Consolidation Settlement

When a saturated soil is loaded externally,

What is Consolidation?

saturated clay

GL

the water is squeezed out of the soil and the soil shrinks, over a long time may be up to several years depending upon the permeability of the soil this whole phenomena is called consolidation.

As the time passes the water comes out of the soil voids and so the volume of soil decreases, this is the cause of the settlement.

Concept of Consolidation

The settlement is directly proportional to the decrease in volume of soil= The volume of squeezed out water

Settlement is the vertically downward movement of structure due to the shrinkage in volume of soil due to consolidation.

Settlement due to consolidation

He

esettlement

o

1

This leads to settlements occurring over a long time,

Settlement vs. Time

End of Primary Consolidation

Logarithm of Time

Se

ttle

me

nt

Cα

which could be several years, depending upon the permiability of the soil.

• It occurs due to the process of consolidation.• Clay and Organic soil are most prone to

consolidation settlement.• Consolidation is the process of reduction in volume

due to expulsion of water under an increased load.• It is a time related process occurring in saturated

soil by draining water from void.• It is often confused with Compaction.• Consolidation theory is required to predict both rate

and magnitude of settlement.

Consolidation Settlement

Since water flows out in any direction, the process is three dimensional.

But, soil is confined laterally. Hence, vertical one dimensional consolidation theory is acceptable.

Spring analogy explains consolidation settlement.

Permeability of soil influences consolidation.

Equating the two expressions for average vertical strain,

Settlement-Void ratio Relation

oe

e

1

oH

H

consolidation settlement

initial thickness of clay layer

initial void ratio

change in void ratio

Settlement computations

=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

Settlement by Mv Method

~co-eff of volume change=mv

~volumetric strain in a soil element per unit increase in stress

volumeoriginal

volumein change

VV

mvi.e.,

no units

kPa or MPakPa-1 or MPa-

1

Virgin Consolidation Line

log v’

void

rati

ovirgin consolidation line

p’

original state

vo’

eo

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

'

''log

vo

vocCe

initial

vo’

eo

vo’+

e

If the soil is normally consolidated,

the entire loading path is along the virgin compression line.

Settlement computations

'

''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

Practical Applications

Prefabricated Vertical Drains to Accelerate Consolidation

Installation

Cross section of PVD

Consolidation Test

Increment of load

Topcap

porous stonesample

water confiningring

settlementdial gauge

At any degree of consolidation, comparison in time taken for consolidation between laboratory & field with respect to drainage path is    

t = Time takend = Drainage path

2

field

lab

field

lab

d

d

t

t

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