Bearing Capacity Ofsoil

8/9/2019 Bearing Capacity Ofsoil

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earing Capacity Of Shallow

Foundation


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Bearing Capacity Of Shallow Foundation

* A foundation is required for distributing

the loads of the superstructure on a largearea.*The foundation should be designed

such that


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Basic "efinitions #

$)%lti!ate Bearing Capacity &qu)#

The ulti!ate bearing capacity is thegross pressure at the base of thefoundation at which soil fails in shear.

')(et ulti!ate Bearing Capacity &qnu) #


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3) Net Safe Bearing Capacity (qns) :

It is the net soil pressure which can be

safely applied to the soil considering only shear

failure. hus! qns " qnu #$%S

$%S & $actor of safety usually ta'en as . &3.


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/)(et Safe Settle!ent 0ressure &qnp) #t is the net pressure which the soil cancarry without e1ceeding allowablesettle!ent.

6) Net Allowable Bearing Pressure (qna ):

It is the net bearing pressure which can be

used for design of foundation.


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2odes of shear Failure #

3esic &$456) classified shear failure ofsoil under a foundation base into threecategories depending on the type ofsoil location of foundation.$)7eneral Shear failure.')8ocal Shear failure.

3) 0unching Shear failure


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7eneral Shear failure ,

Strip footing resting on surface 1oad 2settle,ent cure

of dense sand or stiff clay

*

he load & Settle,ent cure in case of footing resting on surface of dense sandor stiff clays shows pronounced pea' 4 failure occurs at ery s,all stain.

* 5 loaded base on such soils sin's or tilts suddenly in to the ground showing a

surface heae of ad6oining soil

*

he shearing strength is fully ,obili7edall along the slip surface 4 hencefailure planes are well defined.

*he failure occurs at ery s,all ertical strains acco,panied by large lateral

strains.

*

I08 9 !N83! ; 8 39

! e < .


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') 8ocal Shear failure 9

* :hen load is equal to a certain alue qu&$)

=The foundation !oe!ent is acco!panied by sudden ;er


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6) 0unching Share failure 9

=The loaded base sin


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Ter=aghis Bearing Capacity Analysis ,er7aghi (?@*3) analysed a shallow continuous footing by

,a'ing so,e assu,ptions 2


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=The failure =ones do not e1tend aboe thehori=ontal plane passing through base of footing

=The failure occurs when the down ward pressuree1erted by loads on the soil ad;oining the inclinedsurfaces on soil wedge is equal to upwardpressure.

="ownward forces are due to the load &+quD B) the weight of soil wedge &$E -B'tanG)

=%pward forces are the ertical co!ponents of

resultant passie pressure &0p) the cohesion &c)acting along the inclined surfaces.


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For equilibriu!#

HF + ?$ - B'tan I J qu1B + '0p J'C D 8i sinI

where 8i + length of inclined surface CB & + BE' EcosI)

Therefore


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quA B" (p)r(p)

c(p)

qD BcEtanFE&G/ BtanFE

SubstitutingH (p)r & GrBtanF? " B A / BNr

(p)q " B A / 0Nq 4 (p)c Bc?tanF? " B A C?NcH

Je get!

qu "CENc / 0f Nq . / B N /


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I 7eneral Shear Failure 8ocal Shear Failure

(c (q (r (c (q (r

? /.5 $.? ?.? /.5 $.? ?.?

$/ $'.4 . './ 4.5 '.5 ?.4

/ $5'.6 $56.6 '45./ /$.' 6/.$ 65.5


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I,portant points :

= er7aghiEs Bearing Capacity equation is applicable

for general shear failure.

=er7aghi has suggested following e,pirical reduction to

actual c 4 F in case of local shear failure >obilised cohesion C, " #3 C

>obilised angle of F, " tan 2?(KtanF)

hus! Nc

E

!Nq

E

4 Nr

E

are B.C. factors for local shear failure


16/46

Lffect of water table on BearingCapacity #

= The equation for ulti!ate bearing

capacity by Ter=aghi has beendeeloped based on assu!ption thatwater table is located at a great depth .

=f the water table is located close to


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i) :hen water table is located aboe the base offooting 9

* The effectie surcharge is reduced as the


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Thus

qu + c(c J N-

sub"f J&- 9 -

sub)"w (q J ?./ -

subB(r

:hen "w +?

qu +c(c J -sub

(c J ?./ -sub

B(r

when 1 + ?


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ii) :hen water table is located at depth y below base #

*Surcharge ter! is not affected.*%nit weight in ter! is - + -

subJ y & - , -

sub)

B Thus

qu + c(c J -"f (q J ?./B -(r

:hen y + B M :.T. at B below base of footing.

qu + c(c J - "f (q J ?./ B - (r

Pence when ground water table is at b Q B the equation is not


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Pansens Bearing Capacity Lquation#

Pansens Bearing capacity equation is #

qu + c(cScdcic J q(qSqdqiq J ?./ - B(rSrdr irwhere(c(q (r are Pansens B.C factors which areso!e what s!aller than Ter=aghis B.C. factors.Sc.Sq Sr are shape factors which are

independent of angle of shearing resistanceM


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The sa!e for! of equation has beenadopted by .S. >?6 ,$45$ !ay be usedfor general for! as

qnu + c (c Sc dc ic J q&(q9$)Sqdqiq J ?./ - B(rSrdr ir :


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Settle!ent of foundation:

a) Settle!ent under loadsSettle!ent of foundation can be classified as9$.Llastic settle!ent &Si)#Llastic or i!!ediatesettle!ent ta


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ThusTotal settle!ent &s) + SiJ Sc J Ssb) Settle!ent due to other causes$. Structural collapse of soil.

'. %nderground erosion.


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Llastic settle!ent of foundation #a) On Cohesie soilsAccording to schleicher the ertical settle!entunder unifor!ly distributed fle1ible area is

Si " q B ?& M#s I

where

q 9unifor!ly distributed load.B 9 characteristic len th of loaded area


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b) On Cohesionless SoilsAccording to Stuart!ann Part!an i!!ediatesettle!ent on Cohesionless soils is gien by 9

:here C$9Correction factor for depth of foundation

( ) =

=ZB

Z S

iE

IqqCCS

0

221


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in which qC 9 static cone resistance

?

9!ean effectie oerburden pressure

U 9 ncrease is pressure at center of layer

due to net foundation pressure. P 9 thic


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0ifferential Settle,ent := he difference between the ,agnitudes of

settle,ents at any two points is 'nown as

differential settle,ent.

= If there is large differential settle,entbetween arious part of a structure! distortion

,ay occur due to additional ,o,ents

deeloped.=


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= t is difficult to predict the differentialsettle!ent.=t is generally obsered indirectlyfro! the !a1i!u! settle!ent.

= t is obsered that the differentialsettle!ent is less than /?W of the!a1i!u! settle!ent is !ost of the

cases.


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S d h d 0l ti l


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Sand hardClay

0lastic clay

2a1.Settle. "iff.Settl Angulardistortion

2a1.Settle "iff.Settle.

Angulardistortion

solatedfoundation

i) steel structii) XCC struct

/?!!/?!!

?.??668?.??$/8

$E6??$E>>>

/?!!5/!!

?.??668?.??$/8

$E6??$E>>>

Xaftfoundation

i) steel structii) Xcc struct.

5/!!5/!!

?.??668?.??'8

$E6??$E/??

$??!!$??!!

?.??668?.??'8

$E6??$E/??

heoretically! no da,age is done to the superstructure

if the soil settles unifor,ly.

Ooweer! settle,ents e-ceeding ?,, ,ay cause

trouble to utilities such as water pipe lines! sewers!

telephone lines 4 also is access fro, streets.


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Consolidation Settle!ent #

= Co!pressibility of soil is the property of the soil due towhich a decrease in olu!e occurs under co!pressieforces.

=The co!pression of soils can occurs due to9A) Co!pression of solid particles water in the oids.

B) Co!pression e1pulsion of air in the oids.C) L1pulsion of water in the oids.

= The co!pression of a saturated soil under a steady

pressure is


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Consolidation of laterally confined soil#:hen a pressure is applied to a saturated soilsa!ple of unit cross9 sectional area the pressure isshared by the solid particles water as

J u + nitially ;ust after the application of pressure theentire load is ta


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1aboratory Consolidation est:

= he consolidation test is conducted in a laboratory studythe co,pressibility of soil.

= Consolidation test apparatus! 'nown as consolido,eter oran odo,eter consists a loading deice 4 a cylindrical

container called as consolidation cell. Consolidation cell are

of two types! i) free ring or floating ring cell 4

ii) fi-ed ring cell


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= 5n initial setting load of about 'N# , is applied to sa,ple.

= he first incre,ent of load to gie a pressure of ? PN# ,is thenapplied to the speci,en! the dial gauge readings are ta'en after .!

?.! !*!@!?9!QQ etc up to the * hours.

= he second incre,ent of load is then applied. he successiepressures usually applied are !*! R! ?9 4 3 PN# , etc till the

desired ,a-i,u, load intensity is reached.

( 5ctual loading on soil after construction of structure)


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Consolidation test results$) "ial gauge reading ti!e plot #

0lotted for each load incre!ent Xequired for deter!ining the coefficient of consolidation. %seful for obtaining the rate of consolidation in field.


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')Final oid ratio , effectie stress plot#lotted for entire consolidation process underdesired load.

Xequired for deter!ination of the !agnitude of

the consolidation settle!ents in field.


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6) final oid ratio , log plot


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)8oading unloading reloading plot


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!portant "efinations$) Coefficient of co!pressibility & a) is defined asdecrease in oid ratio per unit increase in effectie stress.a + 9deEd + 9eE & slope of e 9 cure units , ! ' E( )

') Coefficient of olu!e change & !)is defined as the

olu!etric strain per unit increase in effectie stress.! + 9 & E o)E in which

o , initial olu!e , change in olu!e

9 change in effectie stress


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6)Co!pression inde1 & Cc)is equal to the slopeof the linear portion of the oid ration ersus logplot.

Cc + 9 eE log$?

&?J ) E

?

in which ?+ initial effectie stress.9 change in effectie stress.

L!pirical relationship after Ter=aghi 0ec


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(or!ally consolidated soil #9 A nor!allyconsolidated soil is one which had not beensub;ected to a pressure greater than thepresent e1isting pressure. The portion AB

of loading ,unloading cure represent thesoil in nor!ally consolidated condition.Oer consolidated soil# 9A soil is said tooer consolidated if it had been sub;ected in

the past to a pressure in e1cess of the


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(or!ally consolidated soils Oerconsolidated soils are not different types ofsoils but these are conditions in which a soil

e1ists.0reconsolidation 0ressure9The !a1i!u!pressure to which an oerconsolidated soil hadbeen sub;ected in the past is


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Final Settle!ent Of Soil "eposit n The Field

For co!putation of final settle!ent the coefficient of

olu!e change or co!pression inde1 &Cc) is required. Forti!e rate of co!putation the Ter=aghis theory is used.

Final settle!ent using coefficient of olu!e change # 8et Po + initial thic


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Final Settle!ent %sing 3oid Xatio

The alue of e corresponding to thegien load incre!ent is read off fro! e ,

plot substituted in ,

P + Po & e E $ J eo )

i.e Sf + Po & e E $ J eo )YY. &$)


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a)(or!ally Consolidated Soils9 The co!pression inde1 ofa nor!ally considered soil is constant.

Cc ? J U

Sf + Po 8og$Je

?

$?

?

b)0re Consolidated Soils 9

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Bearing Capacity Ofsoil