expansive soil stabilization

1. Index
1. Introduction to Expansive soil
2. Different mineral content in clay soil
a)TETRAHEDRAL SHEET
b)Octahedral sheet
3. Clay-water interaction
4. Clay particle interaction
5. Identification of expansive soil
6. Treatment of expansive soil
7. Foundation on expansive soil

2. INTRODUCTION
 Expansive soil are those soil which
swell considerably on absorption of
water and shrink on removal of
water.
 When expansive soil swells cohesion
decreases.
 The variation in volume of the soil is
to the extent of 20% to 30% of the
original volume.
 In India the expansive soils cover
approximately 20% of the total land
area.

3. Continue……
 Expansive soils are very small in size and have a large surface area that attract the free water .
 Because of these characteristics expansive soil (clay) exhibits extreme change in volume.
One pound of
montmorillonite
particle would have
An incredible surface
area = 325 ha with
which to attract
water.

4. Different mineral content in clay soil
 Expensive soil or clayey soil are made up of different mineral content which is
evolved from chemical weathering.
 Clay mineral particles are tiny crystalline substances, very small in size and flaky in
shape.
 These particles are microscopic.
 The particle size of clay is < 2micron in diameter but all the particles having size <
2micron need not to be a clay particle because the clay particles have electrical
charges on their surface.

5.  So let’s discuss clay mineralogy…..
 Almost all clay minerals are made up of two fundamental crystal sheets joined
through different bonds.
 One is a tetrahedral Sheet which is also called a silica sheet and another one is an
octahedral Sheet also called an Alumina Sheet or Gibbsite Sheet.

6. TETRAHEDRAL SHEET
• A Tetrahedral sheet is made up of several Silica tetrahedral units combined together.
• One Silica Tetrahedral Unit consists of a silicon ion (Si4+) surrounded by four oxygen
ions (O2-) forming a shape of tetrahedron. Silicon sits at the centre and oxygen ions
sit at the tips of tetrahedron.
• Each oxygen is shared with two units of tetrahedron.
• There is a hexagonal opening in the sheet.

7. Continue…
 Each oxygen at base is shared
in two units so carry -1 charge.
 Oxygen at top has -2 charge.
 Silicon ions has +4 .
 Therefore net charge on each
unit is –
-1-1-1-2+4=-1
+4

8. Octahedral sheet or Gibbsite Sheet
 Made up of Octahedral Units.
 One octahedral unit consists of six
hydroxyls forming a configuration of
an octahedron and having one
aluminium ion at the centre.
 1 OH is shared by 3 units of
octahedral and each OH has -1
charge.
 So net charge due to 6 OH is-
= 6 × (−1) × (
1
3
)
= -2
Charge on Al= +3
 So net charge on each unit
= +3-2
= +1

9. Three important clay minerals
1. Kaolinite
 Kaolinite mineral is made up of silica
and gibbsite sheets.
 In its basic structural unit these are
stacked one over the other and tips of
silica sheets are embedded in the
gibbsite sheet.
 The thickness of such structural unit is
about 7 Angstrom.
 As this mineral is formed by stacking of
one layer of each sheet it is sometimes
called a 1:1 clay mineral.
 Kaolinite mineral is formed by staking
one over the other such several basic
units. This unit extends indefinitely in
other dimensions.

10. Continue….
 These structural units join together by hydrogen bond
between hydroxyls of alumina sheet and oxygen of silica
sheet.
 As the hydrogen bond is sufficiently strong, the kaolinite
mineral is stable and water cannot easily enter between the
structural units and cause expansion. kaolinite is the least
active of all clay minerals.

11. 2. Montmorillonite
 The basic structural unit of
montmorillonite consists of a
gibbsite sheet sandwiched
between two silica sheets to form
a single layer. The thickness of this
layer is about 10 A and clearly it is
a 2:1 mineral.
 Montmorillonite mineral is formed
by many such structural units
joined together by Vander Waals
force which is a very weak force
compared to hydrogen bond.
 Clay soils that attract more water
have more plasticity, more swelling
or shrinkage.

12. Continue…..
 Water molecules are dipolar and negatively charged surface of silica sheet attracts these molecules
in the space between two structural units causing the layers of the mineral to be further separated
which results in the expansion of the mineral.
 That is why soils containing clay mineral montmorillonite exhibit high volume change. They swell as
the water enters into the structure and shrink as the water is removed.
 In India, clay soils containing montmorillonite mineral are commonly known as black cotton soils. It
covers about country’s 20 % area in the states of Andhra Pradesh, Karnataka, Madhya Pradesh,
Maharashtra and Uttar Pradesh.

13. 3. Illite
 Its basic structural unit is similar to that of
montmorillonite, so it is also a 2:1 mineral and layer
thickness is also about 10
 In this mineral there is isomorphous substitution of
silicon ions in silica sheet by aluminium ion. Also the
potassium ions occupy the space between different
structural units and do not allow water to take its
place.
 Potassium ion bonds the two layers together more
firmly which was not the case in the
montmorillonite. Therefore, illite does not swell as
much in the presence of water as montmorillonite,
but still it does much more than kaolinite.
 Soils containing illite swell more than that of soil
containing kaolinite but less than that of soil
containing montmorillonite.

14. Clay-water interaction
 Clay particles generally have Negative charges on them except at edge.
 Water molecules can attract with clay in the following ways:
+ + +
+ + +
-
-
-
-
-
-
-
-
-
+ -
+ -
+ -
+ -
a) Attraction due to electrostatic forces
k
Cation
- +
- +
b) Attraction through cation attachment
On surface of clay particle

15. Clay particles interaction
1. Flocculated structure:
 Flocculated structure is formed when net force
between clay particle is attractive.
 This is edge to face interaction.
 It has high quantity of voids.
 The type of structure has high seepage velocity.
NOTE:
Marine clay have flocculated structure due to
presence of salts.

16. 2. Dispersed structure:
 This type of structure is formed when
net force between clay particles is
repulsive.
 It has face to face interaction.
 Generally, one dimensional seepage
velocity is high.
 It has low voids as compared to
flocculated structure.
Note:
Lacustrine soil has dispersed structure.

17. Identification of expansive soil
Identification of expansive soil
Mineralogical
identification
Physical
properties
X ray
diffraction
Differential
thermal analysis
Electron
microscopy
Free swell
test
Differential Free
swell test
Swelling pressure
test

18. Mineralogical identification
1. Differential thermal analysis (DTA) :
 The DTA method is based on the fact that certain characteristic reactions take place at specific
temperature for different minerals.
When these minerals heated to high temperatures, resulting in a loss of
or gain in heat.
 A specimen of the soil with the unknown mineral is heated continuously along with an inert
substances in a electric oven and a record of change in temperature of the mineral plotted against
oven temperature is obtained.
 By comparing this with the available records of several known clay minerals, the type clay mineral
present and its amount can be known.

19. 2. X ray diffraction method
 Different minerals with different regular patterns of crystalline structures
will diffract x-ray to yield different x-ray diffraction patterns.
 With the x-ray diffraction patterns of common clay minerals being known,
it is possible to know which types of mineral are present and in what
proportion.
3. Electron microscopy
 In electron microscopy, the soil is observed under polarized light in an
electron microscope .
 The method requires skill and experience.
 Certain characteristics stains etc. are indications of the nature of the clay
minerals present.

20. 1. Free swell test
 Holts and Gibbs(1956) suggested free swell
test.
 10 𝑐𝑚3
of dry soil passing through 425 micron
sieve is poured into a 100 𝑐𝑚3 graduated
cylinder filled with water.
 The volume of settled soil is measured after 24
hours.
 The free swell % is determined as ,
𝑺𝒇=( 𝐕𝒇-𝐕𝐢)/(𝐕𝒊) × 𝟏𝟎𝟎
Where,
𝑉𝑖= initial dry volume of poured soil (10 𝑐𝑚3
)
 Bentonite , a highly swelling soil which contains
montmorillonite may have a free swell value
ranging from 1200 to 2000% , Kaolinite about
80% and illite from 30 to 80%.
Identification based on physical properties

21. Continue…..
 The free swell value increases with plasticity index.
• Soils having a free swell value as low as 100% can cause considerable
damage to lightly loaded structures and soils having a free swell value
below 50% seldom exhibit appreciable volume change even light loadings.

22.  Two samples of dried soil weighing 10g each, passing through 425 𝜇 sieve are
taken.
 One is put in a 50cc graduated glass cylinder containing kerosene oil ( a
nonpolar liquid ). The other sample is put in a similar cylinder containing
distilled water.
 Both the samples are left undisturbed for 24 hours and then their volumes
are noted.
 The DFS is expressed as,
DFS =
(𝑺𝒐𝒊𝒍 𝒗𝒐𝒍𝒖𝒎𝒆 𝒊𝒏 𝒘𝒂𝒕𝒆𝒓)−(𝑺𝒐𝒊𝒍 𝒗𝒐𝒍𝒖𝒎𝒆 𝒊𝒏 𝒌𝒆𝒓𝒐𝒔𝒆𝒏𝒆)
𝑺𝒐𝒊𝒍 𝒔𝒂𝒎𝒑𝒍𝒆 𝒊𝒏 𝒌𝒆𝒓𝒐𝒔𝒆𝒏𝒆
 The degree of expansiveness increases with increasing DFS%.
2. Differential free swell test

23. 3. Swelling pressure test
 Swelling pressure can be defined as the maximum force
per unit area required to be placed over a swelling soil to
prevent volume increase.
 Swelling is very useful index of the trouble potential of an
expansive soil.
 A swelling pressure can be defined from two different
types of tests.
(1). Swell Pressure Test by Consolidometer
(2). Swell Pressure Test by Constant Volume Method

24. 1. Swell Pressure Test by Consolidometer
 In this type of test, the specimen is placed in a odometer under a small surcharge of
about 7.0 kN/m2.
 Water is added to the specimen allowing it to swell and reach an equilibrium position
after some time.
 Now pressure on the specimen is gradually increased and the specimen is allowed to
consolidate.
 The plot of specimen deformation (𝛿 ) versus pressure (𝜎′) is drawn

25. 2. Constant volume test
 The constant volume test can be conducted by taking a
specimen in a consolidation ring and applying a pressure
equal to the effective overburden pressure, 𝜎0’ plus the
approximate anticipated surcharge caused by the
foundation 𝜎𝑠’.
 Water is then added to the specimen. As the specimen
start to swell , pressure is applied in small increments to
prevent swelling.
 Pressure is maintained untill full swelling pressure is
developed on the specimen, at that time the total
pressure is ,
𝜎𝑠𝑤′= 𝜎0’ + 𝜎𝑠’+ 𝜎1’
where,
𝜎𝑠𝑤′ = Total pressure applied to prevent swelling
𝜎0’ = effective overburden pressure
𝜎𝑠’ = surcharge caused by foundation
𝜎1’ = additional pressure applied to prevent swelling after
addition of water.

26. Treatment of expansive soil
 The swelling of a soil has two injurious effects on a structure founded on it .
 One is the reduction in the strength of the soil and second is the movement of the
structure.
 The following measures may be taken to reduce the swelling potential of soil and
increases the strength.
1. Replacement of expansive soil:
 A simple and easy solution for slabs and footing on expansive soils is to replace
the foundation soil with non-swelling soils.
 Experiences indicates that there is no danger of foundation movement if the sub
soil consists of 1.5 m of non- swelling soil underlain by highly expansive soil.

27. 2.Moisture barriers :
 Moisture control method are applied around the perimeter of the structure .
 Moisture barriers may be vertical or horizontal but, vertical barriers are more
effective.
 A vertical trench, about 15 cm wide, 1.5m deep and filled with gravel, lean
concrete or lime-fly ash have been quite effective moisture barrier.
 The moisture barrier should be supplemented with adequate drainage system.

28. 3. Soil stabilization
 Soil stabilization is a process by
which a soils physical property are
transformed to provide long-term
permanent strength gains.
 Stabilization is accomplished by
increasing the shear strength and
the overall bearing capacity of a soil.
 Once stabilized, a solid monolith is
formed that decreases the
permeability, which in turn reduces
the shrink/swell potential and
harmful effects of freeze/thaw cycles
 Different method are available for
soil stabilization .

29. • Soil-lime Stabilization
 Lime stabilization improves the strength, stiffness and
durability of fine grained materials.
 In addition, lime is sometimes used to improve the
properties of the fine grained fraction of granular soils. Lime
has been used as a stabilizer for soils in the base courses of
pavement systems, under concrete foundations, on
embankment slopes and canal linings.
 Addition of about 5-7% of lime reduces the swelling and
shrinkage characteristic of expansive soil.
• Soil-Cement Stabilization:
• Soil-cement is the reaction product of an intimate mixture of
pulverized soil and measured amounts of Portland cement
and water, compacted to high density.
• As the cement hydrates, the mixture becomes a hard,
durable structural material. Hardened soil-cement has the
capacity to bridge over local weak points in a sub grade.
• When properly made, it does not soften when exposed to
wetting and drying, or freezing and thawing cycles

30. Foundation on expansive soil
 It is necessary to note that all parts of building will not equally be affected be the
swelling potential of the soil.
 Beneath the center of a building where the soil is protected from sun and rain, the
moisture changes are small and the soil movements are least.
 Beneath outside the wall, the moisture changes and soil movements are greater.
 Hence, damage to building is greater on the outside walls.
• The following types of foundation are provided in expansive soils.
1. Shallow foundation isolated from swelling soils.
2. Waffle slab
3. Drilled pier foundation
4. Under reamed pile

31. 1. Shallow foundation isolated from swelling soils.
 Figure shows a typical typical foundation under a outside wall.
 The granular material provided around the foundation mitigates the effects of
expansion of the soils.

32. 2. Waffle slab
 In this type of construction , the ribs of hold the structure load.
 The waffle voids allow the expansion of soil.

33. 3. Drilled pier foundation
 Drilled piers are commonly used to
resist uplift forces caused by the
swelling of soils.
 Drilled piers when made with
enlarged base are called belled piers,
and made without an enlarged base
are called straight shaft piers.
 The bottom of the shaft should be
placed below the active zone of the
expansive soil.

34. 4. Under reamed piles
 The under reamed piles are successfully developed by
C.B.R.I, Roorkee, for serving as foundation for black
cotton soils, filled up ground and other types of soils
having poor bearing capacity.
 The principle of this types of foundation is to anchor
the structure at a depth where ground movement due
to seasonal moisture changes is negligible.
 The under reamed piles are bored cast in situ concrete
piles having one or more bulbs or under reams in its
lower portion.
 The bulbs are formed by under reaming tool.
 The length of pile is 3 to 8m.
 The dia. Of pile is 0.2 to 0.5m.
 The dia. of bulb is 2 to 3 times the dia. of pile.
 The spacing of pile vary from 2 to 4m.

35. DIFFERENTIAL SETTLEMENT OF STUCTURE