Effect of expansive soils on buildings and its prevention

1. BY
SAILISH CEPHAS E
17MK09
M.E. INFRASTRUCTURE
ENGINEERING
EFFECT OF EXPANSIVE SOILS ON
BUILDING STRUCTURES AND IT’S
PREVENTION
15CN23 FOUNDATION
STRUCTURES
(ASSIGNMENT PRESENTATION)

2. OUTLINE OF PRESENTATION
• Definition
• Causes of expansion
• Problems
• solutions

3. EXPANSIVE SOILS
• Expansive soils are soils that expand(swell) when
water is added, and shrink when they dry out.
• The expansive soil has considerable strength in dry
state, but the strength goes on reducing on
absorption of water. The soil exerts considerable
pressure on foundations during swelling.
• Expansive soils contain minerals such as
Montmorillionite, Illite, Kaolinite clays that are
capable of absorbing water.

4. • In India these expansive soils are known by local
names such as Black Cotton soils (BC) in central
India, Bentonite in Rajasthan and Kashmir, Mar or
Kabar in Uttar Pradesh. These soils occupy about 30
to 40 % of the land area of India.

6. CAUSE OF EXPANSION OF EXPANSIVE
SOILS
Rain
Lawn and garden watering creates a moist zone on
the exterior of a foundation.
Cesspools
Septic tanks
leaky pipes and
swimming pools

7. PROBLEMS
• When expansive soils are present they will generally
not cause a problem if their water content remains
constant. The situation where greatest damage
occurs is when there are significant or repeated
moisture content changes.
• Expansive soils usually wont have problem with
heavy structures but with light structures and
pavement.

8. DEEP EXCAVATION FOR FOUNDATION:
• Black cotton soils are residual soils resulting from
weathering of Igneous rock (Basalt).
• The thickness of soil stratum can be high as 3 to 10
m.
• Laying the foundation on a firm non-swelling
stratum involves deep excavation in stiff clay and
increases the cost of construction.

9. ASSUMPTION OF LOW BEARING
CAPACITY:
• The correct estimation of allowable bearing capacity of
BC soils is complicated by various factors such as
swelling pressure, ground water table variations, site
conditions etc.
• This leads to assumption of lower bearing capacity.
• But if the probable swelling is higher than the assumed
bearing capacity, the foundations are subjected
differential settlements.
• Cracking of single storied buildings is very common than
that of double storied buildings.

10. NON UNIFORM SWELLING OR
SHRINKAGE:
• The equilibrium water content is not same below
the foundation. This leads to differential settlements
and diagonal cracking of masonry superstructure.

11. HIGH COST AND LOW RELIABILITY OF
REHABILITATION
• Remedial measures for damaged structure are
costly and not reliable in long term. Hence
prevention is better than cure.

12. LIFT OF FLOOR SLABS

13. LOSS OF SKIN FRICTION OF PIERS

14. CRACKS IN INTERIOR WALLS AS A
RESULT OF UPLIFT PRESSURE

15. SEVERE DIAGONAL DEEP CRACK IN
THE WALL

16. SOLUTION
• Replacing the expansion soil under the foundation.
• Changing the nature of the expansive soil.
• Strengthening the structures to withstand heave,
constructing structures that are flexible enough to
withstand the differential soil heave without failure,
or constructing isolated deep foundations below
the depth of the active zone.

17. REPLACING THE EXPANSIVE SOIL
• The first and very simple method is to remove the
entire layer of expansive soil up to firm and non-
expansive stratum.

18. CHANGING THE NATURE OF
EXPANSION SOIL
• Compaction
• Pre-wetting
• Installation of moisture barriers
• Stabilization of soil

19. COMPACTION
• The heave can be minimized by compacting to a
lower unit weight on the high side of the optimum
moisture content(possibly 3-4% above the optimum
moisture content).

20. PRE-WETTING
• Wetting the soil before construction so that most of
the heave formed before construction.
• This technique may be time consuming because
seepage of water through highly plastic clays is
slow.
• So, after ponding, 4-5% of hydrated lime may be
added to the top layer of the soil to make it less
plastic and more workable.

21. INSTALLATION OF MOISTURE
BARRIERS
• The long term effect of the differential heave can
be reduced by controlling the moisture variation in
the soil.
• This is achieved by providing vertical moisture
barriers around the perimeter.
• These moisture barriers may be constructed in
trenches filled with gravel, lean concrete, or
impervious membranes.

23. CHEMICAL STABILIZATION OF SOIL
• Chemical stabilization with the aid of lime and cement.
• Lime or cement and water are mixed with the top layer
of soil and compacted.
• The addition of lime or cement will decrease the liquid
limit, the plasticity index, and the swell characteristics of
the soil.
• This can be done up to a depth of 1 to 1.5 m
• Hydrated high-calcium lime and dolomite lime are
generally used for lime stabilization.

24. PRESSURE INJECTION OF LIME SLURRY
OR LIME FLY ASH SLURRY
• Stabilizing by adding lime slurry usually to a depth of
4-5m and occasionally deeper to cover the active
zone.

25. UNDER-REAMED PILES
• Under-reamed piles are also used.

26. CONCLUSION
• It is possible to build successfully and safely on expansive
soils if stable moisture content can be maintained or if
the building can be insulated from any soil volume
change that might occur. The procedure for success is
as follows:
• Testing to identify any problems
• Design to minimize moisture content changes and insulate
from soil volume changes
• Build in a way that will not change the moisture conditions of
the soil
• Maintain a constant moisture environment after construction

27. REFERENCES
• https://pdfs.semanticscholar.org/fb0f/ad84d77876987c86e5f1c3d
224f6b973ee34.pdf
• http://www.foundation-repair-guide.com/expansive-soil.html
• https://geology.com/articles/expansive-soil.shtml
• Braja.M.Das, principles of foundation engineering, fifth edition.
• https://s3.amazonaws.com/ppt-download/expansivesoilsofindia-
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