Retaining Wall | Types of Retaining wall | Modes of Failure of Retaining wall | How to calculate voulme of concrete for retaining wall

Retaining wall

Introduction: 

The retaining wall are used to retain earth fill so that ground surfaces at different elevations are maintained on either of the retaining wall.

Terms used in retaining wall:
1. Stem: The vertical wall above the base slab.
2. Heel slab: The portion of base slab behind the stem, where soil is retained above it.
3. Toe slab: The portion of base slab in front of stem, where soil is away from it.
4. Shear key: A deepened portion of base slab to resist sliding of retaining wall.
5. Backfill: The process of refilling the soil in layers behind the stem.
6. Surcharge: Load applied on the soil, either in front or behind the wall.

Figure-1

Uses of Retaining wall:

1. Retaining wall is must in the construction of building having basement.

2. In bridge work, the wing wall and abutments are designed as retaining wall.

3. Construction of under ground water tanks

4. Roads in hilly ares.

5. Swimming pools.

Types of Retaining wall:

1. Gravity retaining wall: It depends entirely on its own weight to provide the necessary stability.
2. Cantilever retaining wall: It is a reinforced concrete wall utilizing the weight of the soil itself to provide the desired weight. T shaped & L shaped cantilever retaining wall are the types of cantilever retaining wall.
3. Counterfort retaining wall: It looks like a T shaped & L shaped cantilever retaining wall and uses the weight of the soil for stability. The counterfort supports the stem and heel slab of a retaining wall as shown in figure -2 (e)
4. Buttress retaining wall: It is like a covered counterfort type with ribs or walls. The toe and stem are a continuous slab. (Refer Figure - 3)

Figure -2

 

Figure - 3

Assumptions made in the design of retaining wall  as per Rankine's theory:

1. The soil mass is semi- infinite, homogeneous, dry and cohesionless.
2. The back of the retaining wall is vertical and smooth.
3. The ground surface is plane which may be horizontal or inclined.

4. The wall yields about the base.

Forces acting on retaining wall:

1. Weight W₁ of the stem.

2. Weight W₂ of the base slab.

3. Weight W₃ of soil supported on the heel slab.

4. Horizontal force P_a equal to active earth pressure acting at H/3 above the base.

Figure - 4

Modes of Failure of Retaining wall: 
1. Overturning about the toe.

2. Sliding

3. Failure of soil due to excessive pressure at toe or tension at heel

4. Bending failure at stem or base of slab or heel slab.

Arrangement of Reinforcement in retaining wall:

Figure - 5

Note:

1. The factor of safety due to overturning is taken as 2.

2. The factor of safety due to sliding is taken as 1.5.

3. The retaining wall with height exceeding 6m are designed as counterfort retaining wall.

4. The spacing of counterfort is 1/3 to 1/2 of the height of retaining wall. Generally it is kept between 3 to 3.5m.



 Calculation of volume of concrete for retaining wall:
1. T - shaped retaining wall:

Figure - 6

When we carefully look at retaining wall we can find that base of retaining wall is in rectangular shape and stem is in trapezium shape.
From  figure -6
Rectangular base slab:
Length of base slab = 15m
Breadth of base slab = 2m
Thickness of base slab = 200mm = 0.2m

Volume of rectangular base slab = L*B*H = 15*2*0.2 = 6m³

Trapezoidal shaped stem:
a = 200mm = 0.2m
b = 500mm = 0.5m 
height of stem = 5m
Surface area of stem = [(sum of parallel sides * height)/2] = [(0.2+0.5)*5/2] = 1.75m²

Volume of stem = surface area of stem*length = 1.75*15 = 26.25m³

Total volume of T - shaped retaining wall = 6 + 26.25 = 32.25m³ 

2. Counterfort retaining wall:

Figure -7

When we carefully look at retaining wall we can find that base of retaining wall is in rectangular shape and stem & counterfort are in trapezium shape.
From figure -7
Rectangular base slab:
Length of base slab = 10m
Breadth of base slab = 1.5m
Thickness of base slab = 200mm = 0.2m

Volume of base slab = L*B*H = 10*1.5*0.2 = 3m³

Trapezoidal shaped stem:
a = 200mm = 0.2m
b = 500mm = 0.5m 
height of stem = 7m
Volume of stem = Surface area of trapezoid * length = (sum of parallel sides * height)/2 * length 

                           = [(0.2+0.5)*7/2]*10 = 24.5m³

Trapezoidal shaped counterfort:
No. of counterfort provided = 2 
Volume of counterfort =2*Surface area of trapezoid*length=[(sum of parallel sides * height)/2]* length 

                                       = 2* [(0.4+1)*7/2]*0.2 = 1.96m³

Total volume of concrete for counterfort retaining wall = 3 + 24.5 + 1.96 = 29.46m³