Consolidation sivakugan (Complete Soil Mech. Undestanding Pakage: ABHAY)

1
Consolidation of Clays
N. Sivakugan
Duration: 17 minutes

What is Consolidation?
When a saturated clay is loaded externally,
saturated clay
GL
the water is squeezed out of the clay over a long time
(due to low permeability of the clay).
SIVA Copyright©2001 2

What is Consolidation?
This leads to settlements occurring over a long time,
time settlement
which could be several years.

In granular soils…
Granular soils are freely drained, and thus the
settlement is instantaneous.
time
settlement

During consolidation…
Due to a surcharge q applied at the GL,
the stresses and pore pressures are increased at A.
GL
q kPa
A
Ds
Du
Ds’
saturated clay
..and, they vary
with time.

During consolidation…
Ds remains the same (=q) during consolidation.
while Ds’ increases,
transferring the load from water to the soil.
GL
Du decreases (due to drainage)
q kPa
A
Ds
Du
Ds’
saturated clay
Ds
Du
Ds’
q

One Dimensional Consolidation
~ drainage and deformations are vertical (none laterally)
~ a simplification for solving consolidation problems
q kPa
saturated clay
GL
reasonable
simplification if the
surcharge is of
large lateral extent
water squeezed out

DH -De Relation
q kPa
saturated clay
GL
saturated clay
e = eo
e = eo - De
DH
GL
q kPa
Ho
Time = 0+
DH
Time = µ
average vertical strain =
o H

DH -De Relation
Consider an element where Vs = 1 initially.
De
eo
1
Time = 0+ Time = ¥
average vertical strain =
e
+
o e
D
1

DH -De Relation
Equating the two expressions for average
vertical strain,
e
+
o e
D
1
H
D =
o H
consolidation
settlement
initial thickness of
clay layer
change in void ratio
initial void ratio

Coefficient of volume compressibility
~ denoted by mv
~ is the volumetric strain in a clay element per
unit increase in stress
= change in volume
original volume
V
D
= V
Ds
mv
i.e.,
no units
kPa-1 or MPa-1 kPa or MPa

Consolidation Test
~ simulation of 1-D field consolidation in lab.
field
GL
lab
undisturbed soil
specimen
Dia = 50-75 mm
Height = 20-30 mm
porous stone
metal ring
(oedometer)

Consolidation Test
allowing full consolidation before next increment
DH1
loading in increments
Dq1
eo- De1
e
De = DH +
1 (1 )
1 o
o
H
Ho eo
Dq2
D = 2 e

Consolidation Test
unloading

e – log sv’ plot
loading
sv’ increases &
e decreases
log sv’
void ratio
- from the above data
unloading
sv’ decreases &
e increases (swelling)

Compression and recompression indices
Cc ~ compression index
log sv’
1
C
r
void ratio 1
Cc
Cr ~ recompression index
(or swelling index)
1
Cr

Preconsolidation pressure
SIVA Copyright©2001
log sv’
void ratio
sp’
preconsolidation
pressure
is the maximum
vertical effective
stress the soil
element has
ever been
subjected to

Virgin Consolidation Line
virgin consolidation line
log sv’
eo
void ratio
original
state
s sp’ vo’
eo, svo’

Overconsolidation ratio (OCR)
svo’
log sv’
void ratio
virgin consolidation line
'
s
=
OCR p
s
s sp’ vo’
eo
original
state
Field
'
vo

Overconsolidation ratio (OCR)
log sv’
void ratio
OCR=1
OCR=2
OCR=2
OCR=13
OCR=13
~current state
VCL
Normally
consolidated
clay
Slightly
overconsolidated
clay
Heavily
overconsolidated
clay

More to come…

Settlement computations
q kPa
Ds=q
eo, svo’, Cc,
Cr, sp’, mv
-oedometer
test
H
Two different ways to estimate the
consolidation settlement:
(a) using mv
settlement = mv Ds H
(b) using e-log sv’ plot
settlement e
H
e
= D
1
o +
next slide

~ computing De using e-log sv’ plot
If the clay is normally consolidated,
the entire loading path is along the VCL.
D = s +Ds
log ' '
'
vo
vo
c e C
s
initial
svo’
eo
svo’+ Ds
De

If the clay is overconsolidated, and remains so by
the end of consolidation,
initial
D = s +Ds
log ' '
'
vo
vo
r e C
s
svo’
eo
svo’+ Ds
De
VC
L
note the use of Cr

If an overconsolidated clay becomes normally
consolidated by the end of consolidation,
initial
s +D D = +
s s
log ' '
'
'
r e C C
'
log
p
vo
c
p
vo
s
s
svo’
eo
VC
L
svo’+ Ds
sp’
De

Preloading
Preloading at West Kowloon Expressway, Hong Kong.
(5-10 m embankments for 2-5 months)

Preloading
Piezometers measure pore pressures and thus
indicate when the consolidation is over.

Preloading
Installation
Cross section of PVD
Prefabricated Vertical Drains to Accelerate Consolidation

Prefabricated Vertical Drains
Installation of PVDs

Consolidation sivakugan (Complete Soil Mech. Undestanding Pakage: ABHAY)

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