Transcript Document

Consolidation of Clays
N. Sivakugan
Duration: 17 minutes
1
What is Consolidation?
When a saturated clay is loaded externally,
GL
saturated clay
the water is squeezed out of the clay over a long time
(due to low permeability of the clay).
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2
What is Consolidation?
settlement
This leads to settlements occurring over a long time,
time
which could be several years.
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3
In granular soils…
settlement
Granular soils are freely drained, and thus the
settlement is instantaneous.
time
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4
During consolidation…
Due to a surcharge q applied at the GL,
the stresses and pore pressures are increased at A.
q kPa

A
GL
..and, they vary
with time.
u
’
saturated clay
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During consolidation…
 remains the same (=q) during consolidation.
u decreases (due to drainage) while ’ increases,
transferring the load from water to the soil.

q kPa
u

A
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’
u
’
saturated clay
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GL
q
6
One Dimensional Consolidation
~ drainage and deformations are vertical (none laterally)
~ a simplification for solving consolidation problems
q kPa
GL
water squeezed out
saturated clay
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reasonable
simplification if the
surcharge is of
large lateral extent
7
H -e Relation
H
average vertical strain =
Ho
q kPa
GL
saturated clay
e = eo
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Time = 0+
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H
q kPa
GL
Ho
saturated clay
e = eo - e
Time = 
8
H -e Relation
Consider an element where Vs = 1 initially.
e
eo
1
Time = 0+
Time = 
average vertical strain =
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e
1  eo
9
H -e Relation
Equating the two expressions for average
vertical strain,
consolidation
settlement
initial thickness of
clay layer
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change in void ratio
H
e

Ho
1  eo
initial void ratio
10
Coefficient of volume compressibility
~ denoted by mv
~ is the volumetric strain in a clay element per
unit increase in stress
no units

changein volume
original volume
i.e.,
mv 
kPa-1 or MPa-1
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V
V

kPa or MPa
11
Consolidation Test
~ simulation of 1-D field consolidation in lab.
GL
porous stone
undisturbed soil
specimen
metal ring
Dia = 50-75 mm
(oedometer)
Height = 20-30 mm
field
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lab
12
Consolidation Test
loading in increments
allowing full consolidation before next increment
q1
q2
H1
eo- e1
H o eo
H1
e1 
(1  eo )
Ho
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e2 
13
Consolidation Test
unloading
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e – log v’ plot
- from the above data
void ratio
loading
v’ increases &
unloading
e decreases
v’ decreases &
e increases (swelling)
log v’
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Compression and recompression indices
Cr
void ratio
1
Cc ~ compression index
Cc
Cr ~ recompression index
(or swelling index)
Cr
1
1
log v’
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16
Preconsolidation pressure
void ratio
is the maximum
vertical effective
stress the soil
element has
ever been
subjected to
pressure
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p’
log v’
Virgin Consolidation Line
original
state
void ratio
eo
virgin consolidation line
eo, vo’
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vo’
p’
log v’
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Overconsolidation ratio (OCR)
original
state
vo’
Field
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void ratio
eo
virgin consolidation line
 p'
OCR 
 vo '
vo’
p’
log v’
Overconsolidation ratio (OCR)
VCL
OCR=1
~current state
void ratio
OCR=13
Normally
consolidated
clay
OCR=2
OCR=13
Heavily
overconsolidated clay
OCR=2
Slightly
overconsolidated clay
log v’
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More to come…
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Settlement computations
Two different ways to estimate the
consolidation settlement:
q kPa
(a) using mv
settlement = mv  H
H
=q
(b) using e-log v’ plot
next slide
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eo, vo’, Cc,
Cr, p’, mv
-oedometer
test
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e
settlem ent
H
1  eo
Settlement computations
~ computing e using e-log v’ plot
If the clay is normally consolidated,
the entire loading path is along the VCL.
initial
eo
 vo '  '
e  Cc log
 vo '
e
vo’
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vo’+ 
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Settlement computations
~ computing e using e-log v’ plot
If the clay is overconsolidated, and remains so by
the end of consolidation,
eo
 vo '  '
e  Cr log
 vo '
initial
e
note the use of Cr
VCL
vo’
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vo’+ 
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Settlement computations
~ computing e using e-log v’ plot
If an overconsolidated clay becomes normally
consolidated by the end of consolidation,
eo
 p'
 vo ' '
e  Cr log
 Cc log
 vo '
 p'
initial
e
VCL
vo’
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p’
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vo’+ 
Preloading
Preloading at West Kowloon Expressway, Hong Kong.
(5-10 m embankments for 2-5 months)
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Preloading
Piezometers measure pore pressures and thus
indicate when the consolidation is over.
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Preloading
Cross section of PVD
Installation
Prefabricated Vertical Drains to Accelerate Consolidation
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Prefabricated Vertical Drains
Installation of PVDs
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