2. What is Consolidation?
When a saturated soil is loaded externally,
GL
saturated clay
the water is squeezed out of the soil and the soil shrinks, over a long time may be up
to several years depending upon the permeability of the soil this whole phenomena
is called consolidation.
3. Concept of Consolidation
As the time passes the water comes out of the soil
voids and so the volume of soil decreases, this is the
cause of the settlement.
The settlement is directly
proportional to the decrease in
volume of soil= The volume of
squeezed out water
4. Settlement due to consolidation
Settlement is the vertically downward movement of
structure due to the shrinkage in volume of soil due
to consolidation.
e
settlement H
1 eo
5. Settlement vs. Time
This leads to settlements occurring over a long time,
Settlement
End of Primary Consolidation
Cα
Logarithm of Time
which could be several years, depending upon the permiability of the soil.
6. Consolidation Settlement
• It occurs due to the process of consolidation.
• Clay and Organic soil are most prone to consolidation
settlement.
• Consolidation is the process of reduction in volume due to
expulsion of water under an increased load.
• It is a time related process occurring in saturated soil by
draining water from void.
• It is often confused with Compaction.
• Consolidation theory is required to predict both rate and
magnitude of settlement.
7. Since water flows out in any direction, the process is
three dimensional.
But, soil is confined laterally. Hence, vertical one
dimensional consolidation theory is acceptable.
Spring analogy explains consolidation settlement.
Permeability of soil influences consolidation.
8. Settlement-Void ratio Relation
Equating the two expressions for average vertical
strain,
consolidation change in void ratio
settlement
H e
Ho 1 eo
initial thickness of initial void ratio
clay layer
9. Settlement computations
Two different ways to estimate the
consolidation settlement:
q kPa
(a) using mv
settlement = mv H
=q
H
(b) using e-log v’ plot
e
settlement H
1 eo
10. Settlement by Mv Method
~co-eff of volume change=mv
~volumetric strain in a soil element per
unit increase in stress no units
change in volume
original volume V
i.e.,
mv V
kPa-1 or MPa-1 kPa or MPa
12. Settlement computations
~ computing e using e-log v’ plot
If the soil is normally consolidated,
the entire loading path is along the virgin
compression line.
initial
eo
e
vo’+
vo’
13. Settlement computations
If an overconsolidated clay becomes normally
consolidated by the end of consolidation,
initial
eo
e
VCL
vo’ p’ vo’+
14. Practical Applications
Cross section of PVD
Installation
Prefabricated Vertical Drains to Accelerate Consolidation
15. Consolidation Test
settlement Increment of load At any degree of
consolidation, comparison in time taken
dial gauge
for consolidation between laboratory &
Topcap
field with respect to drainage path is
water confining
ring
t = Time taken
d = Drainage path
sample porous stone
