CONSOLIDATION OF SOIL
COMPRESSIBILITY
OF SOIL
•
When a soil is
subjected to compressive force, its volume decreases. This is compressibility
of soil.
•
It is caused
because of following reasons
•
Compression of
soil particles and water in voids
•
Compression and
expulsion of air in voids
•
Expulsion of
water present in the voids
Explanation
of cause of compressibility
•
Compression of
soil particles is negligible and water compression is also extremely small for
stress considered in Soil Engineering. So, it is not much significant.
•
Compression and
expulsion of water in voids occur when soil is not fully saturated. But here we
will be dealing with saturated soil only. So, it is also not significant.
•
As saturated soil
contains only water in voids, expulsion of water from the voids is possible.
Therefore, this is significant cause.
Definition
of Consolidation of soil
•
It is compression
of saturated soil under steady static pressure
•
It is entirely
due to expulsion of water from voids
•
When compressive
force is applied on saturated soil, water escapes and shifting of soil
particles take place.
• Decrease in volume occurs not due to compression of
solids or water but due to shifting of position of particles as the water
escapes.
Settlement
of structure
•
It is vertical,
downward movement due to a volume decrease of the soil on which it is built
•
It is sinking of
structure due to compression of soil below
Consolidation
vs compaction
•
Consolidation
occurs only in saturated soil due to expulsion of water present in the voids
•
Compaction occurs
in unsaturated soil due to expulsion of air present in the voids
•
But both process
compresses the soil
Type of
consolidation
•
Initial
consolidation
•
Primary
consolidation
•
Secondary
consolidation
Initial consolidation
•
When load is
applied to partially saturated soil, a decrease in volume occurs due to
expulsion and compression of air in voids
•
Small decrease in
volume also occurs due to compression of solid particles
•
The reduction in
volume of soil just after the application of the load is known as initial
consolidation
•
No change in
moisture content occurs
Primary
consolidation
•
After initial
consolidation, further reduction in volume occurs due to expulsion of water
from voids
•
When saturated
soil is subjected to a pressure, initially all applied pressure is taken up by
water as an excess pore water pressure, as water is incompressible
•
Hydraulic
gradient develops and water starts flowing out and hence decrease in volume
occurs
•
It is time
dependent
•
In fine-grained
soil, it takes long time due to less permeability
•
In coarse-grained
soil, it is quick due to high permeability
Secondary
consolidation
•
Even after
primary consolidation, there is decrease in volume of soil at slow rate
•
This additional
reduction of volume of soil is called secondary consolidation
•
Its cause is not
fully established
•
It occurs due to
plastic adjustment of soil fabrics
Most important Consolidation
•
Primary consolidation is the most important of all.
Normally
consolidated
•
A normally
consolidated is soil which has not been subjected to a pressure greater than
the present existing pressure
•
Settlement in
such soils are more
Over-consolidated
•
A soil is said to
be over-consolidated if it had been subjected in the past to a pressure in
excess of the present pressure.
•
Settlement in
such soils are small
Over
consolidation ratio (OCR)
•
It is the ratio
of highest stress experienced by the soil to the current stress
•
OCR = highest
stress experienced/current stress
•
For normally
consolidated soil, current stress is the highest stress as those soils have
never been subjected to pressure greater than the current pressure so OCR = 1.
Pre-consolidated
pressure
•
It is also known
as over-consolidation pressure
•
It is defined as
the maximum pressure to which an over consolidated soil had been subjected in
the past
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