Analysis and Design of Retaining Structures

1. GEOTECHNICAL ENGINEERINGECG 503LECTURE NOTE 07 TOPIC : 3.0 ANALYSIS AND DESIGN OF RETAINING STRUCTURES

2. LEARNING OUTCOMES Learning outcomes: At the end of this lecture/week the students would be able to: • Understand natural slope and made engineered soil slope assessment which include rainfall induced failure and role of suction.

3. TOPIC TO BE COVERED • Types of Retaining Structures • Sheet Pile Wall – Cantilever and Anchored Sheet Pile

4. LATERAL EARTH PRESSURE Introduction & Overview 2.1 Introduction and overview Retaining structures such as retaining walls, basementwalls, and bulkheads are commonly encountered in foundation engineering, and they may support slopes of earth mass. Proper design and construction of these structures require a thorough knowledge of the lateral forces that act between the retaining structures and the soil mass being retained.

5. Retaining walls are used to prevent the retained material from assuming its natural slope. Wall structures are commonly use to support earth are piles. Retaining walls may be classified according to how they produce stability as reinforced earth, gravity wall, cantilever wall and anchored wall. At present, the reinforced earth structure is the most used particularly for roadwork

6. 3 basic components of retaining structure • Facing unit – not necessary but usually used to maintain appearance and avoid soil erosion between the reinforces. • Reinforcement – strips or rods of metal, strips or sheets of geotextiles, wire grids, or chain link fence or geogrids fastened to the facing unit and extending into the backfill some distance. • The earth fill – usually select granular material with than 15% passing the no. 200 sieve.

7. Component of E.R. Wall

8. EARTH RETAINING STRUCTURES Types of Retaining Wall Retaining Wall The various types of earth-retaining structures fall into three broad groups. Gravity Walls Embedded walls Reinforced and anchored earth

9. EARTH RETAINING STRUCTURES Gravity Walls Gravity Walls Masonry walls Gabion walls Crib walls RC walls Counterfort walls Buttressed walls

10. EARTH RETAINING STRUCTURES Gravity Walls Unreinforced masonry wall

11. EARTH RETAINING STRUCTURES Gravity Walls Gabion wall

12. EARTH RETAINING STRUCTURES Gravity Walls Crib wall

13. EARTH RETAINING STRUCTURES Gravity Walls Types of RC Gravity Walls

14. EARTH RETAINING STRUCTURES Embedded Walls Embedded walls Driven sheet-pile walls Braced or propped walls Contiguous bored-pile walls Secant bored-pile walls Diaphram walls

15. EARTH RETAINING STRUCTURES Embedded Walls Types of embedded walls

16. EARTH RETAINING STRUCTURES Reinforced and Anchored Earth Reinforced and anchored earth Reinforced earth wall Soil nailing Ground anchors

17. EARTH RETAINING STRUCTURES Reinforced and anchored earth Reinforced earth and soil nailing

20. EARTH RETAINING STRUCTURES Stability Criteria Stability of Rigid Walls Failures of the rigid gravity wall may occur due to any of the followings: • Overturning failure • Sliding failure • Bearing capacity failure • Tension failure in joints • Rotational slip failure In designing the structures at least the first three of the design criteria must be analysed and satisfied.

21. LATERAL EARTH PRESSURE Types of Lateral Pressure States of Equilibrium Hydrostatic Pressure and Lateral Thrust Earth Pressure at Rest Active Earth Pressure Passive Earth pressure

22. LATERAL EARTH PRESSURE Types of Lateral Pressure Hydrostatic pressure and lateral thrust Horizontal pressure due to a liquid

23. A z σv σh = Koσv B Earth pressure at rest LATERAL EARTH PRESSURE Earth Pressure at Rest Earth pressure at rest If wall AB remains static – soil mass will be in a state of elastic equilibrium – horizontal strain is zero. Ratio of horizontal stress to vertical stress is called coefficient of earth pressure at rest, Ko, or Unit weight of soil = γ

24. LATERAL EARTH PRESSURE Earth Pressure at Rest Earth pressure at rest .. cont.

25. LATERAL EARTH PRESSURE Active Earth Pressure Active earth pressure A Plastic equilibrium in soil refers to the condition where every point in a soil mass is on the verge of failure. If wall AB is allowed to move away from the soil mass gradually, horizontal stress will decrease. This is represented by Mohr’s circle in the subsequent slide. Unit weight of soil = γ z σv σh B Earth pressure at rest

26. σz σX = Koσz ACTIVE EARTH PRESSURE (RANKINE’S) (in simple stress field for c=0 soil) – Fig. 1 ø σz Ko σz σx’A

28. LATERAL EARTH PRESSURE Active Earth Pressure Based on the diagram : (Ka is the ratio of the effective stresses) Therefore : It can be shown that :

29. zo z LATERAL EARTH PRESSURE Active Earth Pressure Active pressure distribution

30. LATERAL EARTH PRESSURE Active Earth Pressure Active pressure distribution Based on the previous slide, using similar triangles show that : where zo is depth of tension crack For pure cohesive soil, i.e. when  = 0 :

31. z LATERAL EARTH PRESSURE Active Earth Pressure Active pressure distribution For cohesionless soil, c = 0

32. LATERAL EARTH PRESSURE Passive Earth Pressure 2.2.4 Passive earth pressure A If the wall is pushed into the soil mass, the principal stress σh will increase. On the verge of failure the stress condition on the soil element can be expressed by Mohr’s circle b. The lateral earth pressure, σp, which is the major principal stress, is called Rankine’s passive earth pressure Unit weight of soil = γ z σv σh B Earth pressure at rest

33. σz σX = Koσz PASSIVE EARTH PRESSURE (RANKINE’S) (in simple stress field for c=0 soil) – Fig. 2 ø Ko σz σz σx’P

35. Mohr’s circle representing Rankine’s passive state. Shear stress D b c σp A O Koσv σv C Normal stress a D’ LATERAL EARTH PRESSURE Passive Earth Pressure

36. LATERAL EARTH PRESSURE Passive Earth Pressure Referring to previous slide, it can be shown that : For cohesionless soil :

37. z LATERAL EARTH PRESSURE Passive Earth Pressure Passive pressure distribution For cohesionless soil,

38. Earth Pressure Passive pressure At-rest pressure Active pressure Wall tilt LATERAL EARTH PRESSURE Earth Pressure In conclusion Wall tilt

39. LATERAL EARTH PRESSURE Types of Lateral Pressure Rankine’s Theory • Initial work done in 1857 • Develop based on semi infinite “loose granular” soil mass for which the soil movement is uniform. • Used stress states of soil mass to determine lateral pressures on a frictionless wall Assumptions : • Vertical frictionless wall • Dry homogeneous soil • Horizontal surface

40. LATERAL EARTH PRESSURE Types of Lateral Pressure Active pressure for cohesionless soil

41. LATERAL EARTH PRESSURE Types of Lateral Pressure Effect of surcharge Effect of a stratified soil

42. LATERAL EARTH PRESSURE Types of Lateral Pressure Effect of sloping surface

43. LATERAL EARTH PRESSURE Types of Lateral Pressure Active pressure, Passive pressure, where and

44. LATERAL EARTH PRESSURE Types of Lateral Pressure Tension cracks in cohesive soils

45. LATERAL EARTH PRESSURE Types of Lateral Pressure Effect of surcharge (undrained)

46. LATERAL EARTH PRESSURE Types of Lateral Pressure Passive resistance in undrained clay

47. LATERAL EARTH PRESSURE Stability Criteria The stability of the retaining wall should be checked against : • FOS against overturning (recommended FOS = 2.0) (ii) FOS against sliding (recommended FOS = 2.0)

48. ∑ V Ph A LATERAL EARTH PRESSURE Stability Analysis The stability of the retaining wall should be checked against : 2.3.1 FOS against overturning (recommended FOS = 2.0) Pp .. overturning about A

49. ∑ V Ph Pp LATERAL EARTH PRESSURE Stability Criteria 2.3.2 FOS against sliding (recommended FOS = 2.0) Friction & wall base adhesion

50. LATERAL EARTH PRESSURE Stability Criteria • 2.3.3 For base pressure (to be compared against the bearing capacity of the founding soil. Recommended FOS = 3.0) Now, Lever arm of base resultant Thus eccentricity