What are the different types of retaining wall?
by Andrew Lees, on 21-Jan-2021 05:13:02
Retaining walls come in all types, shapes and sizes – from simple gravity walls to bored pile walls for basements and reinforced soil walls using geogrids – to suit a wide range of project needs, and site conditions.
In my last blog I looked at the basics of retaining walls, what they do, how they work and how they are designed. This time around, I’m going to give a brief overview of the main types of retaining walls available and give an insight to how they work, as well as their benefits and limitations.
Ground Coffee Ask Andrew Episode 2: Andrew Lees explores the different types of retaining wall available to engineers.
Gravity walls are the simplest, and earliest recorded, type of retaining wall. Built of masonry, brick, concrete blocks or mass cast-in-situ concrete, these hard-wearing structures rely on their large weight to resist toppling and sliding caused by the lateral earth pressure (see my last blog) from the soil behind them.
Gravity walls are typically wider at their base, with sloped faces, enabling them to resist the higher lateral earth pressures at depth. This means that, while they are easy to build and suitable for retained heights of up to about 3m, any higher and they tend to take up too much space and can end up being too heavy for the ground below, leading to bearing capacity failure. This can lead to failure of the soil being retained by the wall.
Cantilever retaining walls
Cantilever walls are built using reinforced concrete, with an L-shaped, or inverted T-shaped, foundation. The vertical stress behind the wall is transferred onto the foundation, preventing toppling due to lateral earth pressure from the same soil mass.
Source: AsmithNJIT at English Wikipedia
Additionally, a T-shaped foundation benefits from the weight of soil (and therefore vertical stress) in front of the wall, providing further stability. Foundations sometimes include a ‘key’ in their base, that sticks into the ground to prevent sliding failure.
A big advantage of cantilever walls is that they take up little space once built, and are suitable for retained heights of up to 5m. However, construction does require space behind the wall, so they are not particularly suited to retaining existing slopes, unless temporary support is provided during construction.
Embedded retaining walls
Embedded retaining walls are used to form near-surface underground structures, such as basements, car parks and metro stations. Walls can be huge – those for Westminster underground station on the Jubilee Line in London, next to the Houses of Parliament, are 40m deep, for example.
They are built using a number of different methods, depending on ground conditions, how watertight the excavation has to be, constructability (ie time, cost and excavation method) and the retained depth required.
Example of an embedded retaining wall
Source: Störfix at English Wikipedia
For deep excavations, methods include diaphragm walls and panels, as well as bored concrete piles, where piles are either interlocking (secant) or installed next to one another (contiguous). For shallow and temporary excavations, sheet piles and king post walls are commonly used, as shown in the image above.
Embedded retaining walls act like cantilever walls, extending deeper than the excavation to take advantage of passive earth pressure of the ground below to, at least partly, counteract the active earth pressure being exerted on the wall above. Additional support is provided by internal propping – usually from the base slab, ground slab and any intermediate floor slabs – or by ground anchors installed through the wall.
Reinforced soil, or mechanically stabilised earth, retaining walls
Last, but not least, there are reinforced soil walls, sometimes referred to as mechanically-stabilised earth walls, use layers of geogrid to reinforce soil, increasing bearing capacity and increasing resistance to differential settlement. I’ll discuss these walls in more detail next time…
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