Solutions come plentifully for retaining walls which is both good and bad! The type of retaining wall chosen for a project should be a decision took from someone with experience in the area, someone with a civil, structural or geotechnical background.
By the end of this post you will know the different types of retaining walls, the advantages and pitfalls for each of the different types and some of the factors that come into play when during the decision process.
Use of Retaining Walls
Retaining walls retain earth behind them. They are designed to withstand the pressures and allow structures to be built on uneven ground, slopes, near embankments and allow us to excavate to build basements etc.
As land becomes more of a premium in such places like London, Hong Kong, Japan, New York etc. building down can be a very desirable investment. Currently in London a lot of basements are being built and not just in commercial properties but also residential for the extra room.
Factors for Designing Retaining Walls
So, we realise that we need a retaining wall. What on earth (no pun intended) affects the design and chose. Ground conditions, load conditions, location construction (site access) and size of the overall project.
In the design of retaining walls, factors of safety typically 1.5 – 2 are used to take into consideration unknown factors in the design stage and the fact that materials are non-linear, similar to other areas of design for engineers.
Modes of Failure
Retaining walls have five modes of failure to be aware of when designing them, three of them will be explained briefly below.
Sliding Failure – This is a shear failure at the base of the wall, as the name suggests the retaining wall will slide. Not the ideal scenario when there is a lot of soil behind it. Friction and self-weight of the wall are the main component that prevents sliding failure.
Bearing Capacity Failure – Think of bearing capacity failure as someone sinking in quicksand. A heavy wall on weak soil may well be part of the titanic remake (cheaper to make too! Any movie produces out there, please get in touch…).
Overturning Failure – Caused by rotation at the toe of the wall with the rotation being caused from bending in the stem.
On top of the above, shallow and deep failure also occur, due to it being a more advanced topic this will be ignored in this post.
Loading On A Retaining Wall
Loading on a retaining wall occurs from active and passive pressures. Active pressure must be equal to the passive pressure to avoid overturning of the retaining wall. A surcharge may also be applied to the wall from various scenarios such as highway loads and nearby buildings.
The pressures will cause both bending and shear force in the retaining wall, the wall must have the capacity to resist the forces. This is from the section properties of the material (think – reinforcement in concrete for tension resistance or sheet pile second moment of area). After all the components from the loading are broke down and the forces on the wall are clear, the design of the wall is not complicated at all.
Below is a sketch from the typical forces from pressure that may be seen on the wall. Note the location of the maximum force, it is useful to understand how they look and why before plowing into the design of them.
Gravity Retaining Walls
The first and arguably the most common type of retaining wall we are going to look into is the gravity retaining wall. The name comes the fact that the self-weight of the wall resists the pressure from the soil back and of course the self-weight is due to gravity. They do not rely anything else for stability.
When designing gravity retaining walls, the following needs to be taken into account… Sliding, overturning and bearing capacity. On top of this the capacity of the wall needs to be considered, these can be made from a lot of different materials, most commonly we see a lot of reinforced concrete walls.
Gabion Retaining Walls
A gabion retaining wall is also a type of gravity retaining wall, the best way to think of a gabion retaining wall is a large scale lego set. They come in rectangular or square blocks using crushed materials tied together with mesh. They are flexible and can be built very fast, being fabricated off-site. Commonly seen for smaller scale jobs such as residential gardens. These typically don’t require a lot of design input structurally.
Buttress Retaining Walls
You’ve seen castles and the use of buttress walls, they provide stability to the wall to stop it sliding and overturning. The same principle applies to earth retaining structures as well. They are great for walls that are very tall and can reduce the size of the wall by incorporating buttresses into them.
Piled Retaining Walls – Sheet Piles and Bored Pile
Two types of solutions are possible for different ground conditions, continuously piled retaining walls and secant piled retaining walls. Both of these provide great solutions for something that causes a lot of headache for engineers… Water! Piled training walls, both continuous and secant are also good for granular soils.
First of all let’s explore continuously piled retaining walls, these can also be called sheet piles, and are bored into the ground (literally pushed). The continuous nature of the piles will stop the flow of water through, although seepage will need to checked if the water table is high. They are cheap to erect and easy to transport. Lots of different sizes are also available and normally in stock as well, bonus! One complication with sheet piles is the construction / erection method, vibrations can damage nearby structures or annoy the local residents. Sheet piles are also used for temporary work situations until more permanent solutions are built, some are sacrificial and left in the ground. Minimal excavation is required.
Piled retaining walls also come with the advantage as sheet piles in that they can stop water entering the construction area. They have a higher construction cost and the process takes longer, plus more excavation is required.
Both sheet piles and piled retaining walls work in many conditions, however with tough ground work the construction process becomes difficult.
Anchored Retaining Walls
These types of retaining walls although similar to gravity retaining walls gain additional strength from anchors / rods and cables that are in the ground behind the structure. They are found in situations with weak soil or rock. They are also commonly used when using a cantilever retaining wall is not possible such as with the presence of bedrock.
Prestressed anchors are drilled into the ground and then filled with grout at pressure which causes the grout to expand. The expansion and the grout help with the tensile resistance due to the friction between the grout and soil.
Blockwork Retaining Walls
This is a solution that came about in my current job (although we are not taking ownership of the design, it is more the fact I’ve never seen them built like this elsewhere), these retaining walls are similar in construction to load bearing walls in houses, they have an inner and outer leaf as well as a cavity, the cavity is filled with reinforced concrete to provide the strength.
Logically they would be used near residential houses and look the same. Basements and in gardens etc.
A major advantage of having these tied into the house floor slab or garage is to prevent sliding of the wall.
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