What are retaining walls?
Retaining walls, quite simply, retain the ground behind them. They are designed to counteract the force of the ground’s load, either via their own mass, or by an anchoring system.
What are the different types of retaining wall?
There are four main types of retaining wall, which can be created with different materials, and each come with their own advantages and disadvantages.
1. Gravity Retaining Walls
Gravity retaining walls rely on their mass to resist pressure from the load behind, and are therefore made from heavy materials such as stone or concrete. Gabion baskets (steel baskets filled with stones) are an excellent example of this, as they do not require concrete footing, unlike most gravity retaining wall systems. Consequently they are easy to install and simple to remove when required.
2. Sheet Pile Retaining Walls
Sheet pile retaining walls are often used in areas where there is soft soil and tight spaces.
Sheet pile retaining walls are constructed by driving, vibrating or pressing sheet piles into the ground(7.3). A degree of robustness is thus required and in this regard steel is used in the manufacture of the sheet piles.
Anchors to anchored sheet pile walling are typically ground anchors, where the anchor comprises a tendon utilising steel bar, or strand tendon centrally located in cement grout, which can be, but not generally, injected under high pressure. The cement grout provides the integrity of the anchor and bond between tendon and surrounding soil.
Other forms of ‘tie back’ anchor can be employed utilising screw anchors or ‘dead man’ anchors, with associated steel rod or strand ties.
The aesthetic appearance of sheet pile walls can be enhanced by the use of a gabion type facing.
Sheet pile walls can be expensive to install as heavy piling machinery is required, which is often hard to manoeuvre in a small space. As an alternative, gabion baskets of a minimum depth of 300mm can be used in small spaces if held by metal or concrete support posts, but a Consultant engineer will be required to assess the installation.
3. Cantilever Retaining Walls
Cantilever retaining walls are made from reinforced concrete or mortared masonry. Their reinforced base means that pressure from the ground behind the wall acts vertically rather than horizontally, creating firm structure. However, their installation may be complex and costly, making gabion baskets a great alternative.
The use of the description ‘cantilever retaining walls’ can be applied to both gravity and embedded retaining walls. The former relates to the inverted ‘T-shaped’ walls or reverse cantilever walls, also known as ‘L-shaped’ walls.
In the case of the former, the cantilever can comprise reinforced concrete or reinforced masonry.
Brick has historically been used to form gravity earth retaining walls, and there are many examples of Victorian engineering and construction, notably associated with railways, of both retaining walls and bridge abutments. Reinforcement is not required for a gravity retaining wall.
Timber has been used in both gravity and embedded cantilever retaining walls. Advancement and knowledge in types of timber and treatment process, notably that in New Zealand, resulted in the use of timber in the construction of crib retaining walls. There are many examples throughout the world, including the UK, where timber crib earth retaining walls have been extensively used.
Like the gabion wall, the granular infill to the crib wall maintains free passage of water.
Migration of particles typically occurs where there is a flow of water and where soil particles are sufficiently small to be conveyed by the passage of water. Particle migration can occur in sands, silts and clays. Geotextile membranes are specified to the rear of the wall to address such occurrence’s but their presence must be taken into account in the design of the retaining wall as appropriate.
4. Anchored Retaining Walls
Anchored retaining walls are literally anchored by cables, which are driven into the ground behind the wall and secured by a mechanical expansion, or by injecting pressurised concrete. This method is good for walls with high loads or for reinforcing thin walls, but again installation is likely to be more complex than that for gabion walls.
What are the main materials used in retaining walls?
A failed retaining wall made from brick
Brick retaining walls can be used for low or planter walls, but in general they lack reinforcement and suffer from hydrostatic issues. Weep holes and drainage trenches will aid water movement, but this will involve additional costs.
A failed retaining wall made from timber sleepers
Timber is often used for short crib retaining walls but it is susceptible to a build-up of water, which can lead to a dramatic retaining wall failure.
Although very sturdy, concrete can be complex to install, and even harder to remove. It is also not very sympathetic to the surrounding landscape.
Failed concrete retaining wall
Steel is perfect for sheet pile retaining walls as it can be driven into the ground easily, but again the overall installation process may be quite costly.
A retaining wall made from gabion baskets
Gabion baskets have very few drawbacks apart from access to local stone, and in the UK this is rarely an issue, as a multitude of suitable stone types are readily available from local quarries. Unlike other materials, gabion baskets are not vulnerable to drainage issues as they are porous, so water cannot build up and cause structural failure to the retaining wall. When gabion baskets are used on ground such as red clay, where there is the potential that material could wash through the retaining wall, a geotextile cloth is used behind to prevent this from occurring.
Gabion baskets are also perfect for creating a retaining wall, which is sympathetic to natural surroundings, as vegetation is able to grow through the baskets, restoring the landscape to its original glory.
You can find out more about the pricing of our Gabion baskets on our website, along with everything you need to know about design, installation and recommended installers. You can also read our gabion blog for examples of different uses. If you are having any difficulty sourcing gabion stone for your baskets, or have any other questions, please get in touch by phoning 01902 810 310, by email or by connecting to our live online chat.