Guide to Retaining Walls | Working, Types & Components

Retaining walls are structures designed to restrain soil or any engineered fill material at a slope or angle steeper than it can hold naturally. The earth retained by retaining walls may be natural soil or any filling material. The material retained or supported by the retaining wall is called "backfill". 

Retaining walls can be designed in concrete, modular block, stones, or timber in various fields of civil engineering like irrigation, hydraulics, highways, railways, tunnels, and military engineering. 

In this article, we will delve into the basic working of retaining walls, components, their types, selection criteria, and more.

🔽You Will Find

• Working Principle of Retaining Wall
• Components of Retaining Wall
• Types of Retaining Wall
• Factors Affecting the Selection of Retaining Wall

 

Working Principle of Retaining Walls

Retaining walls are vertical or near-vertical earth retaining structures designed to resist the lateral pressure of soil (earth) or other materials. The soil retained applies external lateral pressure or horizontal pressure on the wall surface, trying to move the walls from their original position.  This stress imposed is a function of the height and density of the backfill. All retaining walls are designed to sustain this lateral pressure and keep the wall stable in position. 

Loads On Retaining Walls
Image Credits: JLC Online

Retaining walls are designed by considering various lateral pressure theories and considering the possible retaining wall failure modes expected for a durable and safe structure.

With various design considerations, the main purpose of retaining walls is to help create level areas in sloping terrain or provide additional support for naturally formed sloped and also help build terraces for infrastructure. In essence, retaining walls, help to maximize space on site that would have stayed ideal due to the sloped terrain. 

Retaining walls can fail in several ways. The mode of failure of a retaining wall is considered for the effective design of the structure.  Overturning occurs when the wall bends or topples over due to excessive outward force. Sliding failure happens when the wall moves horizontally, often due to non-cohesive soil. Bearing check examines if the soil under the wall can support its weight. Overstress refers to excessive bending or shearing forces on the wall. General stability checks overall stability, including slope failure and base stability. 

Retaining Wall Modes of Failure

By considering these failure modes, engineers ensure retaining walls are designed to withstand the forces they face and remain stable over time.

Components of Retaining Walls

The whole structure of a retaining wall is a combination of different design components, that work together to provide stability and support. When evaluating the load intensity acting on the structure, the greatest lateral pressure is exerted on the base of the retaining wall, which decreases as it moves to the top of the retaining wall. 

For better understanding, let's familiarize the basic components of a retaining wall:

1. Stem
2. Toe
3. Heel
4. Shear Key

Components of Retaining Wall

1. Stem

The stem forms the vertical portion of the retaining wall. 

• The upper portion of the step is free and the bottom is connected to the base footing: the toe and heel slab as shown above.
• Have a greater slenderness ratio and support a majority of the fill material. 
• The thickness of the stem can be either uniform or increase from top to bottom based on the design requirements. 

2. Toe & Toe Slab

The portion of the footing lying over the compacted soil mass and projecting at the front of the wall forms the "toe" or "toe slab".

• Lies on the top of the compacted soil, where the soil mass is not retained.
• Also mentioned as the base footing of the retaining wall
• Toe of the retaining wall helps distribute the weight of the wall and applied load uniformly on the ground.
• It can be extended beyond the face of the wall to increase stability. 

3. Heel or Heel Slab

The portion of the footing lying over the compacted soil mass and projecting out at the backfill side forms the "heel" or "heel slab".

• Lies on the top of the compacted soil, where the soil mass is retained.
• The length of the heel is larger than the toe slab due to the following reasons:
• To increase the self-weight of the retaining wall.
• To increase the counter moment or restoring moment against the overturning moment caused due to lateral pressure from the retained mass.
• To make the soil above the heel a part of the wall and impose gravity self-weight load on the base of the footing.

4. Shear Key

Shear keys are small projections provided at the base of the retaining walls to increase their sliding resistance. 

• The shear key is placed exactly below the base footing in-line with the stem so that the placement of reinforcement in the stem can be extended directly into the shear key.
• The shear key is designed depending on the applied loads for bending and shear.
• As per ACI 317-17, the width of the shear key should be at least twice its depth.

Types of Retaining Wall

Retaining walls are classified either based on the mode of resisting earth pressure or based on the shape. The principal types of retaining walls are classified as:

1. Gravity Retaining Walls
2. Cantilever Retaining Walls
3. Counterfort / Buttressed Walls
4. Anchored Retaining Walls
5. Piled Retaining Walls
6. Mechanically Stabilized Earth (MSE) Retaining Wall
7. Hybrid Systems

1. Gravity Retaining Walls

Gravity-retaining walls are rigid earth-retaining structures that resist the earth pressure from the backfill by the dead weight of the wall. When reinforcement is introduced in these walls to minimize the size of the wall section. These are called semigravity walls.

• Made of masonry or concrete or stones.
• Massive structures as they use self-weight as a gravitational force to counteract the soil lateral pressure. 
• Proportioned and designed such that no tension is developed anywhere in the structure and the resultant forces remain within the middle third of the base. 

Fig.3. Gravity Retaining Walls

• Stability of the gravity wall is dependent on its self-weight and the weight of the soil it bears. 
• It is economical up to a height of 3 meters.
• Gabion, crib, and bin retaining walls are the major types of gravity retaining walls. 

2. Cantilever Retaining Walls

Cantilever retaining walls resist the horizontal earth pressure and other vertical pressure by the bending of various components (stem, heel slab, and toe slab) that act as cantilevers. 

• Consists of a thin stem and a base slab (footing) that sits under the backfill.
• Made from reinforced concrete, precast concrete, or prestressed concrete.
• They can be T-shaped or L-shaped cantilevers.

Cantilever Retaining Walls

• Under the action of backfill pressure, these components (AB, BC, and DB) bend as a cantilever about a point (B). 
• The components are reinforced at their tension phase similar to the reinforcement details of a cantilever beam.
• Economical up to a height of 10 m.
• Compared to gravity walls, the quantity of material is less but requires proper design and construction.

3. Counterfort /Buttressed Retaining Walls

Counterforts are retaining walls whose vertical stem and heel slab are strengthened by providing counterforts at some suitable intervals. Buttressed walls are retaining walls whose stem is supported by buttresses instead of counterforts.

Counterfort /Buttressed Retaining Wall

• Counterfort or buttresses help the stem and base slab to act as a single continuous slab.
• They help reduce shear and bending moments on the wall. 
• The spacing of the counterforts is equal or greater than half of the counterfort height.
• It can achieve heights between 8-12 meters.

4. Anchored Retaining Walls

Anchored Retaining Walls

Anchored retaining walls are retaining walls additionally strengthened using cables or stays anchored in the rock or soil behind them. 

• Consists of a facing (the visible part of the wall) and a series of anchors or tiebacks that extend into the soil behind the wall. 
• Anchors are typically made of steel and are embedded deep into the ground or attached to a stable structure such as a concrete foundation or rock mass.
• Suitable for steep slopes, highway and railway embankments, waterfront structures, and basement walls.

5. Piled Retaining Wall

Pile Retaining Wall
Image Credits: Fussey Piling

Pile retaining walls consist of reinforced concrete piles driven adjacent to each other. These walls are used to provide support and stability in excavations or slopes.

• The depth of the piles is chosen to counteract the lateral forces that could push over the wall.
• Pile retaining walls can be used for both temporary and permanent construction projects.
• They offer high stiffness and can withstand significant lateral pressure.
• These walls cause minimal disturbance to surrounding structures or properties.
• Steel sheet pile walls, another type of retaining wall, are constructed using steel sheets and are suitable for slopes or excavations up to a certain depth.
• However, sheet pile walls are not as capable of withstanding very high pressure.
• Pile retaining walls are economically viable for heights up to 6 meters.

6. Mechanically Stabilized Earth Retaining Walls (MSE)

MSE Retaining Walls
Image Credits: FDOT

A Mechanically Stabilized Earth (MSE) retaining wall is a type of retaining wall system that utilizes reinforcement elements, such as geosynthetic materials (like geogrids or geotextiles), to enhance the stability and load-bearing capacity of the wall.

• MSE combines the strength of the soil mass with the tensile strength of the reinforcement to create a stable and efficient structure.
• Most economical and most commonly constructed retaining walls.
• Mechanically stabilized earth retaining wall is supported by selected fills (granular) and held together by reinforcements, which can be either metallic strips or plastic meshes
• The panel, concrete block, and temporary earth retaining walls are the common types of MSE retaining walls.

7. Hybrid Systems

Image Credits: Durisol’s Hybrid system

Hybrid systems retaining walls integrate multiple construction methods and materials, often combining elements such as concrete, steel, geosynthetics, or other materials. 

• These systems leverage the unique properties of each component to optimize the performance and efficiency of the retaining wall.
• Hybrid systems can incorporate a variety of materials and techniques, depending on the specific design requirements. 
• For example, they may combine the strength of concrete or steel with the reinforcement properties of geosynthetics or use precast concrete elements in combination with soil nails or ground anchors.

In conclusion, retaining walls serve the purpose of holding back soil or other materials on steep slopes. They come in various types, including gravity walls, cantilever walls, and anchored walls, and can be constructed using different materials. By preventing soil movement, these walls allow for the creation of level areas in sloping terrains.

It is important to consider failure modes such as overturning, sliding, bearing check failure, and overstress when designing and constructing retaining walls to ensure their stability. Additionally, hybrid systems offer an innovative approach by combining different materials and techniques to enhance the performance and efficiency of the retaining wall structures.

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