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1Retaining wall

1Definition: Retaining walls are usually built to hold back soil mass. However, retaining walls can also be constructed for aesthetic landscaping purposes. Retaining walls are structures that are constructed to retail soil or any such materials which are unable to stand vertically by themselves. They are also provided to maintain the grounds at two different levels.Types of retaining walls1. Gravity Retaining Walls2. Semi-Gravity Retaining Walls3. Cantilever Retaining Walls4. Counter fort Retaining Walls

Gravity WallsThe gravity wall provides stability by virtue of its own weight , and therefore, is rather massive in size. It is usually built in stone masonry, and occasionally in plain concrete

The thickness of wall is also governed by need to eliminate or limit the resulting tensile stress to its permissible limit .Plain concrete gravity walls are not used for heights exceeding about 3m, for obvious economic reasons.Stress developed is very low.These walls are so proportioned that no tension is developed anywhere and the resultant of forces remain within the middle third of the base.

Semi-Gravity WallsSemi-gravity walls resist external forces by the combined action of self weight, weight of soil above footing and the flexural resistance of the wall components.

Brick retaining wallStone retaining wallCantilever Retaining Wall:stem H2 H1 Htoe heel yshear key b

The Cantilever wall is the most common type of retaining structure and is generally economical for heights up to about 8m.The structure consists of vertical stem , and a base slab, made up of two distinct regions, viz., a heel slab and a toe slab

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BatterDrainage HoleToeCantilever Retaining wall with shear key11Two basic forms:-A base with a large heelA cantilever with a large toe

Cantilever LCantilever TForces acting on the retaining wall:Lateral forces: Earth pressure due to backfill and surcharge.Vertical forces: Acting downwards: Self weight of the retaining wall ; Weight of soil above heel slab. Acting upwards: Force due to soil pressure underneath the base slab.Earth Pressure (P)

Earth pressure is the pressure exerted by the retaining material on the retaining wall. This pressure tends to deflect the wall outward.

Types of earth pressure :

Active earth pressure or earth pressure (Pa) and Passive earth pressure (Pp).

Active earth pressure tends to deflect the wall away from the backfill. 14PaGLVariation of Earth pressure14Factors affecting earth pressureEarth pressure depends on type of backfill, the height of wall and the soil conditions

Soil conditions: The different soil conditions are

Dry leveled back fillMoist leveled backfillSubmerged leveled backfillLeveled backfill with uniform surchargeBackfill with sloping surface1515Analysis for dry back fills16Maximum pressure at any height, p=kah Total pressure at any height from top, pa=1/2[kah]h = [kah2]/2

Bending moment at any height M=paxh/3= [kah3]/6 Total pressure, Pa= [kaH2]/2 Total Bending moment at bottom, M = [kaH3]/6

PaHhkaHMGLGLH=stem height16Where, ka= Coefficient of active earth pressure= (1-sin)/(1+sin)=tan2 = 1/kp, coefficient of passive earth pressure= Angle of internal friction or angle of repose=Unit weigh or density of backfill

If = 30, ka=1/3 and kp=3. Thus ka is 9 times kp

1717pa= ka H at the bottom and is parallel to inclined surface of backfill

ka=

Where =Angle of surchargeTotal pressure at bottom=Pa= ka H2/2

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Backfill with sloping surface

GL18Incase of backfill with surcharge;The surcharge on backfill may be due to traffic load on top of back fill or due to a structure near it.If ws is the surcharge pressure on horizontally finished back fill, then uniform effect of surcharge on stem is given by; ps = ka wsps pawsStability Conditions:A retaining wall must be stable as a whole, and it must have sufficient strength to resist the forces acting on it.In order that the wall may be stable, the following conditions should be satisfied:

The wall must be strong enough to resist the bending moment and shear force.The wall should not overturn.Maximum pressure at base should not exceed the SBC of soil.The wall should not slide due to lateral pressure.

Stability requirements of RW

Following conditions must be satisfied for stability of wall (IS:456-2000).

It should not overturnIt should not slideIt should not subside, i.e Max. pressure at the toe should not exceed the safe bearing capacity of the soil under working condition

2121Check against overturningFactor of safety against overturning = MR / MO 1.55 (=1.4/0.9)Where, MR =Stabilising moment or restoring moment MO =overturning moment

As per IS:456-2000,MR>1.2 MO, ch. DL + 1.4 MO, ch. IL0.9 MR 1.4 MO, ch IL22

22Check against SlidingFOS against sliding = Resisting force to sliding/ Horizontal force causing sliding= W/Pa 1.55 (=1.4/0.9)

As per IS:456:20001.4 = ( 0.9W)/Pa 23

Friction W SLIDING OF WALL23In case the wall is unsafe against sliding

pp= p tan2 (45 +/2) = p kpwhere pp= Unit passive pressure on soil above shearing plane ABp= Earth pressure at BC

R=Total passive resistance=ppxa

24Design of Shear key=45 + /2appRABWka(H+a)PAH+aHC24Design of Shear key-Contd.,If W= Total vertical force acting at the key base= shearing angle of passive resistanceR= Total passive force = pp x aPA=Active horizontal pressure at key base for H+aW=Total frictional force under flat base

For equilibrium, R + W =FOS x PA

FOS= (R + W)/ PA 1.55

2525Maximum pressure at the toe26Pressure below the Retaining Wall Tx1x2W1W2W3W4b/2b/6exbH/3PaWHhPmaxPmin.R26Let the resultant R due to W and Pa lie at a distance x from the toe.X = M/W, M = sum of all moments about toe.

Eccentricity of the load = e = (b/2-x) b/6

Minimum pressure at heel= >Zero.

For zero pressure, e=b/6, resultant should cut the base within the middle third.Maximum pressure at toe= SBC of soil.

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27Depth of foundationRankines formula:

Df =

=

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Df28Preliminary Proportioning (T shaped wall)

Stem: Top width 200 mm to 400 mmBase slab width b= 0.4H to 0.6H, 0.6H to 0.75H for surcharged wallBase slab thickness= H/10 to H/14Toe projection= (1/3-1/4) Base width

29H200b= 0.4H to 0.6Htp= (1/3-1/4)bH/10 H/1429Behaviour or structural action and design of stem, heel and toe slabs are same as that of any cantilever slab.30Behaviour or structural action

30Design of Cantilever RWStem, toe and heel acts as cantilever slabs

Stem design: Mu=psf (ka H3/6)Determine the depth d from Mu = Mu, lim=Qbd2

Design as balanced section or URS and find steel

Mu=0.87 fy Ast[d-fyAst/(fckb)]3131Curtailment of bars32Ast ProvidedAst/2AstDist.fromtoph2Every alternate bar cutAst Ast/2 h2 Ldth1c h1 Cross section Curtailment curveEffective depth (d) is Proportional to hBending moment is proportional to h3Ast is l to (BM/d) and is l to h2

32Design of Heel and ToeHeel slab and toe slab should also be designed as cantilever. For this stability analysis should be performed as explained and determine the maximum bending moments at the junction. Determine the reinforcement. Also check for shear at the junction. Provide enough development length.Provide the distribution steel

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DRAINAGE OF THE BACK FILLDRAINAGESand + Stone FilterWeepersOrWeep HolesDRAINAGEDrainage Pipes f 100-200 mm @ 2.5 to 4 mDRAINAGE (Alternate)Perforated PipeSuited for short wallsCounterfort WallStem and Heel slab are strengthened by providing counterforts at some suitable intervals.The stability of the wall is maintained essentially by the weight of the earth on the heel slab plus the self weight of the structure.

For large heights, in a cantilever retaining wall, the bending moments developed in the stem, heel slab and toe slab become very large and require large thickness. The bending moments can be considerably reduced by introducing transverse supports, called counterforts.

Retaining wall failure at the Shin-Kang Dam

Precast concrete retaining wallsapplication