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Intorduction

Retaining wall is a rigid wall used to retain the soil at different levels. Its basic function is to retain soil at a slope which is greater than it would naturally assume, usually at a vertical or near vertical position.

Terminologies

  1. Backfill : The soil placed behind a
  2. Backfill slope : Often the backfill slopes upward from the back face of the The slope is usually expressed as a ratio of horizontal to vertical.
  3. Batter : The slope of the face of the stem from a vertical plane, usually on the inside (earth) face
  4. Footing (or foundation) : That part of the structure below the stem that supports and transmits vertical and horizontal forces into the soil
  5. Footing key : A deepened portion of the footing to provide greater sliding resistance
  6. Heel : That portion of the footing extending behind the wall (under the retained soil).
  7. Stem : The vertical wall above the
  8. Surcharge : Any load placed in or on top of the soil, either in front or behind the
  9. Toe : That portion of footing which extends in front of the front face of the stem (away from the retained earth).
  10. Weep holes : Holes provided at the base of the stem for drainage. Weep holes usually have gravel or crushed rock behind the openings to act as a sieve and prevent clogging. Poor drainage of weep holes is the result of weep holes becoming clogged with weeds, thereby increasing the lateral pressure against the wall. Unless properly designed and maintained, weep holes seldom “weep”. Alternatively, perforated pipe surrounded with gravel and encased within a geotextile can be used to provide drainage of the backfill.

Angle of repose

The natural slope taken up by any soil is called its angle of repose and is measured in relationship 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. 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. 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 on the wall.

Basic considerations

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 should not occur
  2. Forward sliding should not occur
  3. Materials used are should be suitable
  4. The bearing capacity of the soil should be considered
  5. 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

What are the different forces acting on retaining wall?

There are three types of pressures acting on retaining walls-

  • 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

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.

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.

Types of retaining walls

Mass retaining walls

  • Sometimes called gravity walls and rely upon their own mass therefore, is rather massive in
  • 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 8 m
  • Mass walls can be constructed of semi-engineering quality bricks bedded in a 1:3 cement mortar or of mass concrete
  • 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
  • Plain concrete gravity walls are not used for heights exceeding about 3m, for obvious economic
  • Stress developed is very
  • These walls are so proportioned that no tension is developed anywhere and the resultant of forces remain within the middle third of the

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

    • A base with a large heel
    • A cantilever with a large toe
  • Economic height range of 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
  • Two basic forms

  • T- Shaped Cantilever walls
  • 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
  • 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.

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
  • The stability of the wall is maintained essentially by the weight of the earth on the heel slab plus the self-weight of the
  • 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
  • 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
  • The bending moments can be considerably reduced by introducing transverse supports, called
  • 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

Precast concrete retaining wall

  • Manufactured from high-grade pre cast concrete on the cantilever
  • 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

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

Stone pitching

  • This is used to retain earth along shorter slopes along road or river side.
  • The soil slope is topped with stones to retain the
  • Some of these stones are key stones which are embedded to a sufficient length in the soil while other stones are supported on these key stones making it a rigid stone retaining wall

Riprap

  • Riprap is an alternative method for providing river bank and scour protection. It consists of sized and graded rock placed in a layer or in the shape of a berm

Revetment

Riprap revetments to be used as channel bank protection and channel linings on larger streams and rivers Riprap has been described as a layer or facing of rock, dumped or hand-placed to prevent erosion, scour, or sloughing of a structure or embankment. Materials other than rock are also referred to as riprap; for example, rubble, broken concrete slabs, and preformed concrete shapes (slabs, blocks, rectangular prisms, etc.). These materials are similar to rock in that they can be hand-placed or dumped onto an embankment to form a flexible revetment. The types of slope protection or revetment:

  • Rock
  • Rubble
  • Wire-enclosed rock (Gabions).
  • Pre-formed
  • Grouted
  • Paved

Rock Drape

Wire Mesh Drapery is generally defined as double twisted wire mesh draped over a slope area and anchored at the top with soil or rock anchors. Wire mesh is used where rocks are generally less than 2 feet in diameter and used to prevent rocks from reaching travel ways or other protected areas or property. The wire mesh drapery system is applied to slopes which exhibit potential for rockfall, the double twist wire mesh system allows for rockfall to occur but in a controlled manor. The drapery is applied to the slope and rockfall is controlled by the wire mesh preventing freefall and bouncing of the rocks on the slope, thereby preventing uncontrolled rockfall into protected areas. This consists of panels of double twisted hexagonal Wire mesh draped over a rock slope. The system is anchored at the top and attached to a Wire rope cable support grid. Each panel is attached to the next with wire fasteners forming one large blanket on the slope. This system is generally designed to control rockfalls by providing resistance to the moving rock by having enough flexibility to allow the rock to slowly trickle its way down the slope and fall harmlessly into a ditch area

Reinforced Earth Wall

It is a combination of earth and linear reinforcing strips that are capable of bearing large tensile stresses. Components of reinforced earth wall are-

  1. Soil
  2. Skin
  • Skin is the facing element of reinforced soil wall
  • These elements keep the reinforcement in desired elevation in the reinforced soil wall and also protect the granular at the edge from falling
  • Made of either metal units or precast concrete panels
  1. Reinforcement
  • A variety of materials can be used as reinforcing materials like- steel, concrete, fiberglass, wood, rubber, aluminum
  • Reinforcement can take the form of strips, grids, anchors and sheet materials, chains, planks, ropes, vegetation and combination of these and other material forms.
  1. In reinforcement earth wall two types of stability checks are important-
  • External Stability

It considers the reinforcement structure as whole and check the stability for sliding, overturning, bearing/tilt and slip by considering the effect of dead loads and forces acting on the structure.

  • Internal Stability

It covers internal mechanism such as shear within the structure, arrangement and behavior of the reinforcement and backfill. It checks the stability for each reinforcement layers and stability of wedges within the reinforced fill.

Provisions for joints in retaining wall

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

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.

What is backfill Drainage?

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.

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.

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