Consolidation of Soil

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Consolidation of SoilDR. M. Zayed

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Consolidation of Soil

SIVA

Copyright2001

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Consolidation of Soil 1002Copyright3

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Consolidation of Soil * : . * . * . * . * . * . * . * (). SIVA1002Copyright4

Types of ground movements

Compressibility

.The volume of soil mass is decreased under stress this decrease is known as Compression, and the copacity of soil to decrease in volume under stress .is known as compressibility . () . .5

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Mechanical modeling of the consolidation process10kg 10kgWater out

piston Water Spring(a)

Stop cock

10kgWater out

10kgWater out

10kg

(b)

(c)

(d)

(e)

(f)

Spring load (kg): Water load (kg): Consolidation %:

0 10 0

2.5 7.5 25

5 5 50

7.5 2.5 75

10 0 1006

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ConsolidationWhen soil is loaded undrained, the pore pressures increase. Then, under site conditions, the excess pore pressures dissipate and water leaves the soil, resulting in consolidation settlement. This process takes time, and the rate of settlement decreases over time.eo 1

Time = 0+

Time = 7

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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(. SIVACopyright20018

What is Consolidation?This leads to settlements occurring over a long time,settlemen t

time

which could be several years.

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In granular soilsGranular soils are freely drained, and thus the settlement is instantaneous.settlemen t

time

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During consolidationDue to a surcharge q applied at the GL, the stresses and pore pressures are increased at A.q kPa GL

..and, they vary with time.

Asaturated clay

u -

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During consolidation remains the same )=q( during consolidation. u decreases )due to while drainage( the load from water to the soil. increases, transferringq kPa saturated clay GL u

A

u q

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One Dimensional Consolidation drainage and deformations are )none vertical laterally( a simplification for solving consolidation q kPa problemsGL

water squeezed out

saturated clay

reasonable simplification if the surcharge is of large lateral extent

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H -e Relationaverage vertical strain =q kPa GL H q kPa GL

H Ho

saturated clay e=eo Time = 0+

Ho

saturated clay= eo - e eTime = 14

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H -e RelationConsider an element where Vs = 1 initially.eo e

1

Time = 0+

Time =

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e average vertical strain 1 + eo =

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H -e RelationEquating the two expressions for average vertical strain, consolidati on settlement initial thickness of clay layer

H e = Ho 1 + eo

change in void ratio

initial void ratio

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Definitions

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Coefficient of compressibility denoted by av is the ratio of change in void ratio to the corresponding chang in stress e e0 e1

=

Change void change in volume

original stress Changevolume

aSIVACopyright2001

mv =

V e

no units

V0 1 18

Coefficient of volume compressibility denoted by mv is the volumetric strain per unit increase in stress

=

change in volume original volume

V mv =

V

av = 1+e0

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Compression index C c void ratio - e e = e0 - Cc log10 [ / 0]1.0

0.6

straight line phase pressure kNm-2100 1000

e, log10 cc e0-e1 -e Cc = = log10 [ / 0] Log SIVACopyright200120

Casagrande tangent to max curvature1.0

C P

S

line from tangent bisector

Q

T B0.6

R

straight line phase lab. virgin curve

p0

100

pc

1000

pressure kNm-2

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Casagrande : 1- e - log 2- p ( ). 3- PQ P .PT 4- QPT .PR 5- CB S . PCSIVA1002Copyright22

Casagrande : Normally consolidated clay0=PC . Preconsolidated clay 0 PC> . - underconslidsted clay 0 PC0.1 0.1 0.02 0.02 0.005 0.005 0.002