Earth retaining structures are used to retain earth. They are highly applicable in Civil Engineering in the areas of
- Hydraulics and irrigation structures
- Highways (Bridges and flyovers)
- Mining and Military Engineering
Complete design of retaining walls begins with the determination of shear forces and bending moments acting on the structure through the analysis of lateral pressure from soils retained by the structure. With the shear forces and bending moments, the full structural design is carried out.
Types of retaining walls
- Gravity retaining walls: These types depend on their weight for stability. They are usually constructed with plain concrete or masonry and are not economical for large heights.
- Semi-gravity retaining walls: Some amount of reinforcement is provided in this type near the back face, thus enabling reduction of its size.
- Cantilever retaining walls: They are reinforced and economical up to 6-8m
- Counterfort retaining walls: Counterforts are introduced here to tie the vertical stem with the base slab in order to reduce the shear force and bending moments in the vertical stem and the base slab. They are economical for heights more than 6 to 8m. Counterforts are usually on the side of the backfill.
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Principles of the design of retaining wall
Before the actual design of the retaining wall, the soil parameters that influence the earth pressure and the bearing capacity of the soil must be evaluated through suitable soil tests. These include:
- Soil unit weight
- Angle of shearing resistance
- The cohesion intercepts
- The angle of wall friction
With the knowledge of these parameters, the lateral earth pressure and the bearing capacity of the soil can be determined.
Requirements for safe design of retaining walls
For a safe design of retaining walls, the following requirements must be satisfied:
NO SLIDING: The wall must be safe against sliding i.e. μRV ˃ RH where RV and RH are vertical and horizontal components of R respectively. Factor of safety against sliding, FoSsliding = μRV/RH where μ is the coefficient of friction between the base of the wall and the soil (μ = tanδ). Minimum value of FoSsliding = 1.5.
NO OVERTURNING: Wall must be safe against overturning about the toe. The factor of safety against overturning, FoSovertuning = ΣMR/ΣMO
MR = sum of resisting moment about the toe, and
MO = sum of overturning moment about the toe
FoSoverturning should be between 1.5 and 2
NO BEARING CAPACITY FAILURE: The pressure caused by RV at the toe of the wall must not exceed the allowable bearing capacity of the soil. The pressure distribution at the base is assumed to be linear and the maximum pressure at the base is given by
Pmax = RV/B (1 + 6e/B)
e = eccentricity
B = breadth of the retaining wall
The factor of safety against bearing capacity failure is given by
FoSbc = qna/Pmax
where qna = allowable bearing capacity
The FoSbc of 3 is specified provided the settlement is also within the allowable limit
NO TENSION: There should be no tension at the base of the wall. When the eccentricity (e) is greater than B/6, tension develops at the heel of the retaining wall. Tension is not desirable because the tensile strength of soil is very small and tensile crack would develop and the effective base area is reduced. In such situation, the maximum pressure should be given as:
Pmax = 4/3 (RV/ (B – 2e))
Procedure for the design of earth retaining structures
Gravity retaining wall
Step 1: Choose a trial section
- Top width should be ≥ 0.3m for ease of placement of section
- Depth of foundation (D) below the surface of natural soil should be ≥ 0.6m
- Base width of the wall should be 0.5H to 0.7H, where H is the height of the retaining wall and an average of 2H/3
Step 2: Compute earth pressure using Rankine or Coulomb’s theory. To check whether to use Rankine or Coulomb theory, check that
η = (45o + β/2) – ϕ/2 – sin-1 (sin β/ sinϕ) where β reprensent inclination of backfill and ϕ is the angle of wall friction.
Step 3: Check for stability using weight of soil, earth pressure and weight of wall based on the component dimensions of the structure.
Semi-gravity retaining wall
Same procedure applies for as gravity retaining wall except that the base width is slightly small.
Cantilever retaining wall
Here, top width should be ≥ 0.3H
Width of base slab = 2H/3
Width of stem and bottom, the thickness of base slab and the length of the projection should be kept at about 0.1H
Other procedures apply as in gravity retaining wall
NB: If the required FoSsliding is less 1.5, a base key is provided directly under the stem of the retaining wall. This increases the passive resistance. Some main steel from the stem is taken into the key.
Counterfort retaining wall
The counterforts are about 0.3m thick and have C/C spacing of 0.3H to 0.7H. The analysis is also similar to that of a cantilever retaining wall with slight differences due to the presence of counterforts.
Retaining walls can be designed manually or using softwares such as Tekla Tedds.
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