ce 632 retaining wall design part-1 ppt

CE-632CE 632Foundation Analysis and DesignDesign

Retaining Wall DesignRetaining Wall DesignRetaining Wall DesignRetaining Wall Design

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Foundation Analysis and Design: Dr. Amit Prashant

Conventional Retaining WallsConventional Retaining WallsggGravity Retaining Structures

Stability depends on the self weight of the wallNot economical for designNot economical for design

Semi-gravity Retaining StructuresMinimum amount of reinforcement may be used in the wall to reduce the size of wall

Cantilever Retaining WallsReinforced concrete is used in wall design with thin stem and slab baseRelatively economical for designy g

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Conventional Retaining WallsConventional Retaining Wallsgg

Counterfort/Buttressed Retaining wallsSimilar to Cantilever retaining walls but thin slab stems may beSimilar to Cantilever retaining walls, but thin slab stems may be used at some interval to tie the base slab and stem in order to reduce the shear force and bending moment for more economical designeconomical design

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Retaining Wall Design: ProportioningRetaining Wall Design: Proportioningg g p gg g p g

First, approximate dimensions are chosen for thechosen for the retaining wall.Then, stability of wall is checked for these dimensions

Stem

these dimensions.Section is changed if its undesirable from the stability or economy point ofStem economy point of view.

ToeHeel

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Retaining Wall Design: ProportioningRetaining Wall Design: Proportioningg g p gg g p g

0.3 m min

0.3 m min

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Earth Pressure on Retaining WallEarth Pressure on Retaining Wallgg

Earth pressure may be calculated at the verticalcalculated at the vertical section going through the heel of wall. This is under the constraint that Heel isthe constraint that Heel is

proportioned in such a way that line AC makes an

angle less than or equal to g qη with vertical.

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Earth Pressure on Retaining WallEarth Pressure on Retaining Wallgg

Pa (Rankine)

Pa (Coulomb)

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Equivalent Fluid MethodEquivalent Fluid MethodqqAlong line AB

212h hP K H ′= 21

2v vP K H ′= The units of Kh and Kv are the same as (Ph/H2)

2h h 2

Terzaghi and Peck have produced semi-empirical charts for Kh and Kv for diff t t f il li t d i th t bl b ldifferent types of soils as listed in the table below

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Retaining walls with backfill slope of finite distanceRetaining walls with backfill slope of finite distancegg

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Earth Earth Pressure Pressure ononon on Retaining Retaining walls walls with with backfill backfill l fl fslope of slope of

finite finite distancedistancedistancedistance

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Earth Pressure on Retaining walls with backfill Earth Pressure on Retaining walls with backfill slope of finite distanceslope of finite distance

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Stability of retaining wallStability of retaining wally gy g

OVERTURNING about its toe.

SLIDING along the basealong the base

BEARING CAPACITY failure of supporting

base

Excessive SETTLEMENT may occur if weak soil

layer is located below the yfoundation within 1.5

times foundation width.

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Stability of retaining wallStability of retaining wally gy gDeep seated shear failure may occur if there is a weak soil layer below the foundation within a depth of about 1.5 times width of foundation.

The failure surface may be assumed to have cylindrical shape and critical failure surface for cy d ca s ape a d c ca a u e su ace osliding may be determined through analysis.

For back fill with its slope less than 10º, the critical sliding surface may be

assumed to pass through heel of the retaining wall.

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Check Against Check Against ggOVERTURNNGOVERTURNNG

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Check Against OVERTURNNGCheck Against OVERTURNNGgg

M∑The wall must be safe against overturning about the toe

R

O

MFOS

M= ∑∑

Resisting Moment

Overturning Moment

. .2

. .av i i

ah a P p

P B W xFOS

P y P y+

= ≥−∑ FOS = 1.5, if wind/seismic

forces are consideredah a P py y

Location of Resultant force from toe can determined as

( ).av i R OP W x M M+ = −∑ ∑ ∑M M∑ ∑R O

av i

M Mx

P W−

=+

∑ ∑∑

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In the design of cantilever retaining wall it is preferred that the stem center is right above the location of resultant force at the base (resultant of soil reaction).


Check against SLIDINGCheck against SLIDINGgg

1.75RFFOS

F= ≥∑∑

In most cases passive earth

pressure is ignored SF∑

. tanR b b PF R c B Pδ= + +

while calculating FOS against sliding

R b b P

S ahF P=Base friction and adhesion may be

taken by the FOS 1 5 if i d/ i i following

assumption

⎛ ⎞

FOS = 1.5, if wind/seismic forces are considered

21 2to .2 3bδ φ⎛ ⎞ ′= ⎜ ⎟

⎝ ⎠1 2⎛ ⎞

21 2to .2 3bc c⎛ ⎞ ′= ⎜ ⎟

⎝ ⎠

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Alternatives for Improving FOS against SlidingAlternatives for Improving FOS against Slidingp g g gp g g g

Use base key to increase the passive resistance against

Use a Dead man anchor at the stem to transfer a

Increase the width of base l b ( f bl h l id )

resistance against sliding

part of sliding force to it.

slab (preferably on heel side)

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Check for BEARING CAPACITY failureCheck for BEARING CAPACITY failure

( ) ( )2 2av i ah PR P W P P= + + −∑( ) ( )∑

R OM MCE x

−= = ∑ ∑

B

av i

CE xP W+∑

2Be x= −Eccentricity:

( )P W+⎛ ⎞ ⎛ ⎞∑( )max

6 61 1av iP WQ e eqB B B B

+⎛ ⎞ ⎛ ⎞= + = +⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

∑

( )6 6P WQ +⎛ ⎞ ⎛ ⎞∑( )min

6 61 1av iP WQ e eqB B B B

+⎛ ⎞ ⎛ ⎞= − = −⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

∑

For e > B/6 q becomes negative

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For e > B/6, qmin becomes negative, i.e. tensile force. This is not desirable

and re-proportioning is required


Check for BEARING CAPACITY failureCheck for BEARING CAPACITY failureBearing capacity of soil can be calculated using general bearing capacity equation.

. . . . . . . . 0.5 . . . . .u c c c c q q q qq c N s d i q N s d i B N s d iγ γ γ γγ= + +

Following consideration have to made during the analysisThe eccentricity of load on the foundation can be incorporated using effective area method. The bearing capacity is calculated assuming theeffective area method. The bearing capacity is calculated assuming the width of foundation as B'

2B B e′ = −Inclination of resultant force has to taken into account

tan ah PP Pβ −=

∑av iP Wβ

+∑qF f f i

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[ u

av

qFOSq

=Factor of safety against bearing capacity:

2 for granular soil3 for cohesive soils


Wall JointsWall JointsConstruction Joints: Vertical or horizontal joints are placed between two successive pour of concrete. To increase shear resistance at the joints, keys may used as shown in the figure belowkeys may used as shown in the figure below.

Contraction Joint: These are vertical joints placed in the wall (from top of base slab to the top of wall) that allow theslab to the top of wall) that allow the concrete to shrink without noticeable harm. The groove may be 6-8 mm wide, 12-16 mm deep and they are placed at 8-12 m

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mm deep, and they are placed at 8 12 m spacing.


Wall JointsWall Joints

Expansion Joint: These vertical joints are provided in large retaining walls to allow for the expansion of concrete due to temperature changeswalls to allow for the expansion of concrete due to temperature changes and they are usually extended from top to bottom of the wall. These joints may be filled with flexible joint fillers. Horizontal reinforcing steel bars running across the stem are continuous through all joints Howeverbars running across the stem are continuous through all joints. However, the current thinking is that the large resistance to expansion/contraction on the back face of wall from lateral pressure + the friction resistance of the base these joints are practically uselessthe base, these joints are practically useless.

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Wall DrainageWall DrainageggAccumulation of rain water in the back fill results in its saturation, and thus a considerable increase in the earth pressure acting on the wall. Thi t ll l d t t bl diti T f th ti tThis may eventually lead to unstable conditions. Two of the options to take care of this problem are the following:

Provision of weep holes w/o geo-textile on the back-face of wallPerforated pipe draining system with filter

Filter materialWeep hole Filter materialPerforated pipe

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Wall DrainageWall Drainage Vertical drainsggWeep Holes: They should have a minimum diameter of 10 cm and be adequately spaced d di th b kfill t i l G

Vertical drains

depending on the backfill material. Geo-textile material or a thin layer of some other filter may be used on the back face of wall for the full height in order to avoid the backfor the full height in order to avoid the back fill material entering the weep holes and eventually clogging them.

Inclined drains

Combination of inclined and horizontal drain for

cohesive soils

Top drains for clay backfills

cohesive soils

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Wall DrainageWall Drainagegg

Perforated Pipes: These are provided horizontally along the back face of wall at the bottom of stem The filter material around the perforated pipewall at the bottom of stem. The filter material around the perforated pipe should satisfy the following requirements.

The soil to be protected should note wash into the filter

( )15 5FilterD

D<

Excessive hydraulic pressure head is not created in the soil due to low

( )85 BackfillD

Excessive hydraulic pressure head is not created in the soil due to low permeability.

( )15 FilterD ( )

( )

15

15

4Filter

BackfillD>

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Wall SettlementsWall Settlements

Settlement of soil below the wallImmediate settlement in granular soilImmediate settlement in granular soil. Consolidation settlement in cohesive soil.

Differential settlementHeel settlement is larger when there is substantial increase in backfillbackfillToe settlements are produced by lateral earth pressure. To minimize toe settlements, ground may be strengthened using sand piles rock columns grouting or structural pilespiles, rock columns, grouting, or structural piles.Differential settlements along the length of wall may produce cracks in the wall. This can be watched during construction itself

d ti ti b t k h iand preemptive action may be taken such as ensuring proper compaction of the ground.

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Design of Cantilever Retaining WallDesign of Cantilever Retaining Wall

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ce 632 retaining wall design part-1 ppt

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