Retaining Walls And Their Applications

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INTRODUCTION
Retaining walls are structures which are designed and constructed to resist lateral pressure of soil. When there appears a change in ground elevation that exceeds angle of repose of soil then these walls prove effective. These types of walls are used in construction of buildings having basements, bridges, roads, etc. when it is necessary to retain embankments or earth in a relatively vertical position. These are supported by rock or soil underlying the base slab or supported on piles. When these walls retain earth or any such material in earthen embankments for railways, roads, wing walls of bridges and many more, then such earth or any such material is referred to as backfill. In order to design such walls is mandatory to determine the pressure exerted by and acting on plane faces of soil. Retaining walls can be designed by Working Stress Method or Limit State Method. In both methods Rankine’s theory of earth pressure is used which consists of both passive earth pressure and active earth pressure. By finding stresses we can estimate and design these walls.

TYPES OF RETAINING WALLS
These types of walls are categorized on basis of resistance against the backfill pressure. They are-

1. Gravity Retaining Walls- Such walls are made up of either plain concrete or stone masonry. These walls resist the backfill pressure and remain stable by virtue of its own weight. These walls are required to be enormous to maintain stability and avoid tensile stresses at any section. They are not economical for walls having height more than 3 metres. As per Indian Standard code (IS Code), top width of stem should not be less than 30 cm.

Fig 1: A Gravity type stone retaining wall
COURTESY: WIKIPEDIA

2. Semi Gravity Retaining Walls- These are modified version of gravity retaining walls. These walls have great resistance against external forces by the action of combination of self-weight, weight of soil above footing and flexural resistance of wall components. Here small amounts of reinforcing steel are provided in order to minimize the wall section. They can also be constructed with stone masonry.

Fig 2: Semi Gravity Retaining Wall
COURTESY: BRAINKART

3. Cantilever Retaining Walls– These are used for wall having height not exceeding 6 metres and has the shape of an inverted T-section. It has three components namely the stem, the heel slab and the toe slab. They all act as cantilever. Stability of such wall is based on self-weight and the weight of soil resting on heel slab. In some cases, either toe or heel slab is not provided due to boundary restrictions and hence becomes L shaped which are uneconomical.

Fig 3: Cantilever Retaining Wall
COURTESY: REATINING WALL SOLUTIONS

4. Counterfort Retaining Wall- These are similar to cantilever retaining walls and they are used to retain soils of greater heights. Here the heel slab and vertical slab act as continuous slabs supported by vertical cantilevering counterforts, provided at 3 metre intervals. They have thin concrete slabs in vertical direction behind the wall which are known as counterforts that tie the base slab and wall together. These reduce the bending moment and shear force. The counterfort may be constructed without a toe if additional front clearance is needed and the sliding and overturning requirements for stability are met.

Fig 4: Counterfort retaining wall
COURTESY: THE ENGINEERING COMMUNITY

5. Bridge Abutments- These are special type retaining walls which not only contain the approach fill but also serve as a support for the bridge superstructure. They connect the deck or surface of bridge, to the ground and help its weight both in horizontal and in vertical direction. In case of short bridges, the abutments are placed at either ends of the bridge and connected to the embankment, sometimes including a retaining wall.

Fig 5: Bridge Abutments
COURTESY: TENSAR

6. Crib Walls or Coffer Dams- The units are to be filled with soil or built up members of pre-cast concrete or metal. They are supported by anchor pieces which are embedded in soil for stability.

Fig 6: Precast Concrete Crib Retaining Walls
COURTESY: RETAINING SOLUTIONS

7.Sheet pile walls- These are classified as anchored and cantilevered sheet pile walls. They may be used in single or double sheets walls. Of these only the cantilever type retaining walls and the bridge abutments are mostly used at present since they are of great economic value.

Fig 7: Sheet pile Wall
COURTESY: BASIC CIVIL ENGINEERING

APPLICATIONS:
1. One of the most common use of retaining wall is that it prevents soil erosion. The wall prevents water coming from irrigation canals, or by raining from gushing away the soil from a particular property. Broad aspects in its function include-

• Retention- As evident from the name it retains soil in its position against the anchored wall.
• Drainage- A retaining wall is also designed to properly channel water and drain it thus leading to diverting water from soil.
• Planting- Many times such walls act as raised terraces for planting of trees or making gardens. These plants have roots which adhere to the soil hence preventing erosion.

2. Another application involves stability. To landscape a sloped garden possesses many difficulties. This sloping imposes lateral pressure which leads to the movement of soil in downward direction. By building a retaining wall, it redistributes and accommodates this pressure and prevents soil sliding.

Fig 8: Retaining walls used for landscape
COURTESY: DOLAN LANDSCAPING MAINTENANCE AND CONSTRUCTION

3. Retaining walls prevents flooding to a great extent. In case of monsoon season, heavy flooding destroys crops of farmers which causes them great loss. But by building a retaining wall they divert the excess water away from their farm to the reservoirs. By storing water in the reservoirs, it can be effective during drought season.

4. Retaining walls are also used to make level grounds. These walls act as divider between high and low ground. By using the wall, soil is retained at fairly high position and then levelled and compacted. Then the low ground can be used for many recreational activities.

5.Sometimes these are used to enhance privacy between properties. These are mostly effective when the level height of two adjacent buildings have greater difference.

6.These walls enable to have more usable space especially in case of sloped property.

CONCLUSION
These walls are meant to support coal, earth or other materials whose top level is at a higher level than the level of earth in front of the wall. For design purpose earth material is considered as a granular mass of particles, dry and free from cohesion and moisture. The International Building code required these walls to be designed for safety against sliding, overturning, excessive pressure by foundation and uplift water pressure. Apart from this, such walls give both aesthetic appearance to surroundings as well as full functionality accordingly.

REFERENCES
Wikipedia, “Retaining wall”- https://en.wikipedia.org/wiki/Retaining_wall
Dolan Landscaping Maintenance and Construction, “Retaining Walls to Prevent Soil Erosion and Flooding”- https://dolanlandscaping.com/retaining-walls-to-prevent-soil-erosion-flooding/
Millenium Stone Works, “Uses For Retaining Walls”- https://www.milleniumstoneworks.com/uses-for-retaining-walls/
S. Ramamrutham, “Design of Reinforced Concrete Structures”.
Medium, “Functions of Retaining Walls”- https://medium.com/@retainingperth/functions-of-retaining-walls-8c3e47fb123c

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