A retaining wall is either wood, steel, or reinforced concrete structure designed and constructed to withstand lateral loads and movement of soil materials. The lateral loads could be a result of earth filling, pressure from the water, or other forms of liquids, sand, and granular materials behind the retaining wall structure. There are different types of retaining walls that are constructed to withstand lateral loads and hold back soil movement. Essentially a typical retaining wall is a vertical cantilever.
Types of Retaining Walls
The different types of retaining walls include the following:
- Gravity Retaining Wall
- Mechanically Stabilized Earth Retaining wall
- Piled Retaining Wall
- Gabion Retaining Walls
- Cantilever Retaining Wall
- Anchored Retaining Wall
- Hybrid Systems
- Crib Retaining Wall
- Counter-fort / Buttressed Retaining Wall
Gravity Retaining Wall
This is mostly used for walls up to 2m-3m high but maybe economically higher if locally available materials could be used. The weight of the structure is used to resist both the overturning and sliding forces. These walls need not be homogeneous since it is uncommon to have a hollow concrete wall filled with earth materials to act as a gravity retaining wall. A concrete gravity wall usually contains a nominal amount of reinforcement near the exposed surfaces to control temperature cracking.
The following are the common features of gravity retaining wall:
- Gravity retaining wall is usually massive due to its requirement of significant gravity load to counter lateral soil pressure.
- Gravity retaining wall depends solely on its self-weight to resist lateral soil pressure.
- The design of the gravity retaining wall takes into consideration overturning, bearing, and Sliding forces.
- This type of retaining wall can be constructed using different materials such as stone, masonry units, and concrete.
- It is highly economical for wall heights ranging from 2m to 3m.
- Common retaining walls such as gabions, bin retaining walls, and crib retaining walls are also considered gravity retaining walls.
Mechanically Stabilized Earth Retaining wall
The Mechanically Stabilized Earth (MSE) retaining wall is a composite structure with alternating layers of compacted backfill earth materials and soil reinforcement elements, fixed to the wall facing. The stability of this type of retaining wall system comes from the interaction between the backfill earth materials and the soil reinforcements which produces friction and tension on the wall facing. The wall facing is usually a relatively thin wall, which primary function is to prevent erosion of the structural backfill and to support heavy loads.
The common features of Mechanically Stabilized Earth (MSE) retaining wall are:
- A mechanically stabilized earth retaining wall is one of the most economical and most popularly constructed retaining walls.
- Mechanically stabilized earth retaining wall is usually supported by carefully selected earth fills or granular and bonded by reinforcements, which can be either plastic meshes or metallic strips.
- Examples of these types of retaining walls include concrete blocks, panels, and temporary earth retaining walls.
Piled Retaining Wall
The piled retaining wall is widely used in the construction industry as a result of its availability and ease of construction. Sheet pile walls are constructed using steel sheets driven into a slope or an excavation to a specified depth to withstand low lateral pressure.
It is used as permanent or temporary structures to protect the adjoining structures. It avoids lateral movement of the soil around a foundation structure, especially in weak soil conditions. Also, the piled retaining wall can be employed as shoring in excavation works.
This type of retaining wall is economical eve up to 6m in height.
Gabion Retaining Wall
A typical gabion wall is a type of retaining wall constructed from stacked stone-filled gabions tied together with wire. Gabion walls are commonly battered i.e angled back towards the slope or stepped back with the slope, instead of being stacked vertically.
Some of the characteristics of gabion retaining walls include:
- They are multi-celled rectangular wire mesh boxes, filled with either rocks or other suitable earth materials
- It is commonly used for the construction of erosion control structures
- It is employed in stabilizing steep slopes of either road or bridge embankments
Cantilever Retaining Wall
Cantilever walls are usually constructed using reinforced concrete, with inverted T-shaped, or L-shaped footings. The weight of backfill soil materials behind the wall is directly transferred onto the foundation to prevent toppling resulting from lateral earth pressure from the same soil mass.
Typical cantilever retaining wall has the following features:
- It is made up of a stem and a base slab
- A cantilever retaining wall can either be constructed from reinforced concrete, prestressed concrete, or precast concrete.
- This type of retaining wall is the most common type used as a retaining wall.
- The cantilever retaining wall can either be constructed on-site or prefabricated offsite i.e. precast.
- The part of the base slab underneath the backfill material is known as the heel, and the other part is called the toe.
- The cantilever retaining wall is very economical even up to a height of 6-10m.
- The construction of a cantilever retaining wall requires a smaller quantity of concrete compared to a gravity wall, but the design and construction should be executed with uttermost care.
- During the design just like the gravity wall, sliding, overturning, and bearing pressure are taken into consideration.
Anchored Retaining Wall
This type of retaining wall system is a structural element built into a rock or soil formation to transfer the tensile forces on the structure to the ground. These types of retaining walls are often slimmer compared to other types of retaining walls such as gravity walls and cantilever retaining walls.
Anchored retaining wall systems are also known as a tie-back system, It is made up of cables, anchors, and thin walls. The anchors are installed into the ground to create a balance and transfer tensile loads to the foundation and then the lateral load is supported by the retaining wall, which is firmly held in place by strong cables connecting the anchors and walls to the ground.
Some of the advantages of anchored retaining walls include:
- Stability: With the use of cables and the wall anchored deeply into the soil usually provide your anchored retaining with a great level of structural stability.
- Ability to Support heavy loads: The strength of the cables added with the sizes and location of the anchor provide enough support for much heavier loads.
- Slimmer than other wall types: The anchor walls commonly used when compared to other types of retaining walls are thinner and slimmer, which makes them easier to transport and construct which can save a lot of space.
- Economical: They are cheap to construct and require less maintenance than other types of retaining walls, which makes a great long-term choice.
- Slope protection. The reason for retaining walls is to hold back soil in place, so this option is perfect for slope protection and maintenance.
Hybrid systems retaining walls are the types of retaining walls that use both mass and reinforcement for stability and are also known as composite retaining wall systems.
Hybrid system retaining walls are made up of a lower soil nail wall (SNW) and a mechanically stabilized earth wall (MSEW) to support lateral loads.
Crib Retaining Walls
Crib retaining walls also called crib lock retaining walls are a form of gravity wall deployed to support lateral loads. These types of retaining walls are some of the oldest types of retaining walls. They are made up of cribs or cells made of concrete, timber, and plastic/fibers. The cribs or interlock areas are filled with earth back-fill or free-draining materials that eliminate the hydro-static force and allow a free flow of water.
They are usually constructed from timber or pre-cast concrete of interlocking individual boxes in which the boxes are filled with hardcore, crushed stone, or other coarse granular materials to allow free-draining of water from the structure. Most crib retaining walls are employed to support planter areas and are usually not recommended for slope protection.
The basic materials ideal for crib wall construction are:
- Timber crib retaining walls: This type of crib wall uses timber to form the cells of the crib in which the cells are filled with free-draining materials of crushed stones or granular materials to serve as the mass of the wall and permits free flow of water through the cribs. These types of crib retaining walls can span up to a height of 5-6m and are mostly used for landscaping walls.
- Precast crib retaining walls: This crib system comprises a precast concrete header and stretcher units erected to form precast crib retaining walls.
These types of crib retaining walls are the cheapest types of retaining wall systems and are commonly used for plant terraces, landscape structures, and other works with heights over 10m when properly designed.
Counterfort/Buttressed Retaining Wall
Counterfort/buttressed retaining wall as the name implies is a type of cantilever retaining wall with counterforts, or buttresses fixed to the inside face of the retaining wall to further resist lateral pressure and to hold back death backfill materials. The standard counterfort wall design and installations require the spacing for the counterfort to be slightly larger or equal to half of the counter-fort height. Most counter-fort wall height ranges up to 8-12m/
Some of the materials commonly used for counterfort retaining walls are concrete block systems, poured concrete, treated lumber, brick, and stone.