SIL 211 MEKANIKA TANAH

Retaining Wall Design

DR. IR. ERIZAL, MAGRDEPARTEMEN TEKNIK SIPIL DAN LINGKUNGANFAKULTAS TEKNOLOGI PERTANIANIPB

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Conventional Retaining WallsGravity Retaining Structures

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

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

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

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Conventional Retaining Walls

Counterfort/Buttressed Retaining wallsSimilar to Cantilever retaining walls, but thin slab stems may beused at some interval to tie the base slab and stem in order toreduce the shear force and bending moment for moreeconomical design

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Stem

Retaining Wall Design: Proportioning

First, approximatedimensions arechosen for theretaining wall.Then, stability ofwall is checked forthese dimensions.Section is changedif its undesirablefrom the stability oreconomy point ofview.

ToeHeel

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Retaining Wall Design: Proportioning

0.3 m min

0.3 m min

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Earth Pressure on Retaining Wall

Earth pressure may becalculated at the vertical

section going through theheel of wall. This is underthe constraint that Heel is

proportioned in such a waythat line AC makes an

angle less than or equal toh with vertical.

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Earth Pressure on Retaining Wall

Pa (Rankine)

Pa (Coulomb)

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P h hH’2= P v vH’2=

Equivalent Fluid MethodAlong line AB

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K12

K The units of Kh and Kv arethe same as (Ph/H2)

Terzaghi and Peck have produced semi-empirical charts for Kh and Kv fordifferent types of soils as listed in the table below

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Retaining walls with backfill slope of finite distance

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EarthPressureonRetainingwallswithbackfillslope offinitedistance

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Earth Pressure on Retaining walls with backfillslope of finite distance

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Stability of retaining wall

OVERTURNINGabout its toe.

BEARING CAPACITYfailure of supporting

base

SLIDINGalong the base

Excessive SETTLEMENTmay occur if weak soil

layer is located below thefoundation within 1.5

times foundation width.

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Stability of retaining wall

Deep seated shear failure may occur if there isa weak soil layer below the foundation within a

depth of about 1.5 times width of foundation.

The failure surface may be assumed to havecylindrical shape and critical failure surface forsliding may be determined through analysis.

For back fill with its slopeless than 10º, the criticalsliding surface may be

assumed to pass throughheel of the retaining wall.

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Check AgainstOVERTURNNG

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Check Against OVERTURNNG

The wall must be safe against overturning about the toe

R

O

FOS =SM

SM

Resisting Moment

Overturning Moment

FOS = ≥2 FOS = 1.5, if wind/seismicforces are considered

P av.B+SWi.xi

P ah.ya - P P.yp

Location of Resultant force from toe can determined as

O(P av +SWi).x =SM R -SM

Ox =SM R -SM

P av +SWi

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

SFFOS = ≥1.75

Check against SLIDING

R

SFS

FR = R.tandb +cbB+ P P

FS = P ah

FOS = 1.5, if wind/seismicforces are considered

In most casespassive earth

pressure is ignoredwhile calculating

FOS against sliding

Base friction andadhesion may be

taken by thefollowing

assumption

Alternatives for Improving FOS against Sliding

Use base key toincrease the passive

resistance againstsliding

Use a Dead man anchorat the stem to transfer apart of sliding force to it.

Increase the width of baseslab (preferably on heel side)

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(P av +SWi) +(P ah - P P)

e = - x

Check for BEARING CAPACITY failure

2 2R =

OCE = x =SM R -SM

P av +SWi

Eccentricity:B2

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

and re-proportioning is required

[ 3 for cohesive soils

Check for BEARING CAPACITY failure

Bearing capacity of soil can be calculated using general bearingcapacity equation.

qu = c.Nc.sc.dc.ic +q.Nq.sq.dq.iq +0.5g.B.Ng.sg.dg.ig

Following consideration have to made during the analysis

The eccentricity of load on the foundation can be incorporated usingeffective area method. The bearing capacity is calculated assuming thewidth of foundation as B'

B’ = B-2e

Inclination of resultant force has to taken into account

P ah - P P

P av +SWi

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qu

qav

FOS =Factor of safety againstbearing capacity:

2 for granular soil

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

Contraction Joint: These are verticaljoints placed in the wall (from top of baseslab to the top of wall) that allow the

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concrete to shrink without noticeable harm.The groove may be 6-8 mm wide, 12-16mm deep, and they are placed at 8-12 mspacing.

Wall Joints

Expansion Joint: These vertical joints are provided in large retainingwalls to allow for the expansion of concrete due to temperature changesand they are usually extended from top to bottom of the wall. Thesejoints may be filled with flexible joint fillers. Horizontal reinforcing steelbars running across the stem are continuous through all joints. However,the current thinking is that the large resistance to expansion/contractionon the back face of wall from lateral pressure + the friction resistance ofthe base, these joints are practically useless.

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Wall DrainageAccumulation of rain water in the back fill results in its saturation, andthus a considerable increase in the earth pressure acting on the wall.This may eventually lead to unstable conditions. Two of the options totake care of this problem are the following:

Provision of weep holes w/o geo-textile on the back-face of wall

Perforated pipe draining system with filter

Filter materialWeep hole Filter material

Perforated pipe

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Inclined drains

Wall DrainageWeep Holes: They should have a minimumdiameter of 10 cm and be adequately spaceddepending on the backfill material. Geo-textile material or a thin layer of some otherfilter may be used on the back face of wallfor the full height in order to avoid the backfill material entering the weep holes andeventually clogging them.

Combination of inclined

Vertical drains

Top drains for claybackfillsand horizontal drain for

cohesive soils

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Wall Drainage

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

The soil to be protected should note wash into the filter

<5D15(Filter)

D85(Backfill)

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

D15(Filter)

D15(Backfill)> 4

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Wall Settlements

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

Differential settlementHeel settlement is larger when there is substantial increase inbackfillToe settlements are produced by lateral earth pressure. Tominimize toe settlements, ground may be strengthened using sandpiles, rock columns, grouting, or structural piles.

Differential settlements along the length of wall may producecracks in the wall. This can be watched during construction itselfand preemptive action may be taken such as ensuring propercompaction of the ground.

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Design of Cantilever Retaining Wall

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Contoh