What is a retaining wall?
Retaining walls are constructed to resist the lateral pressure of soil or hold back soil materials. These walls can also withstand the pressure caused by earth filling, liquid pressure, sand, and other granular materials that are located behind the retaining wall structure. The main purpose of retaining walls is to provide support and stability to the soil, preventing it from sliding or collapsing.
There are different types of retaining walls that are designed and constructed for various goals. The selection of a retaining wall type depends on the specific requirements and conditions of the site. Gravity retaining walls, for instance, are made of heavy materials such as concrete or stone and rely on their weight to resist the lateral pressure of the soil. Meanwhile, cantilever retaining walls use a thin stem and a base slab to counteract the pressure from soil and other materials.
Other types of retaining walls include anchored retaining walls, which use cables or other reinforcement to provide additional support, and sheet pile retaining walls, which use steel or vinyl sheets to hold back soil and water. The choice of retaining wall structure is influenced by the type of soil, the height of the wall, and the desired aesthetic appearance. Ultimately, retaining walls are crucial in ensuring the safety and stability of soil in various construction projects.
Types of Retaining Walls
There are various types of retaining walls used in civil engineering, each with its unique features and benefits. One common type is the gravity retaining wall, which relies on the weight and mass of the structure to resist the pressure of soil and water. Another type is the crib retaining wall, which is made up of interconnected concrete or timber blocks, forming a series of cells filled with soil or other materials. Gabion retaining walls, on the other hand, are made of wire mesh baskets filled with rocks or other materials, creating a porous structure that allows water to drain through. Cantilever retaining walls, as the name suggests, are supported by a horizontal projecting beam, which transfers the load of the soil and water to a foundation. Counter-fort or buttressed retaining walls have vertical concrete webs or buttresses that extend from the back of the wall, providing additional support and stability. Anchored retaining walls are supported by cables or other tension members, which are anchored into the soil or rock behind the wall. Piled retaining walls rely on the strength of vertical piles or columns that are driven into the ground to support the wall. Mechanically stabilized earth (MSE) retaining walls are constructed using layers of soil or other materials reinforced with synthetic or metallic materials, such as geogrids or steel strips. Hybrid systems combine two or more types of retaining walls, such as a gabion wall with a cantilever or anchored wall, to create a customized solution for a specific site and situation. Each type of retaining wall has its own advantages and disadvantages, depending on factors such as soil type, water pressure, wall height, and construction costs.
1. Gravity Retaining Wall
A gravity retaining wall is designed to resist lateral earth pressure solely through its self weight. To counteract the pressure of the soil, a significant amount of gravity load is required, making the structure typically quite massive. During the design process, it is important to consider sliding, overturning, and bearing forces. Various materials, such as concrete, stone, and masonry units, can be used to construct this type of retaining wall. For heights up to 3m, a gravity retaining wall is considered to be economical. Additionally, crib retaining walls, gabions, and bin retaining walls are all examples of gravity retaining walls.
Fig. 1: Gravity retaining wall
Fig. 2: Materials used for gravity retaining wall construction
Fig. 3: Pressure acting on gravity retaining wall
2. Crib Retaining Wall
Crib retaining walls are a type of gravity wall that uses individual interlocking boxes made of either timber or pre-cast concrete. These boxes are then filled with coarse granular materials such as crushed stone, creating a structure that is free-draining. Two common types of crib retaining walls are reinforced precast and timber retaining walls.
While crib retaining walls are well-suited for supporting planter areas, they are not recommended for supporting slopes or structures. This is important to keep in mind when deciding whether to use crib retaining walls for a particular project. The walls are designed to resist lateral pressure from the soil, but they are not suitable for all applications.
In summary, crib retaining walls are a type of gravity wall that is constructed using interlocking boxes filled with coarse granular materials. They are commonly used to support planter areas but are not recommended for supporting slopes or structures. When considering using crib retaining walls for a project, it is important to keep in mind their limitations and suitability for the intended application.
Fig. 4: Crib retaining wall
Fig. 5: Timber Crib retaining wall
3. Gabion Retaining Walls
Gabion retaining walls are structures made of rectangular wire mesh boxes, which are filled with rocks or other suitable materials. These boxes are multi-celled, which means that they have several compartments that can be filled with different materials. Gabion retaining walls are often used for erosion control, as they provide a sturdy barrier against the forces of water and can help prevent soil erosion. They are also used to stabilize steep slopes, as the weight of the rocks inside the gabion boxes helps to anchor the soil in place and prevent landslides. Overall, gabion retaining walls are a versatile and effective solution for a variety of construction and environmental challenges.
Fig. 6: Gabion retaining wall
4. Cantilever Retaining Wall
A cantilever retaining wall is a common type of retaining wall that is made up of a stem and a base slab. It can be constructed from reinforced concrete, precast concrete, or prestressed concrete. This type of retaining wall can either be built on site or prefabricated offsite, such as in the case of precast construction.
The base slab of a cantilever retaining wall is divided into two parts: the heel and the toe. The heel refers to the portion of the base slab that is located beneath the backfill material, while the toe is the remaining part.
Cantilever retaining walls are economical up to a height of 10 meters. Although they require a smaller quantity of concrete compared to gravity walls, their design and construction must be carried out with great care. During the design process, considerations such as sliding, overturning, and bearing pressure must be taken into account, similar to those for gravity walls.
Fig.7: Cantilever retaining wall
Fig. 8: Precast retaining wall
Fig. 9: Different pressure on cantilever retaining wall
Fig. 10: Different configuration for cantilever retaining wall
5. Counter-fort / Buttressed Retaining Wall
The described structure is a cantilever retaining wall that has been reinforced with counter forts that are constructed in a monolithic manner with the back and base slabs of the wall. The spacing between the counter forts is either equal to or slightly greater than half the height of each counter fort. The height of the counter-fort wall can range from 8 to 12 meters.
Fig. 11: Counter-fort or buttress retaining wall
6. Anchored Retaining Wall
An anchored retaining wall is a suitable option when limited space or a thin retaining wall is required. It is also effective for loose soil over rocks, and it can support considerably high retaining walls. The structure system involves the insertion of deep cable rods or wires into the ground, which are then filled with concrete to provide an anchor. These anchors, also known as tiebacks, act against the pressure of overturning and sliding. Overall, the use of an anchored retaining wall system is a reliable way to support tall and narrow retaining walls while providing stability against potential soil movement.
Fig. 12: Anchored retaining wall
Fig. 13: Different configuration for anchored retaining wall
7. Piled Retaining Wall
Pile retaining walls are a type of construction where reinforced concrete piles are driven into the ground, adjacent to each other, in order to create a barrier that can withstand lateral pressure. These walls are designed to counteract any force that might push them over, and they are suitable for use in both temporary and permanent works.
Piled walls are known for their high stiffness and their ability to retain lateral pressure in deep excavations without causing any disturbance to the surrounding structures or properties. This makes them a popular choice for a wide range of construction projects.
Sheet pile walls, on the other hand, are built using steel sheets that are driven into the ground to create a barrier that can withstand lateral pressure. While they are effective for slopes or excavations up to a certain depth, they are not suitable for very high pressure situations. Additionally, sheet pile retaining walls are generally only economical up to a height of 6m.
In summary, pile retaining walls and sheet pile walls are two different types of construction techniques that are used to create barriers that can withstand lateral pressure. While piled walls are known for their high stiffness and ability to retain pressure in deep excavations, sheet pile walls are generally only economical up to a height of 6m and are not suitable for very high pressure situations.
Fig. 14: pile retaining wall
Fig. 15: Temporary pile retaining wall
Fig. 16: Sheet pile retaining wall
8. Mechanically Stabilized Earth (MSE) Retaining wall
A cost-effective and frequently used retaining wall is the mechanically stabilized earth (MSE) retaining wall. This type of retaining wall relies on granular fill and reinforcements, which can be made of either metal strips or plastic meshes, to hold it in place. The selected fills and reinforcements work together to provide the necessary support for the wall.
There are different types of MSE retaining walls available, including panel, concrete block, and temporary earth retaining walls. Each type has its own unique features and advantages, making them suitable for various applications. Regardless of the type chosen, an MSE retaining wall is an effective solution for preventing soil erosion and providing structural support.
Fig. 17: Mechanically stabilized earth retaining wall
9. Hybrid Systems
A type of retaining wall system that combines the use of both mass and reinforcement for stability is known as a hybrid or composite retaining wall system. Such a system typically consists of a combination of concrete or masonry blocks, along with reinforcement materials such as steel bars or geosynthetics. The use of both mass and reinforcement in the design of the wall system allows for a more efficient distribution of forces, as well as increased resistance to lateral pressures exerted by the retained soil. The resulting hybrid retaining wall system is often more cost-effective and durable than traditional retaining walls that rely solely on mass for stability. An example of a hybrid retaining wall system is shown in Figure 18.
Fig. 18: Hybrid retaining wall system