1 consolidation of clays n. sivakugan duration: 17 minutes

1

Consolidation of Clays

N. Sivakugan

Duration: 17 minutes

2SIVA Copyright©2001

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).


What is Consolidation?

This leads to settlements occurring over a long time,

which could be several years.

time

sett

lem

en

t


In granular soils…

Granular soils are freely drained, and thus the settlement is instantaneous.

time

sett

lem

en

t


During consolidation…

Due to a surcharge q applied at the GL,

GL

saturated clay

q kPa

A

the stresses and pore pressures are increased at A.

u

’

..and, they vary with time.


During consolidation…

remains the same (=q) during consolidation.

GL

saturated clay

q kPa

A

u

’

u decreases (due to drainage)

u

’

q

transferring the load from water to the soil.

while ’ increases,


One Dimensional Consolidation

~ drainage and deformations are vertical

(none laterally)

saturated clay

GL

q kPa

~ a simplification for solving consolidation problems

reasonable simplification if the surcharge is of large lateral

extent

water squeezed out


H -e Relation

saturated clay

GL

q kPa

saturated clay

GL

q kPa

Ho

Time = 0+

e = eo

H

Time =

e = eo - e

average vertical strain = oH

H


H -e Relation

Consider an element where Vs = 1 initially.

e

1

eo

Time = 0+

Time =

average vertical strain = oe

e

1


H -e Relation

Equating the two expressions for average vertical strain,

oe

e

1

oH

H

consolidation settlement

initial thickness of clay layer

initial void ratio

change in void ratio


Coefficient of volume compressibility

~ denoted by mv

~ is the volumetric strain in a clay element per unit increase in stress

volumeoriginal

volumein change

VV

mvi.e.,

no units

kPa or MPakPa-1 or MPa-

1


Consolidation Test

~ simulation of 1-D field consolidation in lab.

field

GL

lab

undisturbed soil specimen

Dia = 50-75 mm

Height = 20-30 mm

metal ring

(oedometer)

porous stone


Consolidation Test

H1

q1

eo- e1

)1(11 o

o

eH

He

eoHo

q2

loading in increments

2e

allowing full consolidation before next increment


Consolidation Test

unloading


e – log v’ plot

log v’

void

rati

o loading

v’ increases &

e decreasesunloading

v’ decreases &

e increases (swelling)

- from the above data


Compression and recompression indices

log v’

void

rati

o

1

Cc

Cc ~ compression index

Cr ~ recompression index (or swelling

index)

1Cr

1

Cr

SIVA Copyright©2001

Preconsolidation pressure

log v’

void

rati

o

p’preconsolidation pressure

is the maximum

vertical effective

stress the soil element has ever been

subjected to


Virgin Consolidation Line

log v’

void

rati

ovirgin consolidation line

p’vo’

eo

eo, vo’

original state


Overconsolidation ratio (OCR)

log v’

void

rati

o

virgin consolidation line

p’vo’

eo

original state

Field

vo’

'

'

vo

pOCR


Overconsolidation ratio (OCR)

log v’

void

rati

o

OCR=1

OCR=2

OCR=2

OCR=13

OCR=13

~current state

VCL

Normally consolidate

d clay

Slightly overconsolidated clay

Heavily overconsolidated clay


More to come…


Settlement computations

eo, vo’, Cc, Cr, p’,

mv

-oedometer

test

=q

q kPa

H

Two different ways to estimate the consolidation settlement:

(a) using mv

(b) using e-log v’ plot

settlement = mv H

He

esettlement

o

1

next slide


Settlement computations~ computing e using e-log v’ plot

'

''log

vo

vocCe

initial

vo’

eo

vo’+

e

If the clay is normally consolidated,

the entire loading path is along the VCL.



'

''log

vo

vorCe

vo’

initialeo

vo’+

If the clay is overconsolidated, and remains so by the end of consolidation,

e

VCL

note the use of Cr



'

''log

'

'log

p

voc

vo

pr CCe

vo’

initialeo

vo’+

If an overconsolidated clay becomes normally consolidated by the end of consolidation,

VCL

p’

e


Preloading

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


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


Preloading

Prefabricated Vertical Drains to Accelerate Consolidation

Installation

Cross section of PVD


Prefabricated Vertical Drains

Installation of PVDs

1 consolidation of clays n. sivakugan duration: 17 minutes

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