Consolidation of Clays N. Sivakugan Duration: 17 minutes
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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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In granular soils…
settlement
Granular soils are freely drained, and thus the settlement is instantaneous.
time
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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
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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
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H -e Relation H average vertical strain = Ho q kPa
GL
saturated clay e = eo
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Time = 0+ Copyright©2001
H
q kPa GL
Ho
saturated clay e = eo - e Time =
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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
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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 1 eo Ho initial void ratio
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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 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
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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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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’
Virgin Consolidation Line original state
void ratio
eo
virgin consolidation line
eo, vo’
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vo’
p’
log v’ 18
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 Copyright©2001
e settlement 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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