expansive soil lecture note

1. Expansive Soils
Basic questions to be answered
1. What is expansive soils?
2. What are the cause and Damages of
Expansive soils?
3. How we can design for Expansive soils?
4. What are the remedial to reduce the effects
of expansive soils?

2. What is Expansive Soils?
• Expansive Soils: are soils that exhibit volume
change from change in soil moisture.
• They swell when moisture increases and shrink
when moisture decreases.
• Typical expansive soils are highly plastic clays
that often contain colloidal clay minerals such as
montmorillonites.

3. Clay Minerals

4. Damages from differential movements
Differential movements caused by swell or
shrinkage of expansive soils can increase
the probability of damage to the foundation
and superstructure.
Differential rather than total movements of
the foundation soils are generally
responsible for the major structural damage.
Differential movements redistribute the
structural loads causing concentration of
loads on portions of the foundation.

5. Occurrence of damages
• Damages can occur :
within a few months following construction,
may develop slowly over a period of about 5
years,
or may not appear for many years until some
activity occurs to disturb the soil moisture.
• The probability of damages increases:
if the climate, effects of construction, and
effects of occupancy tend to promote moisture
changes in the soil.

6. Structures susceptible to damage
• Types of structures most often damaged
include:
foundations and walls of residential
and light (one- or two-story) buildings,
highways, canal linings, and retaining
walls.
these structures are less able to
suppress the differential heave of the
expansive foundation soil than heavy,
multistory structures.

7. Types of damage
Damages sustained by the structures
include:
 distortion and cracking of pavements;
 distortion and cracking of on-grade
floor slabs;
 cracks in grade beams, walls, and
drilled shafts;
 jammed or misaligned doors and
windows; and
failure of steel or concrete blocks
supporting grade beams.

8. Extent of damage
 The magnitude of damages to
structures can be extensive, impair the
usefulness of the structure, and detract
aesthetically from the environment.
 Maintenance and repair requirements
can be extensive, and the expenses can
grossly exceed the original cost of the
foundation.

9. Examples of damage

10. Depth of Active Zone
• An important criterion when evaluating
expansive soils is Depth of the active
zone, which is the greatest depth of
moisture content fluctuation.

11. Identification & Testing
Three
methods
used
for
identifying
&
classifying
expansive
soils:
2.
Indirect
Method
3.
Direct
Method

12. 1. Mineralogical Identification
Identification of minerals in soils is usually
carried out using at least two of the following
techniques:-
X-ray
diffraction
Differential
thermal
analysis
Electron
microscope
resolution

13. 2. Indirect Method
• Correlation with common soil tests (USBR)

14. 3. Direct Method
• The most satisfactory and convenient method to
determine:
 Swelling Potential
 Swelling Pressure
• Conventional oedometer (1D-consolidometer) apparatus is
used.
i. Swelling Potential
a. Free Swell Test

15. b. Swelling Pressure Test
• Swelling pressure is the pressure that prevents
all swell.
• The swelling pressure can be measured at the
end of the free swell test by simply increasing
the normal load in increments until the sample
returns to its volume.

16. Differential Heave
• The greatest differential heaves are most likely
to occur when swelling is due to such
extraneous influences as:
 Broken waterlines
 Poor surface drainage
 Aggressive tree roots

17. Foundation Considerations
 If the foundation soil is of low expansion
potential, the design follows standard practice.
However, if the soil is of marginal or high
expansion potential, then some safety
measures need to be taken.
 Bearing capacity: Foundation loading
pressures should exceed the soil swell
pressures, if practical, but should be
sufficiently less than the bearing capacity to
maintain foundation displacements within
tolerable amounts.
a
P q
P
S 


18. Basic Safety (Preventive) Measures
Surface Drainage Basement Backfills
Landscaping
Underground Utilities

19. 1. Surface Drainage
 Although good surface drainage is important at all
building sites, it is especially critical where expansive
soils are present.
 The ground surface should slope away from the structure.

20. 2. Basement Backfills
 If the building has a basement the backfill should
consist of non-expansive soils. It should also be
well compacted.
 Install a drain pipe at the bottom of the backfill.
3. Landscaping
 Avoid placing plants and irrigation system
immediately adjacent to the structure.
 Avoid placing irrigation pipes near the structure
(to prevent problems from leaks).
 Direct all spray heads away from the structure.

21. 4. Underground Utilities
 Utility lines such as water or sewer pipes
become distorted due to differential swelling
of expansive soils, resulting in leaks that can
cause more expansion.
 The risk of this potential problem can be
minimized by using flexible pipe materials
(e.g. PVC instead of concrete pipes) and using
some flexible joints.

22. Additional Preventive Measures
 Beyond the basic measures more extensive measures can
also be incorporated. The most common ones are divided
into the following three:
 Altering the expansive soil
 Bypass the expansive soil
 Mitigate the movements in the super structure
I. Altering the expansive soil
i. Replacement:- When shallow expansive soils are
present at the surface they can be removed and
replaced by less expansive soils and then compacted
properly.
ii. Lime treatment:-When lime is mixed with
expansive soils, a chemical reaction occurs and as a
result the swelling potential is reduced, shear
strength is increased, and moisture content is
reduced.

23. iii. Prewetting
 This technique consists of ponding the site with water for
increasing the moisture content of the soil and hence
achieving most of the heave before construction. However,
this technique is time consuming because flow of water
through highly plastic soils is slow.
 After wetting 4 to 5% of lime by weight may be added to
the top layer of the soil to make it less plastic and more
workable.
iv. Moisture barriers:-The long term effect of
differential heave can be reduced by controlling
the moisture variation in the soil. This can be
achieved by providing either vertical or horizontal
moisture barriers.

24. II. Bypassing the expansive soil
i. Drilled Shafts:-In a highly expansive soil one
method of mitigating heave effects is to support the
structure on deeper soils, bypassing some or all of
the active zone. This can be achieved by using
drilled shaft foundation and raised floor.
 Bypassing an expansive clay with a raised floor &
drilled shaft.

25. III. Mitigating movement in the structure
i. Rigid Foundation System:-The philosophy in this
technique is to provide a foundation system that is so
rigid and strong that is moves as a unit. Differential
heaves would then cause the structure to tilt without
distorting. Conventional reinforced mats have been used
for this purpose.
 Conventional reinforced mat foundation-“waffle slab”

26. Cont…