Retaining Wall Design - IPB University

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SIL211 MEKANIKA TANAH, 3(2-3)DESIGN AND DETAILING OF RETAINING WALLSDR. IR. ERIZAL, MAGR.DEPARTEMEN TEKNIK SIPIL DAN LINGKUNGANFAKULTAS TEKNOLOGI PERTANIANIPB

DESIGN AND DETAILINGOF RETAINING WALLSLearning Outcomes: 2After this class students will be able to do thecomplete design and detailing of different types ofretaining walls.

RETAINING WALLRetaining walls are usuallybuilt to hold back soilmass.However, retainingwalls can also be SOILGL1Gravity retaining wall3

Cantilever Retaining wallwith shear keyBatterDrainage HoleToe4

Photos of Retaining walls5

Classification ofRetaining walls Gravity wall-Masonry or Plain concrete Cantilever retaining wall-RCC(Inverted T and L) Counterfort retaining wall-RCC Buttress wall-RCC6

Classification of Retaining wallsBackfillTiledrainGravity RWBackfillL-Shaped RWT-Shaped RWBackfillCounterfortCounterfort RW7ButtressWeepholeButtress RW

Earth Pressure (P) Earth pressure is the pressure exerted by theretaining material on the retaining wall. Thispressure tends to deflect the wall outward.GL Types of earth pressure : Active earth pressure or earth pressure (Pa) and Passive earth pressure (Pp). Active earth pressure tends to deflect the wallaway from the backfill.8PaVariation of Earth pressure

Factors affecting earth pressure Earth pressure depends on type of backfill, theheight of wall and the soil conditionsSoil conditions: The different soil conditions are Dry leveled back fillMoist leveled backfillSubmerged leveled backfillLeveled backfill with uniform surchargeBackfill with sloping surface9

Analysis for dry back fillsMaximum pressure at any height, p ka hTotal pressure at any height from top,pa 1/2[ka h]h [ka h2]/2Bending moment at any heightM paxh/3 [ka h3]/6 Total pressure, Pa [ka H2]/2 Total Bending moment at bottom,M [ka H3]/610GLhHGLPaMka HH stem height

Where, 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 kp11

Backfill with sloping surface pa ka H at the bottom and is parallelto inclined surface of backfillGL ka cos cos 2 cos 2 cos 22 cos cos cos Where Angle of surcharge Total pressure at bottom Pa ka H2/212

Stability requirements of RW Following conditions must be satisfied for stabilityof wall (IS:456-2000). It should not overturn It should not slide It should not subside, i.e Max. pressure at thetoe should not exceed the safe bearing capacity ofthe soil under working condition13

Check against overturningFactor of safety against overturning MR / MO 1.55 ( 1.4/0.9)Where,MR Stabilising moment or restoringmomentMO overturning momentAs per IS:456-2000,MR 1.2 MO, ch. DL 1.4 MO, ch. IL0.9 MR 1.4 MO, ch IL14

Check against Sliding FOS against sliding Resisting force to sliding/ Horizontal force causing sliding W/Pa 1.55( 1.4/0.9) As per IS:456:2000 1.4 ( 0.9 W)/PaFriction WSLIDING OF WALL15

Design of Shear key In case the wall is unsafeagainst slidingpp p tan2 (45 /2) p kpwhere pp Unit passivepressure on soil aboveshearing plane AB p Earth pressure at BC HH aA R CB 45 /216PApp a Wka (H a)R Total passiveresistance ppxa

Design of Shear key-Contd., 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 a W Total frictional force under flat base If For equilibrium, R W FOS x PA FOS (R W)/ PA 1.5517

Maximum pressure at the toex1hx2W4W1H WW2PaRW3Txeb/6bPmaxH/3b/2Pmin.Pressure below theRetaining Wall18

Let the resultant R due to W and Palie 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 Pmin Wb 6e 1 b Zero. For zero pressure, e b/6, resultant should cut the base within themiddle third. Maximum pressure at toe SBC of soil.Pmax W b 6e 1 b 19

Depth of foundation Rankine’s formula: Df SBC 1 sin 1 sin SBC 2ka γ2Df20

Preliminary Proportioning(T shaped wall)200 Stem: Top width 200 mm to 400 mm Base slab width b 0.4H to 0.6H, 0.6Hto 0.75H for surcharged wall Base slab thickness H/10 to H/14 Toe projection (1/3-1/4) Base widthHtp (1/3-1/4)bH/10 –H/14b 0.4H to 0.6H21

Behaviour or structural action Behaviour orstructural action anddesign of stem, heel andtoe slabs are same as thatof any cantilever slab.22

Design of Cantilever RW Stem, 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)]23

Curtailment of barsEffective depth (d) isProportional to hDist.fromtoph1Ast/2h2Bending moment isEvery3proportionaltohalternateh1cbar cutLdtAstAst is αl to (BM/d) and isαl to h2h2Ast/2AstCross section24Curtailment curveAstProvidedAst1 h12i.e. 2Ast 2 h2

Design of Heel and Toe1.2.3.4.5.25Heel slab and toe slab should also be designed as cantilever. For thisstability analysis should be performed as explained and determinethe maximum bending moments at the junction.Determine the reinforcement.Also check for shear at the junction.Provide enough development length.Provide the distribution steel

Design Example Cantilever retaining wallDesign a cantilever retaining wall (T type) to retain earth for aheight of 4m. The backfill is horizontal. The density of soil is18kN/m3. Safe bearing capacity of soil is 200 kN/m2. Take theco-efficient of friction between concrete and soil as 0.6. Theangle of repose is 30 . Use M20 concrete and Fe415 steel.SolutionData: h' 4m, SBC 200 kN/m2, 18 kN/m3, μ 0.6, φ 30 26

Depth of foundation To fix the height of retaining wall [H] H h' Df200 Depth of foundationh1 SBC Df 1 sin 1 sin 1.23m say 1.2m , Therefore H 5.2mh2Dfb27H

Proportioning of wall200 Thickness of base slab (1/10 to1/14)H 0.52m to 0.43m, say 450 mm Width of base slab b (0.5 to 0.6) HH 5200 mm 2.6m to 3.12m say 3mtp 750 mm Toe projection pj (1/3 to ¼)H 1m to 0.75m say 0.75m Provide 450 mm thickness for the stem atthe base and 200 mm at the top28450b 3000 mm

Design of stem Ph ½ x 1/3 x 18 x 4.752 67.68 kN M Ph h/3 0.333 x 18 x 4.753/6 107.1 kN-m Mu 1.5 x M 160.6 kN-m 29Taking 1m length of wall,Mu/bd2 1.004 2.76, URS(Here d 450- eff. Cover 450-50 400 mm)To find steelPt 0.295% 0.96%Ast 0.295x1000x400/100 1180 mm2#12 @ 90 300 mm and 3d okAst provided 1266 mm2 [0.32%]hPaMDfka hOr Mu [ka H3]/6

Curtailment of bars-Stem Curtail 50% steel from top (h1/h2)2 50%/100% ½ (h1/4.75)2 ½, h1 3.36m Actual point of cutoff 3.36-Ld 3.36-47 φbar 3.36-0.564 2.74m from top. Spacing of bars 180 mm c/c 300 mm and 3d okDist.fromtoph1Ast/2Everyalternatebar cuth2h1cLdtAsth2Ast/2Ast30AstProvided

Design of stem-Contd., Development length (Stem steel) Ld 47 φbar 47 x 12 564 mm Secondary steel for stem at front0.12% GA 0.12x450 x 1000/100 540 mm2#10 @ 140 450 mm and 5d ok Distribution steel 0.12% GA 0.12x450 x 1000/100 540 mm2 #10 @ 140 450 mm and 5d ok31200H 5200 mmtp 750 mm450b 3000 mm

Check for shear200 Max. SF at Junction, xx Ph 67.68 kN Ultimate SF Vu 1.5 x 67.68 101.52 kNH 5200 mm Nominal shear stress ζv Vu/bd 101.52 x 1000 / 1000x400 0.25 MPaxx To find ζc: 100Ast/bd 0.32%, From IS:456-2000, ζc 0.38 MPab 3000 mm ζv ζc, Hence safe in shear.32

Stability analysisLoadMagnitude, kNDistancefrom A, mStem W10.2x4.75x1x25 23.751.126.13Stem W2½ x0.25x4.75x1x25 14.840.75 2/3x0.25 0.31613.601.550.632.1323.20B. slab W3 3.0x0.45x1x25 33.75Back fill,W41.8x4.75x1x18 153.9TotalΣW 226.24Earth Pre.PH 0.333x18x5.22/2 PH33BM about AkN-mΣMR 413.55H/3 5.2/3MO 140.05

x1hx2W4W1H WW2PaRTxeb/6b0.75m N/m297.99Pressure below the Retaining Wall34Forces actingon the walland thepressurebelow the wall

Stability checks Check for overturningFOS ΣMR/ MO 2.94 1.55 safe Check for SlidingFOS μ ΣW/ PH 2.94 1.55 safe Check for subsidenceX ΣM/ ΣW 1.20 m b/3 and e b/2 –x 3/2 – 1.2 0.3m b/6 Pressure below the base slabPMax 120.66 kN/m2 SBC, safePMin 30.16 kN/m2 zero, No tension or separation, safe35

0.75m0.45m1.8m30.16 kN/m2120.6 kN/m222.697.9924.1Pressure below the Retaining WallLoadMagnitude,kNDistancefrom C, mBM, MC,kN-mBackfill153.90.9138.51Heel slab0.45x1.8x25 27.250.918.23Pressure dist.rectangle30.16 x 1.8 54.290.9-48.86Pressure dist.Triangle½ x 24.1x1.8 21.691/3x1.8-13.01TotalΣMC 94.86Total Load36Designofheelslab

Design of heel slabContd., Mu 1.5 x 94.86 142.3 kNm200 Mu/bd2 0.89 2.76, URS Pt 0.264% 0.96% Ast 0.264x1000x400/100 1056 mm2 #16@ 190 300 mm and 3d ok Ast provided 1058mm [0.27%]OR Mu 0.87 fy Ast[d - (fyAst/fckb)]37H 5200 mmxxb 3000 mm

Design of heel slabContd.,200 Development length: Ld 47 φbar 47 x 16 752mmH 5200 mm Distribution steel Same, #10 @ 140x 450 mm and 5d okLdt 75238x

Design of heel slab-Contd., Check for shear at junction (Tension) Maximum shear V 105.17 kN, VU,max 157.76 kN, Nominal shear stress ζv Vu/bd 101.52 x 1000 / 1000x400 0.39 MPa 39To find ζc: 100Ast/bd 0.27%,From IS:456-2000, ζc 0.37 MPaζv slightly greater than ζc,Hence slightly unsafe in shear.200xx

Design of toe slabLoadToe slabDistanceMagnitude, kNfrom C, m0.75x0.45x25 Pressure distribution,97.99x0.75rectanglePressure distribution, ½ x22.6trianglex1.0.75Total Load atjunctionBendingmoment,MC, kN-m0.75/2-3.1640.75/227.602/3x1 0.754.24Total BMat junctionΣM 28.6740

Design of toe slab Mu 1.5 x 28.67 43 kN-m Mu/bd2 0.27 2.76, URS200 Pt 0.085% Very small, provide 0.12%GA Ast 540 mm2 #10 @ 140 300 mm and 3d okLdt Development length: Ld 47 φbar 47 x 10 470 mm41

Design of toe slab-Contd., Check for shear: at d from junction (at xx as wall isin compression)200 Net shear force at the section V (120.6 110.04)/2 x 0.35 -0.45x0.35x25 75.45kN VU,max 75.45x1.5 113.18 kN ζv 113.17x1000/(1000x400) 0.28 MPaxdx pt 0.25%, From IS:456-2000, ζc 0.37 MPa ζv ζc, Hence safe in shear.42Ldt

Other deatails Construction joint A key 200 mm wide x 50 mm deep with nominal steel #10 @ 250, 600 mm length in two rows Drainage 100 mm dia. pipes as weep holes at 3m c/c at bottom Also provide 200 mm gravel blanket at the back of the stem for backdrain.43

Drawing and detailing#12 @ 180#10 @ 140#12 @ 90#16 @ 190#10 @ 140C/S OF WALLL/S ELEVATION OF WALL

Drawing and detailingBASE SLAB DETAILSBOTTOMSTEELPLAN OF BASE SLABTOPSTEEL45

Important Points for drawingNote1. Adopt a suitable scale such as 1:202. Show all the details and do neat drawing3. Show the development length for all bars at the junction4. Name the different parts such as stem, toe, heel,backfill, weep holes, blanket, etc.,5. Show the dimensions of all parts6. Detail the steel in all the drawings7. Lines with double headed arrows represents thedevelopment lengths in the cross section46

Design and Detailing ofCounterfort Retaining wall When H exceeds about 6m,CF Stem and heel thickness is more More bending and more steel Cantilever-T type-UneconomicalStem Counterforts-Trapezoidal section 1.5m -3m c/cBase SlabCRW47

Parts of CRW Same as that of Cantilever Retaining wall Plus CounterfortStemCounterfortsHeelToeBase slabCross section48Plan

Design of StemThe stem acts as a continuous slabSoil pressure acts as the load on the slab.Earth pressure varies linearly over the heightThe slab deflects away from the earth facebetween the counterforts The bending moment in the stem ismaximum at the base and reduces towardstop. But the thickness of the wall is kept constantand only the area of steel is reduced. BFp Kaγh49

Maximum Bending moments for stemMaximum ve B.M pl2/16(occurring mid-way between counterforts)andMaximum -ve B.M pl2/12(occurring at inner face of counterforts)- Where ‘l’ is the clear distance between the counterforts and ‘p’ is the intensity of soil pressure50lp

Design of Toe Slab The base width b 0.6 H to 0.7 H The projection 1/3 to 1/4 of base width. The toe slab is subjected to an upward soilreaction and is designed as a cantilever slab fixedat the front face of the stem. Reinforcement is provided on earth face alongthe length of the toe slab. In case the toe slab projection is large i.e. b/3, front counterforts are provided above thetoe slab and the slab is designed as a continuoushorizontal slab spanning between the frontcounterforts.51Hb

Design of Heel Slab The heel slab is designed as a continuous slabspanning over the counterforts and is subjected todownward forces due to weight of soil plus self weight ofslab and an upward force due to soil reaction. Maximum ve B.M pl2/16(mid-way between counterforts)AndMaximum -ve B.M pl2/12(occurring at counterforts)52BF

Design of Counterforts The counterforts are subjected to outwardreaction from the stem. This produces tension along the outer slopingface of the counterforts. The inner face supporting the stem is incompression. Thus counterforts are designedas a T-beam of varying depth. The main steel provided along the slopingface shall be anchored properly at both ends. The depth of the counterfort is measuredperpendicular to the sloping side.53CTd

Behaviour of Counterfort RW-MImportant points M Loads on WallCOUNTERFORTSTEM Deflected shape Nature of BMs Position of steel-MHEEL SLABTOE54 M Counterfort details

55

Earth Pressure (P) 8 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 (P p). Active earth pressure tends to deflect the wall away from the backfill.File Size: 1MBPage Count: 55

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