Retaining walls help keep structures in place to prevent the soil from shifting from one place to another. They protect heavy structures or buildings from soil erosion. They are widely used to build landscapes. In high terrain elevations, structures that protect the roads and landscaping are constructed through retaining walls. This article will discuss the principles, types, design, and other applications in detail.
What is a Retaining Wall?
The ultimate goal of retaining walls is to withstand the earth’s pressure, rock, and other materials to stabilise slopes and prevent soil erosion. The soil on the back is at a high elevation, and the pressure exerted by this holds it back, called backfill. Retaining walls balance different ground levels, especially when the soil at the back is higher than in front. Their extensive uses are to build deep structures to the ground level, such as basements, and wing walls of bridges, and to hold slopes in hilly areas.
Types of Retaining Walls
The four main types of retaining walls that can be chosen based on several factors of each construction project are as follows:
- Gravity Retaining Walls: Gravity retaining walls use their overall weight and mass to counteract the pressure exerted by the soil from backfill. The heavily constructed structure creates a downward force that prevents the wall from falling or moving along the surface. These massive structures are constructed using materials such as concrete, masonry, bricks, blocks, and poured-in-place concrete.
Some features of gravity retaining walls:
- They are wider at the base to counteract horizontal pressure at depth.
- The sloped faces can retain heights of up to 3 meters only.
- Cantilever Retaining Walls: Reinforced concrete is the major component in building cantilever retaining walls. They have specially designed foundations that are L-shaped or inverted T-shaped. The vertical stem and base slab are positioned under the backfill. This design transfers the vertical stress to the foundation and prevents the soil from falling. The inverted T-shape provides stability to the structure. The key structure found in the base helps anchor the heavy structure to the ground to prevent sliding. It is suitable for retained heights up to 5 m.
- Embedded Retaining Walls: Embedded walls can extend up to 40 m deep to the ground to create near-surface, sturdy underground structures like metro stations, car parking, basements, etc. They work by counteracting the active earth pressure being exerted above the wall.
- Reinforced Soil Retaining Walls: Reinforced soil retaining walls are specifically constructed to resist the lateral pressure of the soil and support the soil laterally for balancing different levels of two sides. They are economical and used for constructing highway roads, railroads, and other mass transit systems.
Principles of the Design of Retaining Wall
The design criteria of the retaining wall must satisfy the following:
- Prevent Overturning
The design of the retaining wall must resist rotational movement at ground level. Seismic forces can cause a significant overturning moment, potentially leading to structural failure.
Factor of safety = ΣMR/ΣMO
where,
MR = sum of resisting moments about the toe, and
MO = sum of the overturning moment about the toe
The factor of safety for overturning should be between 1.5 and 2.
- Prevent Sliding
The design must prevent the sliding of the wall at its base.
Factor of safety against sliding = μRV/RH
where,
μ refers to the coefficient friction between the base of the wall and the soil (μ = tanδ),
RV and RH refer to the vertical and horizontal components of R, i.e. μRV ˃ RH
The minimum value of the factor of sliding is 1.5.
- Prevent Tension
Tension should be avoided at the base of the wall while designing a retaining wall. If the eccentricity is greater than B/6, then the tension develops at the heel of the retaining wall.
The maximum pressure should be given as:
pmin=∑Fyb[1−6eb]
\(P_{max} = \frac{\sum F_y}{b} [1+ \frac{6e}{b}]\)
- Prevent Bearing Failure
The pressure caused by the RV at the toe of the wall should not exceed the allowable bearing capacity of the wall. Assume the pressure distribution at the base is linear and calculate the maximum allowable pressure as follows:
Pmax = RV/B (1 + 6e/B)
where,
e = eccentricity
B = breadth of the retaining wall
The factor of safety against the bearing capacity is mentioned as Q/P.
Here Q is the bearing capacity, and the factor of safety for allowable pressure is 3.
How do Retaining Walls Work?
Retaining walls are constructed to withstand the horizontal or lateral earth pressure exerted by the material at a steeper angle than the material’s natural angle. The lateral pressure of the earth depends on the weight of the material situated behind the wall; it depends on the height and density of the backfill.
Design Guidance and Considerations for Retaining Wall Construction
Here we will explain the calculated design guidance and considerations for retaining wall construction as follows:
- Gravity Retaining Walls
Step 1: Choose a trial section
- For easy construction, maintain the top width ≥ 0.3 m.
- Make sure to have a foundation depth (D) ≥ 0.6 m.
- The ideal base width of the wall is between 0.5H to 0.7H (H is the wall height), with an average of 2H/3.
Step 2: Calculate the earth’s pressure using Rankine or Coulomb’s theory
- η = (45° + β/2) – (ϕ/2) – sin⁻¹(sin β / sinϕ)
- Where β = backfill angle, ϕ = wall friction angle.
Step 3: Check stability using the weight of the soil, earth pressure, and wall weight.
- Semi-Gravity Retaining Walls
The same procedure applies to gravity walls, but the only difference is the smaller base width.
- Cantilever Retaining Walls
- Consider the top width ≥ 0.3H.
- Maintain the base slab width at 2H/3.
- Stem, bottom slab thickness, and projection length ≈ 0.1H.
- Follow similar steps as described in the gravity retaining wall.
- If the sliding factor of safety (FOS) is less than 1.5, add a base key under the stem for increased stability.
- Counterfort Retaining Walls
- Counterforts are 0.3m thick and spaced 0.3H to 0.7H apart.
- Designed as a cantilever wall, with slight differences due to the counterforts.
Materials Used in Retaining Wall Design
The most common materials used to construct retaining walls are as follows:
- Wood or Timbers: These are popular choices due to their flexibility and affordability. Wood retaining walls are flexible in design and suitable for curves and angles that complement the landscape.
- Masonry: These are versatile in style and became popular for enhancing the appearance of the landscape.
- Poured Concrete: They are known for their strength and durability. They require minimal maintenance and are ideal for modern landscape work.
- Interlocking Concrete Blocks: They can be used for both curved and straight wall designs. Their lightweight properties and easy installation process make them an ideal choice for many projects.
Steps in Designing a Retaining Wall
Assigning the preliminary dimensions for different components of retaining walls is important before starting any construction work. Let us see the details in below table:
| Features | Measurements |
| Overall height | Depends on the project |
| Base width | Between 0.4 – 0.7 of the overall height |
| Toe width | Between ¼ and ⅓ of the base width |
| Base thickness | Between 0.07 and 0.1 of the overall height and must be greater than 0.3 (12 in) |
| Stem bottom thickness | Between 0.07 and 0.12 of the overall height |
| Stem top thickness | Minimum 0.2 and 0.25 is preferred |
Cost Considerations for Retaining Wall Design
Many factors contribute to the retaining wall costs, including location, land preparation, permits, demolition, soil type, waterproofing, drainage, and labour costs. So, careful planning and consideration of these factors will help you to plan a budget accordingly.
Here is an example of a cost estimation for retaining walls based on above assumptions.
1. RCC Work Cost Estimation
Grade of Concrete: M25
Rate for RCC M25 using RMC (excluding steel): ₹6,500 per cubic meter (cum)
Total RCC Quantity: 117 cum
Calculation:
Cost of RCC Work = Rate × Quantity
Cost of RCC Work = ₹6,500/cum × 117 cum = ₹760,500
2. Steel Work Cost Estimation
Rate of Steel Work (labour only, excluding material): ₹10,200 per metric ton (MT)
Quantity of Steel: 7.65 MT (as per provided data)
Calculation:
Cost of Steel Work = Rate × Quantity
Cost of Steel Work = ₹10,200/metric tons × 7.65 metric tons = ₹78,030
3. Total Cost Estimation Calculation:
Total Cost = Cost of RCC Work + Cost of Steel Work
Total Cost = ₹760,500 + ₹78,030 = ₹838,530
4. Contingency Addition Contingency Percentage: 5%
Calculation:
Contingency Amount = Total Cost × 0.05
Contingency Amount = ₹838,530 × 0.05 = ₹41,926.50
5. Final Total Cost Estimation Calculation:
Final Total Cost = Total Cost + Contingency Amount
Final Total Cost=₹838,530 + ₹41,926.50 = ₹880,456.50
Conclusion
In conclusion, a well-engineered retaining wall provides structural stability and enhances the aesthetic appearance of the mass structure. Ultimately, retaining wall design is crucial for soil erosion, landscape stability, and slope stability. By understanding the key factors, engineers can build retaining walls that stand the test of time.
FAQ’s
The primary purpose of retaining walls is to hold back the soil and other materials in place. It also prevents soil erosion and creates uniform-level surfaces in the terrain or sloped regions.
It completely depends on the quality of the soil, the location of the structure, the intended use, and design preferences.
No, it is not difficult to maintain retaining walls but it requires frequent inspections.