Expansive soil improvement

1. Expansive soil
Out come No.2 - Expansive soil Identification & Remedies

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

4. Clay mineralogy…..
•Almost all the clay minerals are
made up of two fundamental
crystal sheets joined through
different bonds.
•One is tetrahedral Sheet which is
also called silica sheet and
another one is octahedral Sheet
also called Alumina Sheet or
Gibbsite Sheet.

5. 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.

6. •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

7. 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

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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

14. 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.

15. 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.

16. 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

17. 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.

18. 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.

19. 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

20. 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.

21.  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

22. 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

23. 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

24. 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.

25. 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.

26. 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.

27. 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 .

28. • 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

29. DIFFERENTIAL SETTLEMENT OF STUCTURE