Expansive soils and stabilization

Expansive Soils and Stabilization
1
Table of content
Content Page
Abstract 2
Introduction 3
Description 3
Table 1, Figure 1 & Figure 2 7
Engineering Issues 8
Stabilization and Remedial Measures 9
Case Histories 11
Conclusion 12
References 13

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Abstract
Expansive soils pose a potential threat to the structures. They have the ability to
generate tremendous pressure on the structures such as concrete foundations and
pavements. Experiments show that up to 15,000 pounds per square feet pressure can
be generated by expansive soils that is enough to create problems to foundations. This
document highlights the mechanisms involved in the formation of expansive soils. It
explains about the details of clay minerals such as Smectite family, other silicate
minerals and soils rich with sulfate salts. The behavior of these minerals with the
addition or removalof moisture that causes expansion and shrinkage respectively.
It explains about the complications faced by the engineers and damages caused by this
type of soils throughout the world including heaving, differential settlement,
subsidence, development of fissures in the structures.
It presents the remedial measures used in the industry for stabilization of such soils and
to mitigate the issues that can be created by such soils.
Damage caused by expansive soils in different countries and a case study related to the
settlement of a Breakwater in China which was constructed on soft clay has also been
discussed.

3
Introduction
Expansive soils could be defined as soils whose volume changes with introduction of
moisture i.e. swell or shrinkage with increase or decrease in moisture content
respectively. Expandable soils, expansive clays, shrink-swell soils and heavable soils are
some of the synonyms used for these soils. The more water expansive soils absorb, the
more their volume increases. Expansion of ten or more percent is not uncommon. This
change in volume can exert enough force on a building or other structure to cause
damage. This type of soil will also shrink when they dry out. This shrinkage can remove
support from the foundations causing subsidence. It can also develop fissures. These
fissures facilitate the deep penetration of water when moist conditions or runoff occurs.
The spontaneous shrinkage and swelling process can place repetitive stress on
structures.
Description
The expansion potential of any particular expansivesoil is determined by the percentage
and type of clay in the soil. Clay particles are very tiny and their shape is determined by
the arrangement of constituent atoms which form clay crystals. Clay is a silicate mineral
and is composed mainly of Silicone, Aluminum and Oxygen. Silicon atoms are positioned
in the center of a pyramid structure called tetrahedron with one Oxygen atom
occupying each of the four corners. Aluminum atoms are situated in the corner of an
octahedron with an Oxygen atom occupying each of the four corners. Because of
electron sharing, the Silicon tetrahedrons link together with one another to form thin
tetrahedral sheets. Aluminum octahedrons also link in same pattern. The actual clay
crystals are a composite of Aluminum and Silicon sheets which are held together by
intra-molecular forces. The presence and abundance of some other elements or ions i.e.

4
hydrogen, sodium, calcium, magnesium, sulfur, etc can impact the composition and
behavior of the clay minerals.
To understand the concept of swelling or shrinkage, sound knowledge regarding the
formation and constituents of expansive clays is needed. For a group of prominent and
highly expansive clay minerals called Smectites, an octahedral sheet is sandwiched
between two tetrahedral sheets to create the mineral structure. Names and mineralogy
of Smectite family minerals are given in table 1.
A water molecule consists of two hydrogen atoms sharing electrons with a single oxygen
atom. The water molecule is electrically balanced but within the molecule, the offsetting
charges are not evenly distributed. The two positive charged hydrogen atoms are
grouped together on one side of the larger oxygen atom. As a result water molecule
itself behaves like an electrical “dipole” because it has both positive charge on one side
due to two hydrogen atoms as well as a negative charge on the opposite side due to
bare oxygen atom. The electrical charge on water molecule enables it to interact with
other charged particles. The mechanism by which water molecules become attached to
the microscopic clay crystals is called “adsorption”.
In well drained soils, due to high quantity of oxygen these sheets are combined
together through strong hydrogen bond to form Kaolinite and due to this reason
Kaolinite is not more prone to expansion as it has less affinity for water-dipoles.
Montmorillonate, a member of Smectite family is considered one of the most notorious
mineral to expansion (Wyoming Multi-Hazard Mitigation Plan, 2011) because it is
formed in water deficient areas by isomorphous substitution of magnesium or ferrous
iron for aluminum. Elements in Montmorillonite are bound together through weak
Vander Wall’s forces resulting of having electrical charge on its molecule. Water

5
molecules called “dipoles” also contain electrical charges so it is easy for
Montmorillonite to adsorb very large amount of water molecules between its crystalline
sheets and therefore has a large shrink-swellpotential.
The type of soil consisting of Montmorillonite behaves normally under load unless
moisture content increases or decreases in the soil which consequently causes swelling
or shrinkage of the soil. These types of soils are usually found in arid regions due to the
loss of moisture content by evaporation. This prolonged duration of dryness causes an
increase in the area to be affected by the introduction of water. Some other minerals
like Bentonite, Vermiculite, Attapulgite, Nontronite, Illite and Chlorite also possess
swelling characteristics. There are some sulfate salts that will expand with changes in
temperature. When a soil consisting of large amount of expansive minerals it has
potential of significantexpansion and vice versa.
According to Y. Du et al. (1999) dry density, material composition and fabric of the soil
also play an important role the swelling-shrinkage behavior of the soil. Naturally
occurring undisturbed soil develops a certain structure and bond between soil particles
which prevent the occurrence of swelling in the soil in comparison to samples in which
such structural orientation pattern in disturbed due to molding and application of
compressive forces on it. This can be a reason that the fabric of soil does affect the rate
of swelling in the soil.
Soil Engineers or Materials Engineers who identify expansive soils will collect
representative samples at the jobsite and return them to the laboratory for testing. The
tests for calculating percentage of fine particles in a particular sample will be carried
out. If over 50 % of the particles in a sample are able to pass through a number 200
sieve (0.075 mm sieve) is classified as either silt or clay or combination of both. Silt

6
particles are products of mechanical erosion and could contain mineralogy same as sand
particles. Clay particles are products of chemical weathering and are characterized by
their sheet structure and composition. A significant presence of clay minerals in a
sample can indicate a possibleexpansivesoil problem.
“Atterberg limits” is another test by which engineers can know the potential expansion
property of a soil. Clay soils are tested to determine their “plasticity index”. The
plasticity index is a measure of the range over which the clay sample will retain its
plastic characteristics. A dry clay soil sample after addition of water behaves more like a
plastic rather than solid or semi-solid. The percentage of moisture at that point is the
plastic limit. As water is added continuously, at some point the clay will cease acting like
a plastic and will start to act like a liquid. That point is called the liquid limit. The plastic
and liquid limits are referred to as the Atterberg limits after the scientist who defined
them. The difference between the liquid limit and the plastic limit is called the plasticity
index. Clay which has plasticity index greater than 50 percent is considered as highly
plastic. These types of clays often called as “fat clays” are expansive clays.
There are some other laboratory tests designed specially to measure the expansion
potential of a particular sample. The American Society of Testing Materials has
published a test method (ASTM D 4829) to quantify the results of Expansion Index. By
adding water to the sample while measuring its deformation, the soil engineer will
comparethe result to a scale or Expansion Index.
0 – 2: Very Low; 21 – 50: Low; 51 – 90: Medium; 91 – 130: High; ˃ 130: Very High.

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Name of the Mineral Mineralogical Formula
Aliettite Ca0.2Mg6((Si,Al)8O20)(OH)4 · 4H2O
Beidellite (Na,Ca0.5
)0.3
Al2
((Si,Al)4
O10
)(OH)2
· nH2
O
Ferrosaponite Ca0.3
(Fe
2+
,Mg,Fe
3+
)3
((Si,Al)4
O10
)(OH)2
· 4H2
O
Hectorite Na0.3
(Mg,Li)3
(Si4
O10
)(F,OH)2
Montmorillonite (Na,Ca)0.33
(Al,Mg)2
(Si4
O10
)(OH)2
· nH2
O
Nontronite Na0.3
Fe2
((Si,Al)4
O10
)(OH)2
· nH2
O
Pimelite Ni3
Si4
O10
(OH)2
·4H2
O
Saliotite (Li,Na)Al3
(AlSi3
O10
)(OH)5
Saponite Ca0.25
(Mg,Fe)3
((Si,Al)4
O10
)(OH)2
· nH2
O
Sauconite Na0.3
Zn3
((Si,Al)4
O10
)(OH)2
· 4H2
O
Stevensite (Ca,Na)x
Mg3-x
(Si4
O10
)(OH)2
Swinefordite Li(Al,Li,Mg)4
((Si,Al)4
O10
)2
(OH,F)4
· nH2
O
Volkonskoite Ca0.3
(Cr,Mg,Fe)2
((Si,Al)4
O10
)(OH)2
· 4H2
O
Yakhontovite (Ca,Na)0.5
(Cu,Fe,Mg)2
(Si4
O10
)(OH)2
· 3H2
O
Zincsilite Zn3
(Si4
O10
)(OH)2
· 4H2
O
Table 1 : Names and Mineralogy of Smectite family minerals
Figure 1: Montmorillonite Figure 2: A typical case of differential settlement
shown in a structure, figure reproduced from
Mishra (2012)

8
Engineering Issues
There are a number of issues related to the structures constructed on expansive soils. It
has been observed that usually light weight structures are more prone to the swell-
shrink behavior of expansive soil due to their inefficiency to resist the variation of soil
strata underneath them (Institution of Civil Engineers, 2012). These include shallow
foundations, pavements and houses. The most common issue faced by geotechnical
engineers around the world is the heaving or differential settlement of the soil.
The phenomenon of heaving occurs when water is trapped within the sheets of clay
minerals due to which the overall volume of the soil is increased. This results in lifting
the structure upwards from either its ends or at the centre because of the introduction
of moisture content in the soil or because of the differential settlement of the soil due
to shrinkage after drying (Cutler and Richardson, 1991). This pattern, called “differential
heave” can be observed and measured with a level survey.
Light weight structures may quickly respond to soil movement caused by expansive
soils. Sever cracking and dislocation of these structures can be the result. Once the
cracking begins, more invading water feeds the problem. Pavements, building
foundations, retaining walls and other structures resting on expansive soils can move
laterally along with lifting up, a reflection of the principle that expensive soils exert
pressurein all directions.
Trees should not be planted in the vicinities where these types of soils are present as
trees can play a vital role in failure of the structures causing differential settlement of
the soil as shown in the figure . Roots of trees absorb water from soil causing loss of
moisture content and ultimately shrinkage in soil. It has been mentioned by Cutler &
Richardson (1991) that sometimes the roots of a tree can cause blocking or damaging

9
the drain resulting in the moisture content of the soil. The details of the effect of
different trees on the swell-shrink behavior of soil could be found in their book (Cutler &
Richardson, 1991).
The shrink-swell properties of expansive soils will often cause a phenomenon called
“slope creep”. As there is always a horizontal movement of such soil on sloping ground,
the period swelling and shrinkage on a slope together with the forces of gravity will
result in an ongoing creep of soil down the face of the slope. Walls and fences in such
circumstances will rotate down-slope direction. Hillside improvements on creeping soils
must be heavily reinforced and firmly anchored in order to prevent damage and
eventual destruction.
Stabilization and Remedial Measures
Though expansive soils are prohibited to be used in foundations or pavements but if
there is scarcity of suitable soils in a specific locality then immediate attention should be
given to such issues as the cost of introducing a remedial measure is much less than the
cost incurred due to uplift or settlement of a structure.
In new construction where expansive soil is a concern, the engineer may require
controlled pre-wetting of the soil prior to placement of the foundation or pavement.
This will cause pre-expansion of the soil with the idea that further expansion pressure
will be minimized. According to research done in Spain, the issue of swelling could be
handled by eliminating the characteristics of the soil participating in the expansive
behavior along with improving the mechanical properties of the soil (Seco et al., 2011).
Recommendation can be given for removal of several feet of expansive soil layers and
replace those with non-expansive material. Depending upon the severity of expansion
potential, non-expansive soils may be mixed with the expansive soil to lower the

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expansion potential to an acceptable level. Expansive nature of the soils can be reduced
by using additives like Fe2O3, CaO, and some polymers (Mirzababaei et al., 2009). The
latest edition of Ground Improvement (Mitchell and Kelly, 2013) provides a brief history
of the remedial measure adopted by people around the world to mitigate the issue of
expansive soils. These include lime, cement, fly ash and other techniques which mainly
involve recycling the waste materials and reusing it is strengthening the soil. The lime
can cause a reaction called “cations exchange” where “ions” or positively charged atoms
in solution are substituted for other species of ions which are attached to the clay
mineral crystals. Other effects of lime treatment may include “flocculation-
agglomeration” in which small clay particles clump together into larger particles. Lime
also act as a cementing agent increasing the strength of treated soil. High-sulfate soils
do not respond well to lime or other calcium based soil treatment methods. As a result,
highway departments and soil engineering researchers are looking for new and better
option in the formof organic and inorganic polymers.
Piling, thicker foundation slabs, grade beams, concrete piers, screw anchors all with
extra steel reinforcement are another viable options to prevent the transfer of the load
on the expansive part of the soil.
According to Al Rawas and Qamarudin (1998), the extent of damage done to the
structure by expansive soils could be reduced by installing proper drainage. French drain
or subdrain system can be particularly effective where high water tables and subgrade
drainage conditions are bringing high volumes of moisture into the foundation soil. A
French drain is a trench filled with gravel which captures and removes unwanted water.
A perforated pipe in the bottom of trench is also used to accumulate water and forbids
it to saturate the foundation soils.

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Concrete pavements could be executed to minimize the absorption of water into the
underlying soil layers. Another interesting method for soil stabilization is by explosive
replacement method. This method was applied during a highway construction in China.
Crushed stones were piled up near the expansive soils and explosives were placed in the
soil. The explosion causes the soil to move out and the same time pushed the stones in
the cavity. This results in the top soil having higher bearing capacity due to the stones
(Yan & Chu, 2004).
Another way of soil stabilization is by using preloading and prefabricated vertical drains
(PVD). These prefabricated vertical drains increase the consolidation process of the soil
and prevent excessive pore water pressure from developing in the soil and provide a
safepath for water drainage (Yan et al., 2009).
Case Histories
Expansive soils are present throughout the world and are known in every US state.
American Society of Civil Engineers estimates that ¼ of all homes in the United States
have some damage caused by expansive soils. In a typical year in the United States they
cause a greater financial loss to property owners than earthquakes, floods, hurricanes
and tornadoes combined. Every year they cause billions of dollars in damage throughout
the world. It has been mentioned that the annual loss to structures due to expansive
soils is about 400 million pounds a year in the United Kingdom (Driscoll and Crilly, 2000).
According to the hazard mitigation plan, United States of America faces a total loss of
3.2 billion dollars per annum due to expansive nature of soils (Wyoming Office of
Homeland Security, 2011). Whereas China experiences 1 billion dollars loss annually
(Yanjun Du, 1999).

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Yan et al. 2009 have discussed a case of failure of the settlement of an offshore
Breakwater constructed on soft clay in China. A Breakwater was constructed near
Shanghai Port which experienced settlement and lateral movement of the structure due
to floods. Analysis of the structure showed that the strength factor was not taken into
consideration in the original design by the design engineers as floods were not very
common in that place which consequently resulted in the soil having insufficient bearing
capacity. That issue was resolved by installation of prefabricated vertical drains (PVDs).
Those PVDs increased the undrained shear strength of the soil which ultimately
increased the factor of safety from 0.83 to 2.51 making it suitable for construction. This
method proved to be successful as the breakwater sustained more floods after its
reconstruction and has shown no sign of settlement till then.
Conclusion
Expansive soils pose a threat to the safety and economic conditions of people. A
detailed explanation of the mechanisms involved in the soil which causes it to swell or
shrink has been discussed in this document. It highlights the issues faced by engineers
around the world regarding expensive soils and the extent of damage caused by them. It
has been given that the main cause of swelling or shrinkage of the clay soil is due to the
introduction or removal of moisture which depends upon number of factors. They
include:
● Improper drainagesystemof a structure
● Trees planted near foundation of a structure
● Damaged drains along the pavements

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For remedial actions, treatment of soil with additives such as fly ash, iron oxide, calcium
oxide, polymers, lime and cement are prescribed. If the expansive soil is distributed in
relative small area then it is more economical to remove and replace it with non
expansive soil rather than adding additives. Design of proper drainage system is also
recommended in the literature. It has also been observed that the main focus of
research by the engineers is either on introducing new ways to treat expansive soils or
on using an existing method in a more efficient manner. More work is needed to be
done on defining and classification procedure of expansive soils which is a potential
research area.
References
http://www.academia.edu/7599700/Expansive_Soils
http://geology.com/articles/expansive-soil.shtml
http://www.foundation-repair-guide.com/expansive-soil.html
Al-Rawas, A. A., & Qamaruddin, M. (1998).Construction problems of engineering
structures founded on expansive soils and rocks in northern Oman. Building and
Environmental, Vol. 33, 159-171.
Cutler, D. F., & Richardson, I. B. (1991). Tree roots and buildings. Essex: Longman
Scientific& Technical, Longman Group UK Limited.
Driscoll, R., & Crilly, M. (2000). Subsidence damage to domestic buildings. Lessons
learned and questions asked. London: Building Research Establishment
Institution of Civil Engineers, ICE. (2012). ICE Manual of Geotechnical Engineering.
London: Institution of Civil Engineers
Mirzababaei, M., Yasrobi, S., & Al-Rawas, A. (2009). Effect of polymers on swelling
potential of expansive soils. Ground Improvement - Institute of Civil Engineering, 111-
119.

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Mishra, G. (2012). Allowable settlement for different structures. Retrieved 11 28, 2013,
from the Constructor: http://theconstructor.org/geotechnical/allowable-settlement-for-
different-structures/6899/
Mitchell, J. K., & Kelly, R. (2013). Addressing some current challenges in ground
improvement. Ground Improvement - Institution of Civil Engineers, 127-137.
Seco, A., Ramirez, F., Miqueleiz, L., & Garcia, B. (2011). Stabilization of Expansive Soils
for use in construction. Applied Clay Science 51, 348-352.
Wyoming Office of Homeland Security. (2011). Wyoming Multi-Hazard Mitigation Plan.
Wyoming: Wyoming Officeof Homeland Security.
Yanjun Du, S. L. (1999). Swelling-shrinkage properties and soil improvement of
compacted expansivesoils, Nign-Liang Highway, China. Engineering Geology, 351-358
Grim, R.E., Kulbicki, G. (1961) Montmorillonite: high-temperature reactions and
classification. American Mineralogist: 46: 1329-1369

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