Consolidation of Clays N. Sivakugan Duration: 17 minutes What is Consolidation? When a saturated clay is loaded externally, GL saturated clay the water is squeezed out.
Download ReportTranscript Consolidation of Clays N. Sivakugan Duration: 17 minutes What is Consolidation? When a saturated clay is loaded externally, GL saturated clay the water is squeezed out.
Slide 1
Consolidation of Clays
N. Sivakugan
Duration: 17 minutes
1
Slide 2
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).
SIVA
Copyright©2001
2
Slide 3
What is Consolidation?
settlement
This leads to settlements occurring over a long time,
time
which could be several years.
SIVA
Copyright©2001
3
Slide 4
In granular soils…
settlement
Granular soils are freely drained, and thus the
settlement is instantaneous.
time
SIVA
Copyright©2001
4
Slide 5
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
SIVA
Copyright©2001
5
Slide 6
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
Copyright©2001
’
u
’
saturated clay
SIVA
GL
q
6
Slide 7
One Dimensional Consolidation
~ drainage and deformations are vertical (none laterally)
~ a simplification for solving consolidation problems
q kPa
GL
water squeezed out
saturated clay
SIVA
Copyright©2001
reasonable
simplification if the
surcharge is of
large lateral extent
7
Slide 8
H -e Relation
average vertical strain =
H
Ho
q kPa
GL
saturated clay
e = eo
SIVA
Time = 0+
Copyright©2001
H
q kPa
GL
Ho
saturated clay
e = eo - e
Time =
8
Slide 9
H -e Relation
Consider an element where Vs = 1 initially.
e
eo
1
Time = 0+
Time =
average vertical strain =
SIVA
Copyright©2001
e
1 eo
9
Slide 10
H -e Relation
Equating the two expressions for average
vertical strain,
consolidation
settlement
change in void ratio
H
Ho
initial thickness of
clay layer
SIVA
Copyright©2001
e
1 eo
initial void ratio
10
Slide 11
Coefficient of volume compressibility
~ denoted by mv
~ is the volumetric strain in a clay element per
unit increase in stress
no units
change in volume
original
V
volume
i.e.,
mv
kPa-1 or MPa-1
SIVA
Copyright©2001
V
kPa or MPa
11
Slide 12
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
SIVA
Copyright©2001
lab
12
Slide 13
Consolidation Test
loading in increments
allowing full consolidation before next increment
q1
q2
H1
eo- e1
H o eo
e1
SIVA
H 1
Ho
Copyright©2001
(1 e o )
e2
13
Slide 14
Consolidation Test
unloading
SIVA
Copyright©2001
14
Slide 15
e – log v’ plot
- from the above data
void ratio
loading
v’ increases &
unloading
e decreases
v’ decreases &
e increases (swelling)
log v’
SIVA
Copyright©2001
15
Slide 16
Compression and recompression indices
Cr
void ratio
1
Cc ~ compression index
Cc
Cr ~ recompression index
(or swelling index)
Cr
1
1
log v’
SIVA
Copyright©2001
16
Slide 17
Preconsolidation pressure
void ratio
is the maximum
vertical effective
stress the soil
element has
ever been
subjected to
pressure
SIVA preconsolidation
Copyright©2001
p’
log v’
Slide 18
Virgin Consolidation Line
original
state
void ratio
eo
virgin consolidation line
eo, vo’
SIVA
Copyright©2001
vo’
p’
log v’
18
Slide 19
Overconsolidation ratio (OCR)
original
state
virgin consolidation line
vo’
Field
SIVA
Copyright©2001
void ratio
eo
OCR
vo’
p'
vo '
p’
log v’
Slide 20
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’
SIVA
Copyright©2001
Slide 21
More to come…
SIVA
Copyright©2001
21
Slide 22
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
SIVA
eo, vo’, Cc,
Cr, p’, mv
-oedometer
test
Copyright©2001
settlement
e
1 eo
H
Slide 23
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
e
e C c log
vo’
SIVA
vo’+
Copyright©2001
vo ' '
vo '
Slide 24
Settlement computations
~ computing e using e-log v’ plot
If the clay is overconsolidated, and remains so by
the end of consolidation,
e C r log
eo
initial
e
note the use of Cr
VCL
vo’
SIVA
vo ' '
vo’+
Copyright©2001
vo '
Slide 25
Settlement computations
~ computing e using e-log v’ plot
If an overconsolidated clay becomes normally
consolidated by the end of consolidation,
e C r log
eo
initial
e
VCL
vo’
SIVA
p’
Copyright©2001
vo’+
p'
vo '
C c log
vo ' '
p'
Slide 26
Preloading
Preloading at West Kowloon Expressway, Hong Kong.
(5-10 m embankments for 2-5 months)
SIVA
Copyright©2001
26
Slide 27
Preloading
Piezometers measure pore pressures and thus
indicate when the consolidation is over.
SIVA
Copyright©2001
27
Slide 28
Preloading
Cross section of PVD
Installation
Prefabricated Vertical Drains to Accelerate Consolidation
SIVA
Copyright©2001
28
Slide 29
Prefabricated Vertical Drains
Installation of PVDs
SIVA
Copyright©2001
29
Consolidation of Clays
N. Sivakugan
Duration: 17 minutes
1
Slide 2
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).
SIVA
Copyright©2001
2
Slide 3
What is Consolidation?
settlement
This leads to settlements occurring over a long time,
time
which could be several years.
SIVA
Copyright©2001
3
Slide 4
In granular soils…
settlement
Granular soils are freely drained, and thus the
settlement is instantaneous.
time
SIVA
Copyright©2001
4
Slide 5
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
SIVA
Copyright©2001
5
Slide 6
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
Copyright©2001
’
u
’
saturated clay
SIVA
GL
q
6
Slide 7
One Dimensional Consolidation
~ drainage and deformations are vertical (none laterally)
~ a simplification for solving consolidation problems
q kPa
GL
water squeezed out
saturated clay
SIVA
Copyright©2001
reasonable
simplification if the
surcharge is of
large lateral extent
7
Slide 8
H -e Relation
average vertical strain =
H
Ho
q kPa
GL
saturated clay
e = eo
SIVA
Time = 0+
Copyright©2001
H
q kPa
GL
Ho
saturated clay
e = eo - e
Time =
8
Slide 9
H -e Relation
Consider an element where Vs = 1 initially.
e
eo
1
Time = 0+
Time =
average vertical strain =
SIVA
Copyright©2001
e
1 eo
9
Slide 10
H -e Relation
Equating the two expressions for average
vertical strain,
consolidation
settlement
change in void ratio
H
Ho
initial thickness of
clay layer
SIVA
Copyright©2001
e
1 eo
initial void ratio
10
Slide 11
Coefficient of volume compressibility
~ denoted by mv
~ is the volumetric strain in a clay element per
unit increase in stress
no units
change in volume
original
V
volume
i.e.,
mv
kPa-1 or MPa-1
SIVA
Copyright©2001
V
kPa or MPa
11
Slide 12
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
SIVA
Copyright©2001
lab
12
Slide 13
Consolidation Test
loading in increments
allowing full consolidation before next increment
q1
q2
H1
eo- e1
H o eo
e1
SIVA
H 1
Ho
Copyright©2001
(1 e o )
e2
13
Slide 14
Consolidation Test
unloading
SIVA
Copyright©2001
14
Slide 15
e – log v’ plot
- from the above data
void ratio
loading
v’ increases &
unloading
e decreases
v’ decreases &
e increases (swelling)
log v’
SIVA
Copyright©2001
15
Slide 16
Compression and recompression indices
Cr
void ratio
1
Cc ~ compression index
Cc
Cr ~ recompression index
(or swelling index)
Cr
1
1
log v’
SIVA
Copyright©2001
16
Slide 17
Preconsolidation pressure
void ratio
is the maximum
vertical effective
stress the soil
element has
ever been
subjected to
pressure
SIVA preconsolidation
Copyright©2001
p’
log v’
Slide 18
Virgin Consolidation Line
original
state
void ratio
eo
virgin consolidation line
eo, vo’
SIVA
Copyright©2001
vo’
p’
log v’
18
Slide 19
Overconsolidation ratio (OCR)
original
state
virgin consolidation line
vo’
Field
SIVA
Copyright©2001
void ratio
eo
OCR
vo’
p'
vo '
p’
log v’
Slide 20
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’
SIVA
Copyright©2001
Slide 21
More to come…
SIVA
Copyright©2001
21
Slide 22
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
SIVA
eo, vo’, Cc,
Cr, p’, mv
-oedometer
test
Copyright©2001
settlement
e
1 eo
H
Slide 23
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
e
e C c log
vo’
SIVA
vo’+
Copyright©2001
vo ' '
vo '
Slide 24
Settlement computations
~ computing e using e-log v’ plot
If the clay is overconsolidated, and remains so by
the end of consolidation,
e C r log
eo
initial
e
note the use of Cr
VCL
vo’
SIVA
vo ' '
vo’+
Copyright©2001
vo '
Slide 25
Settlement computations
~ computing e using e-log v’ plot
If an overconsolidated clay becomes normally
consolidated by the end of consolidation,
e C r log
eo
initial
e
VCL
vo’
SIVA
p’
Copyright©2001
vo’+
p'
vo '
C c log
vo ' '
p'
Slide 26
Preloading
Preloading at West Kowloon Expressway, Hong Kong.
(5-10 m embankments for 2-5 months)
SIVA
Copyright©2001
26
Slide 27
Preloading
Piezometers measure pore pressures and thus
indicate when the consolidation is over.
SIVA
Copyright©2001
27
Slide 28
Preloading
Cross section of PVD
Installation
Prefabricated Vertical Drains to Accelerate Consolidation
SIVA
Copyright©2001
28
Slide 29
Prefabricated Vertical Drains
Installation of PVDs
SIVA
Copyright©2001
29