Consolidation and Settlement

7/30/2019 Consolidation and Settlement

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CHAPTER 3

CONSOLIDATION AND SETTLEMENT


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LESSON OUTCOMES

At the end of this chapter, students should beable to:

Define consolidation and settlement

Define the types of settlement

Carry out the calculation of consolidation andsettlement rate.


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INTRODUCTION

If the loading on a soil mass increases it iscompressed, i.e. its volume is reduced.

Thus, under foundation loads layer thicknesses arereduced and settlement occurs.

In coarse soils this occurs instantly or as constructionproceeds, but in fine soils settlement may be veryslow and continue long after construction has ceased.


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INCREASE IN VERTICAL EFFECTIVE STRESS

Due to a Placement of a fill

Due to an external load

fillfillzzf H 0

inducedzzzf

0


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COMPACTION VS CONSOLIDATION

If the effective stress on a soil mass is increased areduction in volume (i.e. compression) occurs.

The volume of soil grains remains constant, so thechange in volume is due to a change in volume of

voids - either water or air or a mixture of the two isexpelled

Compaction is a process of volume change where onlyair expelled from dry or partialy saturated soils.

Consolidation is a process where only water isexpelled from saturated soils.


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WHAT IS CONSOLIDATION?

When a saturated clay is loaded externally,

saturated clay

GL

the water is squeezed out of the clay over along time (due to low permeability of theclay).


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WHAT IS CONSOLIDATION?

This leads to settlements occurring over a long time,

which could be several years.

time

settlement


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Skeletal Material

(incompressible)

Pore water

(incompressible)

Voids

Solid

Voids

Solid

Initial State

The consolidation process

Water+

Ds

Deformed State


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Deformation of saturated soil occurs by reduction of porespace & the squeezing out of pore water. The water canonly escape through the pores which for fine-grained soilsare very small

The consolidation process

Effective soil

skeleton spring

water

water squeezed out


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SETTLEMENT

The term Settlement refers to the verticaldownward displacement at the base of afoundation or other structure.

Immediate settlementPrimary Consolidation settlement

Secondary settlement or creep


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IMMEDIATE SETTLEMENT (RI OR SI)

Due to elastic deformation with no change inwater content

occurs rapidly during the application of load

quite a small quantity in dense sands/gravels andstiff/hard clays


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CONSOLIDATION SETTLEMENT (RC OR SC)

Consolidation settlement (rc or sc) due todecrease in voids volume as porewater issqueezed out of the soil

occurs slowly according to the permeability

only significant in clays and silts


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SECONDARY SETTLEMENT OR CREEP (RA OR SA)

due to gradual changes in the particulatestructure of the soil

occurs very slowly, long after consolidation iscompleted

most significant in soft organic soils and peats

Thus, final settlement, r =ri +rc +ra


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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.

DsDuDs

..and, theyvary with time.


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DURING CONSOLIDATION

remains the same (=q) during consolidation.

GL

saturated clay

q kPa

A

DsDuDs

u decreases (due to drainage)

u

q

transferring the load from water to the soil.

while increases,


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IN GRANULAR SOILS

Granular soils are freely drained, and thus thesettlement is instantaneous.

time

settlement


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ONE DIMENSIONAL CONSOLIDATION

~ drainage and deformations are vertical (none laterally)

saturated clay

GL

q kPa

~ a simplification for solving consolidation problems

reasonablesimplification if the

surcharge is of

large lateral extent

water squeezed out


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H -E RELATION

saturated clay

GL

q kPa

saturated clay

GL

q kPa

Ho

Time = 0+

e = eo

DH

Time =

e = eo - e

average vertical strain =

oH

H


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H -E RELATION

Consider an element where Vs = 1 initially.

e

1

eo

Time = 0+ Time =

average vertical strain =

oe

e

1


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H -E RELATION

Equating the two expressions for averagevertical strain,

oe

e

1

oH

H

consolidation

settlement

initial thickness of

clay layer

initial void ratio

change in 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

volumeoriginal

volumeinchange

V

V

mvi.e.,

no units

kPa or MPakPa-1 or MPa-1


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LABORATORY CONSOLIDATION TEST


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~ simulation of 1-D field consolidation in lab.

field

GL

lab

undisturbed soilspecimen

Dia = 50-75 mm

Height = 20-30 mm

metal ring

(oedometer)

porous stone


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PROCEDURE

The saturated specimen is usually 75 mm diameter and 15-20 mm thick,enclosed in a circular metal ring and sandwiched between porous stones.

Vertical static load increments are applied at regular time intervals (e.g. 12,24, 48 hr.).

The load is doubled with each increment up to the required maximum (e.g.

25, 50, 100, 200, 400, 800 kPa). During each load stage thickness changes are recorded against time. After full consolidation is reached under the final load, the loads are

removed (in one or several stages - to a low nominal value close to zero - andthe specimen allowed to swell, after which the specimen is removed and itsthickness and water content determined.

With a porous stone both above and below the soil specimen the drainage

will be two-way (i.e. an open layer in which the drainage path length, d =H/2)


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CONSOLIDATION TEST

H1

q1

eo-e1

)1(11 o

o

eH

He

eoHo

q2

loading in increments

2

e

allowing full consolidation before next increment


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CONSOLIDATION TEST

unloading


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CONSOLIDATION

PLOT


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E LOGV PLOT

log v

void

ratio loading

v increases &

e decreasesunloading

v decreases &

e increases (swelling)

- from the above data


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COMPRESSION AND RECOMPRESSION

INDICES

log v

void

ratio

1

Cc

Cc ~ compression index

Cr ~ recompression index

(or swelling index, Cc)

1Cr

1

Cr


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TEST RESULTS


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TEST RESULTSIdealized Data


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PRECONSOLIDATION PRESSURE

(P OR PC)

log v

vo

idratio

ppreconsolidation pressure

is the maximumvertical effectivestress the soilelement has

ever beensubjected to


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VIRGIN CONSOLIDATION LINE

log v

voidratio

virgin consolidation line

pvo

eo

eo, vo

originalstate


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DETERMINATION OF (P OR PC)

1. e-logpis established by labtesting

2. determine point aat which e-logphas minimum radius ofcurvature

3. Draw horizontal line froma(lineab)

4. Draw tangent to curve at a(lineac)

5. Draw line adto bisect angle bac6. Project the straight-line portion

ofghback to intersect adat f7. Abscissa of point fis the

preconsolidation pressure, pc


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OVERCONSOLIDATION RATIO (OCR)

log v

voidratio

virgin consolidation line

pvo

eo

originalstate

Field

vo

'

'

vo

pOCR


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OVERCONSOLIDATION RATIO (OCR)

log v

voidratio

OCR=1

OCR=2

OCR=2

OCR=13

OCR=13

~current state

VCL

Normally

consolidatedclay

Slightlyoverconsolidated clay

Heavilyoverconsolidated clay


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STRESS HISTORY OF CLAY

Normal consolidated: The present effectiveoverburden pressure is the maximum pressureto which the soil has been subjected in thepast.

Overconsolidated: The present effectiveoverburden pressure is less than that which thesoil has experienced in the past. The maximumeffective past pressure is called the

preconsolidation pressure.


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SETTLEMENT COMPUTATIONS

eo, svo, Cc,Cr, sp, mv-oedometer

test

=q

q kPa

H

Two different ways to estimate theconsolidation settlement:

(a) using mv

(b) using e-log v plot

settlement, S = mv H

He

eSsettlement

o

1

,

next slide


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~ computing mv

k = cvmvgw

where k = coefficient of permeability


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~ 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 theVCL.

'

''log

1,

vo

vo

o

c

e

HCSsettlement


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'

''log

vo

vorCe

vo

initialeo

vo+

If the clay is overconsolidated, and remains so bythe end of consolidation, vo+ p

e

VCLnote the use of Cr


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'''log

''log

p

voc

vo

p

r CCe

vo

initialeo

vo+

If an overconsolidated clay becomes normallyconsolidated by the end of consolidation, vo+ p

VCL

p

e


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TIME RATE OF CONSOLIDATION

Terzaghi (1925) proposed the first theory to considerthe rate of one-dimensional consolidation forsaturated clay soils.

Derivation assumptions

1. Homogeneous clay-water system2. Saturated3. Water and soil grains are incompressible4. Flow of water is unidirectional and in the direction

of consolidation

5. Darcy's law assumed


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RATE OF SETTLEMENT

When soil is loaded undrained, the pore pressuresincrease. Under site conditions, the excess porepressures will dissipate and water leaves the soil,resulting in consolidation settlement.

This process takes time, and the rate of settlementdecreases over time.

The rate at which settlement occurs depends on boththe permeability and compressibility of the soil.


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TERZAGHI'S 1D CONSOLIDATION THEORY

The process of consolidation isabsolutely linked to the dissipation ofexcess pore pressures. Therefore

Terzaghi's equation relates the excesspore pressure at depth z with time, t.

u = excess pore pressure at time t and depth zcv

= coefficient of consolidation a soil parameter (m2/yr)

where


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TIME FACTOR

where d = drainage path length

= H for one-way drainage = H/2 for two-way drainage

2d

tcT

vv


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AVERAGE DEGREE OF

CONSOLIDATION, U

The average degree of consolidation for theentire depth of the clay layer at any time can bewritten as:

%)100log(933.0781.1

)100

%(

4

2

UTv

UTv

S

SU

t

For U = 0% to 60%,

For U =>60%,


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VARIATION OF TV AND U%


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DETERMINATION OF CV FROM OEDOMETER TEST

RESULTS

The coeficient of consolidation, cv, generallydecrease as the liquid limit of soil increase.

Two graphic methods for determining cv from

laboratory 1-D consolidation test:Square-Root-of-Time method

Casagrande or Log-Time method


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SQUARE-ROOT-OF-TIME METHOD

Draw a line AB through theearly portion of the curve.

Draw a line AC such that OC= 1.15OB. The abscissa of

point D, which is theintersection of AC and theconsolidation curve, givesthe square root of time for90% consolidation (t90)

For 90% consolidation, T90

= 0.848, so

2

9090

d

tcT

v

90

2848.0

t

dcv


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CASAGRANDE OR LOG-TIME METHOD

Extend the straight-line portions ofprimary consolidation andsecondary consolidaions tointersect A. (100% primaryconsolidation)

Select times t1 and t2 on the initialcurved portion such that t2 = 4t1.Find x.

Draw horizontal straight line DE sothat vertical distance DE = verticaldistance BC = x. (0% consolidation)

Find d50 and t50 corresponding toit.

For 50% average degree ofconsolidation, Tv = 0.197.

2

5050

d

tcT

v

50

2197.0

t

dcv


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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 thusindicate when the consolidation is over.


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PRELOADING

Prefabricated Vertical Drains to Accelerate Consolidation

Installation

Cross section of PVD


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PREFABRICATED VERTICAL DRAINS

Installation of PVDs

Date post:	14-Apr-2018
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Consolidation and Settlement

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