Retaining Wall_Lecture.ppt
- 2. RETAINING WALL Basic function – to retain soil at a slope which is greater than it would naturally assume, usually at a vertical or near vertical position
- 3. Terminology of Retaining Wall
- 4. Angle of Repose The natural slope taken up by any soil is called its angle of repose and is measured in relation ship to the horizontal It is the wedge of soil resting on this upper plane of the angle of repose which a retaining wall has to support
- 5. ANGLE OF REPOSE The angle of repose or the critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. The angle of repose can range from 0° to 90° At this angle, the material on the slope face is on the verge of sliding.
- 7. The design of retaining wall is basically concerned with the lateral pressures of the retained soil and any subsoil water Greater the angle of repose of a material, the less is the pressure exerted
- 8. Increased pressures must be allowed for when a. there is a surcharge or b. when there are buildings or traffic carrying roads near the top of the wall
- 9. Design of retaining wall Retaining walls have primary function of retaining soils at an angle in excess of the soil’s nature angle of repose. Walls within the design height range are designed to provide the necessary resistance by either their own mass or by the principles of leverage. Design consideration: 1. Overturning of the wall does not occur 2. Forward sliding does not occur 3. Materials used are suitable 4. The subsoil is not overloaded
- 10. Earth Pressures on Retaining wall Pressure at rest Active earth pressure Passive earth pressure Pressure at Rest This is the case when wall has a considerable rigidity. Basement walls generally fall in this category.
- 11. Active Earth Pressure If a retaining wall is allowed to move away from the soil accompanied by a lateral soil expansion, the earth pressure decreases with the increasing expansion. A shear failure of the soil is resulted with any further expansion and a sliding wedge tends to move forward and downward. The earth pressure associated with this state of failure is the minimum pressure and is known as active earth pressure.
- 12. Passive Earth Pressure If a retaining wall is allowed to move towards the soil accompanied by a lateral soil compression, the earth pressure increase with the increasing compression in the soil.
- 13. Factors which designer need to take account Nature and characteristics of the subsoil's Height of water table – the presence of water can create hydrostatic pressure, affect bearing capacity of the subsoil together with its shear strength, reduce the frictional resistance between the underside of the foundation Type of wall Materials to be used in the construction
- 14. Forces acting on Retaining Wall
- 15. FORCES ACTING ON A RETAINING WALL The designer is mainly concerned with the effect of two forms of earth pressure- active & passive
- 17. STABILITY OF RETAINING WALLS The overall stability of a retaining wall is governed by the action and reaction of a number of loads
- 18. Active pressure is exerted by the retained material & water pressure on the back of the wall Passive pressures are the induced loads at the toe and the friction between the underside of the base and the soil Ground water behind a retaining wall can have adverse effects upon the design and stability of the retaining wall
- 19. Typical dimensions of different types of retaining wall
- 20. Types of walls • Mass retaining walls / Gravity Retaining Wall • Cantilever walls • Counterfort retaining walls • Precast concrete retaining walls • Precast concrete crib-retaining walls
- 21. Counterfort Mass retaining Wall / Gravity RW T-Shaped RW L-Shaped RW Backfill Backfill Counterfort RW Buttress Backfill Buttress RW
- 22. Mass retaining walls Sometimes called gravity walls and rely upon their own mass therefore, is rather massive in size. Mass itself, together with the friction on the underside of the base to overcome the tendency to slide or overturn Generally only economic up to 1.8 m Mass walls can be constructed of semi-engineering quality bricks bedded in a 1:3 cement mortar or of mass concrete
- 23. Natural stone is suitable for small walls up to 1m high but generally it is used as a facing material for walls over 1 m, and occasionally constructed 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.
- 24. Typical example of mass retaining walls BRICK MASS RETAINING WALL
- 25. Typical example of mass retaining walls MASS CONCRETE RETAINING WALL WITH STONE FACINGS
- 26. Brick retaining wall Stone retaining wall
- 27. Semi-Gravity Walls Semi-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. Concrete cantilever wall is an example and consists of a reinforced concrete stem and a base footing. These walls are non- proprietary.
- 28. Cantilever walls Usually of reinforced concrete and work on the principle of leverage where the stem is designed as a cantilever fixed at the base and the base is designed as a cantilever fixed at the stem Economic height range of 1.2 m to 6 m using pre-stressing techniques Any durable facing material can be applied to the surface to improve appearance of the wall
- 29. Two basic forms:- • A base with a large heel • A cantilever with a large toe Cantilever L Cantilever T
- 30. The structure consists of vertical stem , and a base slab, made up of two distinct regions, viz., a heel slab and a toe slab “Stem” acts as a vertical cantilever under the lateral earth pressure “Heel slab” acts as a horizontal cantilever under the action of weight of the retained earth (minus soil pressure acting upwards from below) “Toe slab ” acts as a cantilever under the action of resulting soil pressure acting upward. T- Shaped Cantilever walls
- 31. L- Shaped Cantilever walls It resists the horizontal earth pressure as well as other vertical pressure by way of bending of various components acting as cantilevers.
- 32. Shear Key for additional stability
- 37. Counterfort retaining walls Can be constructed of reinforced or prestressed concrete Suitable for over 7 m Stem 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. Counterfort wall are placed at regular intervals of about1/3 to ½ of the wall height, interconnecting the stem with the heel slab The counterforts are concealed within the retained earth on the rear side of the wall.
- 38. 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. The counterforts subdivide the vertical slab (stem) into rectangular panels and support them on two sides(suspender-style), and themselves behave essentially as vertical cantilever beams of T- section and varying depth.
- 39. Butress Retaining Wall Counterfort Retaining Wall
- 42. Precast concrete retaining wall Manufactured from high-grade pre cast concrete on the cantilever principle. Can be erected on a foundation as permanent retaining wall or be free standing to act as dividing wall between heaped materials which it can increase three times the storage volume for any given area Other advantages- reduction in time by eliminating curing period, cost of formwork, time to erect and dismantle the temporary forms Lifting holes are provided which can be utilized for fixing if required
- 43. Application
- 45. Precast concrete retaining walls
- 46. Pre cast concrete crib-retaining walls Designed on the principle of mass retaining walls A system of pre cast concrete or treated timber components comprising headers and stretchers which interlock to form a 3 dimensional framework or crib of pre cast concrete timber units within which soil is retained Constructed with a face batter between 1:6 and 1:8 Subsoil drainage is not required since the open face provides adequate drainage.
- 48. PRE CAST CONCRETE CRIB RETAINING WALL
- 49. BASEMENT WALLS
- 64. Active System with the mesh anchored on the rock facing. Passive System with simple drapery system.
- 65. Provisions for Joints in the Construction of Walls Cast concrete retaining walls may be constructed with any or all of the following joints: Construction Joints: These are vertical or horizontal joints that are used between two successive pours of concrete. Keys are used to increase the shear resistance at the joint. If keys are not used, the surface of the first pour is cleaned and roughened before the next placement of concrete. Keys are almost always formed in the base to give the stem added sliding resistance. The base is formed first, and the stem constructed afterwards
- 66. Contraction joint These are vertical joints or grooves formed or cut into the wall that allows the concrete to shrink without noticeable harm. Contraction joints are usually about 0.25 inches wide and about ½ to ¾ inch deep, and are provided at intervals of not exceeding 30 feet. Expansion Joints: Vertical expansion joints are incorporated into the wall to account for expansion due to temperature changes. These joints may be filled with flexible joint fillers. Greased steel dowels are often cast horizontally into the wall to tie adjacent sections together. Expansion joints should be located at intervals up to 90 feet.
- 67. Backfill Drainage of Retaining Walls One area that can be commonly overlooked, or at least underestimated, is the necessity to drain the backfill of rainwater and/or groundwater. Hydrostatic pressure can cause or induce retaining wall failure, or at least damage. Drainage of water as a result of rainfall or other wet conditions is very important to the stability of a retaining wall. Without proper drainage the backfill can become saturated, which has the dual impact of increasing the pressure on the wall and lessening the resistance of the backfill material to sliding. Granular backfill material offers the benefits of good drainage, easy compaction, and increased sliding resistance.
- 68. Drainage systems usually utilize weep holes and drainage lines. Weep holes actually penetrate the retaining wall and drain the area immediately behind the wall. Weep holes should have a minimum diameter so as to permit free drainage; for large walls, 4 inch weep holes are common. Adequate spacing between weep holes allows uniform drainage from behind the wall. Weep holes should always have some kind of filter material between the wall and the backfill to prevent fines migration, weep hole clogging, and loss of backfill and caving. Drainage lines are often perforated and wrapped in geo textile or buried in a granular filter bed, and serve to carry water to the weep holes from areas deeper within the backfill.
- 69. Sand + Stone Filter Weepers Or Weep Holes Drainage Pipes f 100-200 mm @ 2.5 to 4 m Perforated Pipe Suited for short walls
- 72. Sliding Failure Overturning Failure Bearing capacity Failure Shallow shear Failure Deep shear Failure Five Modes of Failure
- 73. The design of retaining wall must ensure there is no 1.Failure due to overturning 2.Failure due to sliding 3.Failure due to bending The resultant thrust on the soil should be in the middle third of the base
- 74. Sliding Failure Sliding failure is nothing but sliding of wall away from backfill when there is shearing failure at the base of wall The Factor of safety against sliding is:- R Of resultant Vertical & horizontal components - : RV& RH of weight of wall & earth pressure µ = coefficient of friction = tan δ
- 75. Overturning failure Overturning failure is rotation of wall about its toe due to exceeding of moment caused due to overturning forces to resisting forces - : The Factor of safety against overturning is given by = sum of resisting moment about toe = sum of overturning moment about toe
- 76. Bearing capacity failure The pressure exerted by resultant vertical force at toe of wall must not exceed the allowable bearing capacity of the soil , the pressure distribution is assumed to be linear The maximum pressure is given by : :The Factor of safety against bearing failure is