Effective Stress.ppt

8/17/2019 Effective Stress.ppt

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Effective Stress

• Soil is a multi phase system

• To perform any kind of analysis - we must

understand stress distribution

• The concept of effective stress:

• The soil is “loaded” footin! for e"ample#

• The resultin! stress is transmitted to the

soil mass

•  The soil mass supports those stresses at the

 point to point contacts of the individual soil

!rains

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  $

Effective Stress

%ater 

Soil

&

'&

'

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Effective Stress

The total stress at & is calculated from:The wei!ht of the soil above &

The wei!ht of the water above &

  ) 'w * '& - '# sat

  ) Total Stress at &

w  ) +nit %ei!ht of %ater ,$. pcf#

sat  ) Saturated +nit %ei!ht

'&  ) 'ei!ht of & to Top of water 

' ) 'ei!ht of water 

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Effective Stress

•   is the stress applied to the soil by its own

wei!ht

• &s you !o deeper in the soil mass/ the stress

increases

• 0ike in a swimmin! pool/ as you !o deeper/ the

stress of the wei!ht of the water increases

• The soil carries the stress in $ ways:

• & portion is carried by the water acts

eually in all directions#

• & portion is carried by the soil solids at

their point of contact

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Effective Stress•  The sum of the vert components of the forces at

their points of contact per unit of 3- sectional area isthe effective stress

4ross Sectional &rea ) &

a$

51

a( a

.a1

5$

  5(   5.

an ) &rea of points of contact

& ) 4ross Sectional area of soil mass5n ) 6orces actin! at points of contact

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Effective Stress• The sum of vertical components of forces over the area is

the effective stress 7

7) 51v*5$v*5(v *5nv# 8 &

9f as ) a1 * a$ * a( *an

Then the space occupied by water ) & - as

&ssume u ) '&  w 't of water w ) stress due

to water ftpcf ) psf ) stress#

 ) 7 * u& - as# 8 & Since as is very small/assume ;

 ) 7 * u

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  <

Effective Stress

=ecall the followin! euation:

 ) 'w * '& - '# sat

 >ow/ 7 )  - u

Substitutin!: 7 ) ?'w * '& - '# sat@ - '&  w

=earran!in!: 7 ) '& - '#sat - w#

'ei!ht of Soil 4olumn 7 %here 7 ) sat - w

7 ) EFFECTIVE UNIT WEIGHT

Effective Stress is independent of hei!ht of water 

9n the euation:  ) 7 * u7 is the soil skeleton stress

u is the stress in the water/ or porewater ressure