RetainingWalls. Professor of Geotechnical Engineering and Foundations. Faculty of Engineering - Cairo University. By Dr. Ashraf Kamal Hussein

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1 RetainingWalls By Dr. Ashraf Kamal Hussein Professor of Geotechnical Engineering and Foundations

2 1. Introduction Retaining wall: - a structure which retains from failure a soil mass or other materials (grains, liquids, etc.). - located between two different ground levels; upper level and lower level. - constructed in various fields of engineering such as highways, bridges, canals, hydraulic structures, etc. - constructed in situ. Backfill: - the material behind R.W. at the upper elevation. - exerting lateral earth pressure on the wall.

3 2. Types of Gravity Cantilever Counterfort Gravity Wall Cantilever Wall Counterfort Wall

4 Steps of Design: 1- Selection of wall dimensions. 2. Types of 2- Determining forces acting on wall. 3- Checking overall stability. 4- Checking structural stability.

5 3. Gravity Walls Materials: Masonry Bricks Plain concrete Stability: 0.5m to H/12 - from its own massive weight. H Dimensions: - tentatively!! D/2 to D 0.5 to 0.8H D = H/8 to H/6

6 Forces: 3. Gravity Walls - active earth pressure E a (Rankine or Coulomb). - weight of wall (W c ). - weight of soil (W s ) in case of Rankine. - water pressure (E w ). W s - uplift (U). W c E a - passive earth pressure (E p )!!! E p E w2 E w1 O U

7 3. Gravity Walls Overall Stability: 1- Check of Sliding: FS = Resisting Forces Sliding Forces - Resisting Forces = ΣV µ + c w B + E p!!!! W s as ΣV = ΣW + E av U µ = coefficient of friction c w = soil adhesion B = wall breadth E p E w2 O W c E a E w1 - Sliding Forces = E ah + E w1 E w2 U

8 3. Gravity Walls Overall Stability: 2- Check of Overturning: W s - must be safe against rotation about toe (point O) W c E a E p E w2 E w1 FS = Resisting Moments (M Overturning Moments (M r ) OT ) O U - M r = ΣW i x i + E av x E U x U + E p y p!!! - M OT = E ah y E + E w1 y w1 E w2 y w2

9 3. Gravity Walls Overall Stability: Factor of Safety FS = 1.5 in case of cohesionless soils = 2.0 in case of cohesive soils = 2.0 in case of cohesionless soils with passive = 2.5 in case of cohesive soils with passive

10 Overall Stability: 3- Check of Stresses: 3. Gravity Walls M O = Μ r Μ ΟΤ M O C = V B e = C 2 Β V 6 e σ σ = ± min if e max 1 B < q all σ max 6 min B Β if e > 6 σ max 2 V = 3C C ΣV C Β 6 to avoid excessive tilt σ max

11 Structural Stability: 3. Gravity Walls - for critical sections check: compressive stresses tensile stresses shear stresses

12 4. Cantilever Walls - H < 7 m Materials: - Reinforced concrete Stability: - from weight of soil above base. H 0.2 to 0.3 m B/3 D B = 0.4 to 0.7H D = H/12 to H/10 Dimensions: - tentatively!!

13 Forces: 4. Cantilever Walls - active earth pressure E a (Rankine only). - weight of wall (W c ). W c W s - weight of soil (W s ). - water pressure (E w ). E p E w2 E a E w1 - uplift (U). O - passive earth pressure (E p )!!! U

14 4. Cantilever Walls Overall Stability: - check of sliding if unsafe increase heel or use key - check of overturning if unsafe increase toe - check of stresses if unsafe increase toe stem toe heel key

15 4. Cantilever Walls Structural Stability: - Design of critical sections: Stem Heel stem Toe toe heel

16 4. Cantilever Walls Structural Stability: W c Design of Stem E a Design of Heel M 1 Design of Toe W s + W c M 2 W s + W c M 3 σ max σ 3 σ 2 σ min

17 Construction Remarks: 5. Backfill and Drainage - Backfill is preferably well graded cohesionless material for good drainage. - Proper compaction behind the wall should be made. - Heavy compaction equipment should be avoided. - Drainage is necessary to eliminate excess hydrostatic pressure due to rains.

18 5. Backfill and Drainage - Position of filter behind wall affects excess hydrostatic pressure on wall. - If filter is directly behind wall excess pressure is max. - If filter is constructed to get horizontal equipotential lines No excess pressure. Rains Rains E w H 2/3H impervious stratum impervious stratum

19 5. Backfill and Drainage - Weep holes (10 cm in diameter and adequately spaced) or longitudinal perforated pipe. - Coarse grained filters or geotextiles should be placed around entrance of weep holes or drain pipe in order not to be clogged with leached out material. filter material weep hole filter material perforated pipe

20 5. Backfill and Drainage - Filter should satisfy following conditions: D 15(filter) /D 85(soil) < 5 D 15(filter) /D 15(soil) > 5 for Slots: D 85(filter) /Slot width > 1.2 for Circular Holes: D 85(filter) /Hole diameter > 1.0

21 6. Joints - Construction Joints: vertical or horizontal joints. keys or roughened surface. between two successive pours of concrete. increasing shear resistance between successive layers. reinforcement is continuous through joints.

22 6. Joints - Contraction Joints: vertical grooves on wall face (8 to 12 m) weakened planes location of failure is predetermined. grooves 6 to 8 mm wide and 12 to 16 mm deep. cracks due to shrinkage tensile stresses occur across joint. reinforcement is continuous through joints.

23 6. Joints - Expansion Joints: vertical joints: complete separation between two portions of wall. spacing of 25 to 35 m. allow for expansion of concrete due to temperature changes. reinforcement may run across joints but bars are unbonded on one side.