3.7 Foundation Engineering.pptx

7/25/2019 3.7 Foundation Engineering.pptx

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FoundationEngineering

Lateral Earth Pressure

Theories and RetainingWalls


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3.7 Trial Wedge Method for EarthPressure

Step 1- a mass of soil behind the wail is considered as afree body. The force P, which must exist between the free

body and the wall, is found by writing the equation ofequilibrium for the free body as a whole.

Step 2 a dierent free body is considered, having adierent boundary through the soil. nce again therequired force between the wall and the free body is found.

Step 3 the actual force against the !all will be thelargest value of P found as the result of considering all"ossible free bodies.


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Deriation of E!uation "# TrialWedge Method for $%tie &ase

#igure below shows the a""lication of the trial !edgemethod to the "roblem of sim"le retaining wall without wallfriction. $ "lanar failure wedge %&' is considered. There aredistributed normal stresses along %& and &' and distributedshear stress along &'. The resultants of these stresses arecarried out in the analysis. The forces acting at the freebody %&' are(


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Deriation of E!uation "# TrialWedge Method for Passie &ase

The equation is derived as follows. $ "lanarfailure wedge %&' is considered. There aredistributed normal stresses along %& and &' anddistributed along &'. The resultants of these

stresses are carried out in the analysis. %n thiscase, the force # acts above the normal and theangle between the force # and weight ! will be )* +.


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Most %o,,on ,aterials used forretaining -alls are

- !ood sheets0- teel and "lastic interlocing sheets0

- einforced concrete sheets0

- Precast concrete elements 1crib walls and blocwalls20

- 3losely s"aced in-situ soil-cement "iles0

- !ire-mesh boxes 1gabions20

- $nchors into the soil or roc mass 1soil nailing2.


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Rigid Retaining Walls

4nder this category, the wall may be subdividedto four categories.

They are(

5. 6ravity retaining walls7. emi-gravity retaining walls

8. 3antilever retaining walls

9. 3ounterfort retaining walls

:. ;uttressed retaining walls


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&lassi/%ation of Retaining Walls

They can be divided into two ma>or categories(

0a&onentional Retaining Walls

1i2 6ravity etaining !alls Masonry (brick or stone) orPlain concrete

1ii2 emi-gravity etaining !alls - Masonry or Plainconcrete or RCC

1iii23antilever etaining !alls - RCC (Inverted T and L)

1iv23ounterfort etaining !alls - RCC

1v2 ;uttress etaining !alls - RCC

1vi23rib !alls, etc.

0" Me%hani%all# Sta"ilied Earth 0MSE Walls


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2. Se,igrait# Wallso @ot as heavy as gravity walls.

o $ small amount of reinforcement is used forreducing the mass of concrete.

o $ s"ecialiDed form of gravity walls is a semi-gravity retaining wall.

o These have some tension reinforcing steelincluded so as to minimiDe the thicness of thewall without requiring extensive reinforcement.

o They are a blend of the gravity wall and the

cantilever wall designs.


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3. &antileer Retaining Wallso 3onsist of a relatively thin stemand a base slab.

o

;ase slab is the cantilever "ortion.o ;ase is also divided into two "arts, the heeland toe.

o /eel is the "art of the base under the bac?ll.

o Toe is the other "art of the base.

o esists "ressure due to its bending action.

o 4sually made of reinforced cement concrete 1332.

4se much less concrete than monolithic gravity walls,but require more design and careful construction.

6enerally economical u" to about Em 17: ft.2 to 5Bm

187 ft.2 in height. 3an be "recast in a factory or formed on site.

'ore convenient and relatively economical for design.

anineFs and 3oulombFs theories can be used to ?nd

active earth "ressure on the wall.


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5. 'uttressed Wallso imilar to counterfort walls exce"t the bracets or

buttress walls are "rovided on the o""osite sideof the bac?ll.

imilar to 3antilever retaining walls, but thin slabstems may be used at some interval to tie the

base slab and stem in order to reduce the shearforce and bending moment for more economicaldesign


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6. &ri" Walls 3rib walls are made u" of interlocing individual

boxes made from timber or "re-cast concrete. The boxes are then ?lled with crushed stone or

other coarse granular materials to create a freedraining structure.

There are two basic ty"es of crib wall(- Timber, and

- einforced "re-cast concrete.


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7. +a"ion -alls

6abions are multi-celled, welded wire or rectangularwire mesh boxes, which are then roc?lled, andused for construction of erosion control structuresand to stabiliDe stee" slo"es.

Their a""lications include,

- etaining walls,

- ;ridge abutments,

- !ing walls,

- 3ulvert headwalls,

- utlet a"rons,

- hore and beach "rotection walls, and

- Tem"orary chec dams.


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Me%hani%all# Sta"ilied Earth 0MSE Walls

These walls are among the most economical, and mostcommonly constructed.

3ontrary to other ty"es, the 'G walls are su""orted by thesoil, and not the other way around.

They are su""orted by selected ?lls 1granular2 and held

together by reinforcements, which can be either metallicstri"s or "lastic meshes.

The 'G 3ategories are

$2 Panel !alls,

;2 3oncrete ;loc !alls, and

32 Tem"orary Garth !alls


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@owadays. The main com"onents of these ty"es ofwalls are

H BackllIwhich is granular soilH Reinforcement in the bac?ll

H $ cover 1or skin2 on the front face

The reinforcement can be thin galvaniDed steel stri"s,geogrid, or geotextile for descri"tions of geogrid and

geotextile2.

%n most cases, "recast concrete slabs are used as sin.The slabs are grooved to ?t into each other so that soilcannot Jow between the >oints.

Thin galvaniDed steel also can be used as sin when thereinforcements are metallic stri"s.

!hen metal sins are used, they are bolted together,and reinforcing stri"s are "laced between the sins.


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3. Proportioning of Retaining Walls

!hen designing retaining walls, an engineer mustassume some of the dimensions, calledproportioning, which allows the engineer tochec trial sections for stability.

%f the stability checs yield undesirable results,the sections can be changed and recheced.


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Source: BM. Das

$""roximate dimensions for various com"onentsofretaining wall for initial stability checs( 1a2gravity wall0

1b2 cantilever wall Knote( minimum dimension ofis 7 ft 1LB.< m2M


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3.8 Sta"ilit# of Retaining Walls


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Sta"ilit# $nal#sis of RWs

The stability of a gravity wall is due to the self weightof the wall and the "assive resistance develo"ed in frontof the wall.

The gravity walls are designed using 3oulombFs theory.

einforced concrete walls 1cantilever or counterfort ty"es2are more economical than the gravity walls because thebac?ll itself "rovides most of the required dead load.

anineFs theory is used to investigate the stability ofreinforced concrete walls 1cantilever or counterfortty"es2 .


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T#pes of Sta"ilit# $nal#sis of RW

aE9ternal Sta"ilit# $nal#sis Tochec the safety against liding, verturningand ;earing 3a"acity failure. etaining wallsmust be designed to be stable with res"ect tofour "otential external failure modes(

) global stability,

) base sliding,

) overturning, and

)bearing ca"acity.

' Slidi


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'ase Sliding refers to the outwardmovement of the bottom of the retaining wallas a result of the lateral forces generated by

earth "ressure and, if "resent, water "ressure.The force resisting base sliding is the frictionbetween the ?ll in the bottom of ! and thefoundation soil beneath the bottom layer.


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)erturning refers to the ti""ing over of theretaining wall as it rotates about the toe of the structure.

The overturning force is the sum of each destabiliDing

force times its moment arm. The stabiliDing force, orrighting moment, is the "roduct of the weight of theretaining wall and its moment arm, which is thehoriDontal distance from the toe to the center of gravityof the wall. %f calculations show that the righting moment

is less than required, one o"tion is to increase the front-to-bac dimension of the wall, thereby increasing itsoverall weight and the magnitude of its moment arm.

' i & i


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'earing &apa%it# refers to the ability of thefoundation soil to su""ort the weight of the retaining wall"laced u"on it. The analysis is the same as for shallow

foundations. %t is necessary to increase the area of the baseif calculations show that the soil beneath the wall is toowea. This will decrease the "ressure 1force "er unit of area2on the foundation. $nother o"tion is to increase the de"thinto the ground of the retaining wall, thus increasing theability of the foundation soil to resist the im"osed weight.

"*nternal Sta"ilit# $nal#sis


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"*nternal Sta"ilit# $nal#sis) To determine the required ! tensile strength,

and the minimum required length to ensure a

rigid behavior in the reinforced bloc.

) efers to the ability of the individual "arts of the

wall to act as a single unit. The wall must bedesigned so that the individual "ieces of the walldo not "ullout, se"arate, or slide a"art. %n amodular bloc wall, the designer must beconcerned with the "otential of the tiebac failing

under tension or "ulling out from the soil.

L l St "ilit $ l i


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% Lo%al Sta"ilit# $nal#sis

This analysis is carried out for egmental etaining

!alls to ensure that the column of concrete blocunits remains intact without bulging0 local stabilityanalysis are( facing connection, bulging andmaximum unreinforced height.

d +l " l St "ilit $ l i


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d +lo"al Sta"ilit# $nal#sisefers to the stability of the wall, the soil behind it, and

the soil below it. The design engineer must be certain

that the entire area including the wall does notcolla"se. $ thorough soil analysis must be "erformed toeliminate the "ossibility of global failure.

This analysis is "erformed on the overall structureincluding the retained bac?ll and the foundation soil.

This analysis should be "erformed according to theclassical slo"e stability "rocedures, such as ;isho"Fsmodi?ed method of slices.


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To design retaining -alls "ro"erly an engineer


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To design retaining -alls "ro"erly, an engineermust now the basic soil "arameters-that is, theunit weight, angle of friction, and cohesion-for the

soil retained behind the wall and the soil belowthe base slab.

Nnowing the "ro"erties of the soil behind the wallenables the engineer to determine the lateral"ressure distribution that has to be designed for.

There are two "hases in the design ofconventional retaining walls.

#irst, with the lateral earth "ressure nown, thestructure as a whole is checed for stability.

That includes checing for "ossible overturning,sliding, and bearing ca"acity failures.

econd, each com"onent of the structure ischeced for ade!uate stren"th# and the steelreinforcement of each com onent is determined.

To Design a Retaining Wall We


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To Design a Retaining Wall Weshould (no- the Follo-ing

5. #unction of etaining !all Aierent Ty"es #or Aierent

Pur"oses.7. oil Pro"erties

$nit %ei"ht

&n"le of 'riction

Cohesion# C8. Aetermine the tability of the .!.

lidin"

verturnin"

Bearin" Ca*acity verall tability

9. Aesign the etaining !all ections

Check the tren"th

teel Reinforcement or teel Cross section


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For%es $%ting on Retaining Wall

5. !eight of ! 1!2

!eight of stem 1!52

!eight of footing or

base or base slab 1!72

7. !eight of soil above the base slab 1!s2

8. !eight of surcharge 1if any2 1!q2

9. $ctive earth "ressure force 1Pa2

:. Passive earth "ressure force 1P"2

1acting in front of wall is usually ignored because of the"ossibility of disturbance, erosion, etc.2


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Failure of Retaining Wall

$ retaining wall may fail in any of the followingways(

%t may overturn about its toe. 1ee #ig. a2

%t may slide along its base. 1ee #ig. b2

%t may fail due to the loss of bearin" ca*acity ofthe soil su""orting the base. 1ee #ig. c2

%t may undergo dee"-seated shear failure. 1ee#ig. d.2

%t may go through excessive settlement.


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1a2by overturning

1b2by sliding

1c2 by bearingca"acity failure

1d2 by dee"-seatedshear failure

Failure of Retaining Wall

Fa%tor of Safet# 0F)S


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Fa%tor of Safet# 0F)S

For sta"ilit#; a retaining -all should satisf# the

follo-ing %onditions The wall should be sta"le against sliding. The F)S against

sliding shall "e a ,ini,u, of 1.5.

The wall should be sta"le against oerturning.

#or granular bac?ll, the # against overturning shall be a

minimum of 5.:.#or cohesive bac?ll, the # against overturning shall be a

minimum of 7.

The base of the wall should be sta"le against "earing%apa%it# failure.

#or granular bac?ll, the # against bearing ca"acity failureshall be a minimum of 7.

#or cohesive bac?ll, the factor of safety against bearingca"acity failure shall be a minimum of 8.

The resultant of all the forces should fall within the middle third

of the base.


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#or each of these considerations, the resisting orsta"iliing or a%tuating for%es must exceed theforces that would cause failure by a "redetermined

# for each of these considerations.

The selected # should reJect the consequences offailure and the designerFs con?dence in the accuracy

of the in"ut "arameters.

The following # are normally used in the design ofgravity retaining walls(

6lobal tability, # O 5.8

;ase liding, # O 5.:

verturning, # O 7.B

;earing 3a"acity, # O 7.B


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Sta"ilit# $nal#sis of +rait# RW %n 6ravity !s, the use of the anineFs earth "ressure

theory for stability checs involves drawing a vertical line &Bthrough "oint located at the edge of the heel of the baseslab.

The anineFs active condition is assumed to exist along thevertical "lane&B+

anineFs active earth "ressure equations may then be usedto calculate the lateral "ressure on the face&B of the wall.

%n the analysis of the wallFs stability, the force, Pa (Rankine) , the

weight of soil above the heel, and the weight 1c2 of the

concrete all should be taen into consideration.

The assum"tion for the develo"ment of anineFs active"ressure along the soil face &B is theoretically correct if theshear Done bounded by the line &C is not obstructed by thestem of the wall.

The angle , that the line&C maes with the vertical is


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Deter,ination of LEP on +rait# RW

'


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h = f i


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&he%= for )erturning

For%es a%ting on a +rait# and a &antileerRW; "ased on the assu,ption that theRan=ine $%tie Pressure is acting along avertical "lane &B drawn through the heel of the

structure.

F)S against oerturning a"out the toe


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g g0a"out point !)

)erturning Mo,ent;

To %al%ulate the resisting ,o,ent;0negle%ting ; Pp

The weight of the soil above the heel and the weightof

the concrete 1or masonry2 are both forces thatcontribute

to the resisting moment.

#orce, P$also contributes to the resisting moment.

P% Pa!os&

he moment of the force, Pvabout C is

!here, ; O width of !


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+rait# RW


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&antileer RW


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The usual minimum desirable value of the factor ofsafety with res"ect to overturning is 5.: to 7.

&h = f Slidi l th '


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&he%= for Sliding along the 'ase

hear strength of the soil immediately below the base

slab may be re"resented as


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sive force, Ppis also a horiDontal resisting force. /ence

only horiDontal force that will tend to cause the wall to slideivin" force2 is the horiDontal com"onent of the active force,

Therefore,i.e.

'inimum factor of safety of 5.: against sliding is

generally required.


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The magnitudes of 'toe and 'eel can be


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e ag udes o 'toe a d 'eel ca be

determined in the following manner(


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#or maximum and minimum "ressures, the valueof # @ 'A2.


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Cu,eri%al 1Tae q O :


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Tae quO :


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u,eri%al 2


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3.7 Foundation Engineering.pptx

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