hydrometer, shrink-
age limit and unconfined compression tests, the later
method is more accurate and can generally be adopted
for assessing the dispersivity of the soils.
REFERENCES
[1] J. L. Sherard, L. P. Dunnigan and R. S. Decker, “Identifi-
cation and Nature of Dispersive Soils,” ASCE Geotech-
nical Division, Vol. 102, No. 4, 1976, pp. 69-87.
[2] R. T. Heinzen and K. Arulanandan, “Factors Influencing
Dispersive Clays and Methods of Identification,” ASTM
Special Technical Publication, Vol. 623, 1977, pp. 202-
217.
[3] G. G. S. Holmgren and C. P. Flanagan, “Factors Affect-
ing Spontaneous Dispersion of Soil Materials as Evi-
denced by the Crumb Test,” ASTM Special Technical
Publication, Vol. 623, 1977 pp. 219-239.
[4] B. Indraratna, et al., “Stabilization of a Dispersive Soil by
Blending with Fly Ash,” Quarterly Journal of Engineer-
ing Geology, Vol. 24, 1991, pp. 275-290.
doi:10.1144/GSL.QJEG.1991.024.03.03
[5] J. K. Mitchell, “Fundamentals of Soil Behavior,” 2nd
Edition, John Wiley & Sons, Inc, Hoboken, 1993.
[6] S. Bhuvaneshwari, B. Soundra, R. G. Robinson and S. R.
Gandhi, “Stabilization and Microstructural Modification
of Dispersive Clayey Soils,” 1st International Conference
on Soil and Rock Engineering, Srilankan Geotechnical
Society, Columbo, Srilanka, 2007, pp. 1-7.
[7] G. M. Volk, “Method of Determination of the Degree of
Dispersion of the Clay Fraction of Soils,” Proceedings of
Soil Science Society of America, Vol. 2, 1937, p. 561.