consolidation sivakugan (complete soil mech. undestanding pakage: abhay)

1

Consolidation of Clays

N. Sivakugan

Duration: 17 minutes

SIVA Copyright©2001 2

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

settl

emen

t


In granular soils…

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

time

settl

emen

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.

∆σ

∆u

∆σ ’

..and, they vary with time.


During consolidation…

∆σ remains the same (=q) during consolidation.

GL

saturated clay

q kPa

A

∆σ

∆u

∆σ ’

∆u decreases (due to drainage)

∆σ

∆u

∆σ’

q

transferring the load from water to the soil. while ∆σ’ 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

GLq 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=

σ∆

∆= 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 σv’ plot

log σv’

void

ratio

loading

σv’ increases &

e decreasesunloading

σv’ decreases &

e increases (swelling)

- from the above data


Compression and recompression indices

log σv’

void

ratio

1

Cc

Cc ~ compression index

Cr ~ recompression index (or swelling index)

1C r

1Cr

SIVA Copyright©2001

Preconsolidation pressure

log σv’

void

ratio

σp’preconsolidation pressure

is the maximum vertical effective stress the soil element has ever been

subjected to


Virgin Consolidation Line

log σv’

void

ratio

virgin consolidation line

σp’σvo’

eo

eo, σvo’

original state


Overconsolidation ratio (OCR)

log σv’

void

ratio

virgin consolidation line

σp’σvo’

eo

original state

Field

σvo’'

'

vo

pOCRσσ

=


Overconsolidation ratio (OCR)

log σv’

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

eo, σvo’ , Cc, C r, σp’ , mv

-oedometer test

∆σ=q

q kPa

H

Two different ways to estimate the consolidation settlement:

(a) using mv

(b) using e-log σv’ plot

settlement = mv ∆σ H

He

esettlement

o+∆=1

next slide


Settlement computations~ computing ∆e using e-log σv’ plot

'

''log

vo

vocCe

σσσ ∆+=∆

initial

σvo’

eo

σvo’+ ∆σ

∆e

If the clay is normally consolidated,

the entire loading path is along the VCL.



'

''log

vo

vorCe

σσσ ∆+=∆

σvo’

initialeo

σvo’+ ∆σ

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

σσσ

σσ ∆++=∆

σvo’

initialeo

σvo’+ ∆σ

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

VCL

σp’

∆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

consolidation sivakugan (complete soil mech. undestanding pakage: abhay)

Engineering