Home>>Retaining Structures>>How to Design Retaining Wall

How to Design Retaining Wall

The design of the retaining wall is done for the serviceability limit state (SLS) and the ultimate limit state (ULS). Stability analysis and reinforcement design for ultimate limit state loads are done. In this article, we are discussing how to design a retaining wall.

What is Retaining Wall

A retaining wall is a structure constructed to retain or any other material. Mostly as we all know, the retaining walls are constructed to retain the earth. The load applied from the earth is considered for the analysis and design.

There are many types of retaining wall.

Let’s have look at them.

  • Gravity Retaining Walls

Gravity retaining walls rely on their sheer mass to resist the pressure exerted by the soil behind them. They are typically constructed using heavy materials such as concrete or stone.

These walls are suitable for moderate heights and are often found in gardens and residential landscapes. Their stability comes from their weight, which prevents soil from pushing them forward.

  • Cantilever Retaining Walls

Cantilever retaining walls are designed with a horizontal “beam” or slab extending from the base of the wall. This design distributes the weight of the soil and any additional loads over a larger area, increasing stability.

Cantilever walls are commonly used for taller retaining walls and can be constructed using reinforced concrete. They require careful engineering to ensure their effectiveness.

  • Sheet Pile Retaining Walls

Sheet pile retaining walls are typically used in areas with soft soil or where space is limited. They consist of vertically driven steel, vinyl, or wood sheets that interlock to create a barrier against soil pressure. These walls are often seen in waterfront areas, such as along rivers and coastal regions, to prevent erosion.

  • Counterfort Retaining Walls

Counterfort retaining walls are similar to cantilever walls, but they include thin vertical concrete “ribs” on the backside of the wall, known as counterforts. These counterforts provide additional support and strength, allowing the wall to resist higher pressures. They are commonly used for taller walls where the cantilever design might not provide enough stability.

  • Anchored Retaining Walls

Anchored retaining walls use cables or rods to anchor the wall to a stable foundation, typically deep within the ground. This design provides extra resistance against soil pressure and allows for the construction of taller walls in challenging soil conditions. Anchored walls are commonly used in areas with limited space or where a conventional wall may not be feasible.

  • Segmental Retaining Walls

Segmental retaining walls are constructed using individual blocks or segments that interlock with each other. These walls are versatile and can be used for various heights and shapes. They are often used in landscaping projects due to their aesthetic appeal and ease of installation. Segmental retaining walls can be made from concrete blocks, stones, or other materials.

  • Gabion Retaining Walls

Gabion retaining walls are constructed using wire mesh baskets filled with rocks or other materials. These baskets, known as gabions, create a flexible and permeable wall structure. Gabion walls are often used in areas where drainage is crucial and can be an environmentally friendly option, as they allow for the growth of vegetation between the rocks.

  • Hybrid Retaining Walls

Hybrid retaining walls combine different types of retaining wall techniques to address specific site challenges. For example, a hybrid wall might incorporate elements of both gravity and cantilever designs to optimize stability and space efficiency.

  • Tied-Back Retaining Walls

Tied-back retaining walls are similar to anchored walls but use a system of horizontal reinforcing elements, such as cables or rods, to provide additional support. These elements are anchored into the soil behind the wall and help distribute the load more effectively. Tied-back walls are commonly used in areas with high soil pressures and where a taller wall is required.

  • Diaphragm Retaining Walls

Diaphragm retaining walls are often used in tight urban spaces or areas with limited room for a traditional retaining wall. They consist of a continuous vertical wall supported by a horizontal slab. This design helps distribute the soil pressure horizontally, making it suitable for areas where space is at a premium.

  • Sheet Pile Walls

Sheet pile walls are made of interlocking steel, vinyl, or wood sheets driven into the ground vertically. They are commonly used in areas with soft soil, high water tables, or along waterfronts to prevent erosion. Sheet pile walls can be temporary or permanent, depending on the project’s needs.

  • Mechanically Stabilized Earth (MSE) Walls

MSE walls use reinforcing materials, such as geogrids or geotextiles, to reinforce the soil and increase its load-bearing capacity. These walls are often constructed by layering soil and reinforcing materials, creating a stable structure that can resist soil pressure. MSE walls are versatile and can be used for a wide range of heights and applications.

  • Inverted T Retaining Walls

Inverted T retaining walls have a shape that resembles an upside-down “T.” These walls use the horizontal base of the T as the retaining wall’s base, distributing the load over a larger area. Inverted T walls are suitable for situations where space is limited and can provide effective support for moderate heights.

  • Crib Retaining Walls

Crib retaining walls consist of interlocking rectangular or square units stacked together to create a wall structure. These units can be made of wood, concrete, or other materials. Crib walls are commonly used in situations where aesthetics are not a primary concern, such as industrial or commercial areas.

  • Hybrid Mechanisms

Retaining walls can often incorporate a combination of different mechanisms for added stability and efficiency. For example, a retaining wall might utilize both gravity and reinforcement techniques to address specific site conditions and engineering requirements.

Load on Retaining Wall

The following types of loads are applied on the retaining walls.

  1. Self-weight wall
  2. Lateral earth pressure loads. When there is a sloping ground to be retained, there will be vertical and horizontal loads.
  3. Water pressure. This will be controlled by providing weep hole in the retaining wall where possible. Otherwise, we have to design the wall for full lateral water pressure.
  4. Surcharged loads. When the retaining earth have additional loads, it has to consider for the design. For example, when there is a house close to the retaining wall, if there is contribution from that load, it has to be considered for the design.
  5. Earthquake loads.

Desing for SLS

Under the serviceability limit state, stability analysis of the retaining wall is done. The following factor of safeties are considered for the design.

  1. Overturning FOS = 1.5
  2. Sliding FOS = 1.5

However, the above factors may be varied with the project specification as depending on the nature of these factors are adjusted. Further, design conditions such as active pressure, passive pressure and at rest conditions are considered in accordance with the structural behavior.

Stability analysis such as sliding, overturning and bearing checks are done for the serviceability limit state. Reinforcement design or the other design checked are connected with strength of the retaining walls are done to satisfy the requirements in the ultimate limit state.

In addition, in the concrete retaining structures, requirements such as limiting the cracking will also be done as per the SLS requirements.

Design of ULS

All the retaining structure shall be design for the Ultimate Limit State (ULS). After the serviceability requirements are satisfied by the particular structure, its strength at the ultimate limit state will be checked or designed.

Generally, the load factors for ULS are as follows as per the BS 8110. Depending on your relevant standard, you have to select the loads factors for the design. For example, Eurocode 2 consider the factors for dead and live loads are 1.35 and 1.5 respectively as general requirements. These would change depending on the condition of the structure we design.

Spread the love