Written by Tomek Miturski
In this article, we will be discussing retaining walls, what they are, where they can be built, and the common types found on residential projects. Where there may be different soil levels, retaining walls provide lateral support to maintain the different levels. By containing the soil of a higher level, retaining walls prevent it from collapsing or spilling over. These structures are built in various sizes and configurations, and range in scale from large civil projects, retaining massive volumes of soil at tens of metres tall, to small garden features used to create a level difference of a few hundred millimetres.
Image above – a steel-reinforced concrete wall. A hole has been dug into the ground, once the wall is built, soil will be poured into the excavation to fill in the gap.
A common use of retaining walls in residential projects is to create lightwells for floors below ground level or when constructing basements, the walls of which are retaining walls that resist soil from all four elevations. They are also often used in landscaping and gardens. Retaining walls are formed from reinforced concrete, which is either poured using traditional steel-reinforced concrete to create one monolithic structure or out of hollow concrete blocks which are then filled with concrete and steel rebar.
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Structurally, the main purpose of a retaining wall is to resist lateral pressures created by soil retained at two different heights. Disturbed soils come to rest at angled slopes, if a hole is dug into the soil you will find that the walls of the excavation cave in, therefore, forming stable, vertical walls out of the soil is difficult as they can easily collapse into a sloped pile. Retaining walls hold back the soil and form vertical walls, creating the difference in levels.
As the level difference between high and low-level increases i.e. the wall gets taller and therefore retains more soil, the lateral pressures created on the wall by the soil increase, similarly to how pressure increases the deeper one is underwater.
Various forms of retaining walls can be built-in, these utilise different mechanics or systems to derive their resistance to the lateral pressures induced by retaining soil. The type of wall built is scenario-specific as they have varying complexities, require different amounts of space or prior excavation, and varying timeframes for construction. In residential projects, two main types of retaining walls are used: free-standing and propped walls.
Freestanding walls, or cantilever walls, have bases that are considered fixed, resistant to movement, however, the top of the walls are unrestrained to deflect laterally; this is similar to driving a wooden post into the ground and pushing it sideways, the bottom of the post will not move laterally, whereas the top will. Cantilever retaining walls can be formed in “L” shapes and an upside-down “T” shape. The weight of the soil sits on the base, also referred to as the ‘heel’, of the freestanding wall to hold it down and in place. This generates resistance to any sideways movement. Most residential projects that require retaining walls will utilise freestanding walls due to the simpler construction compared to propped walls.
Propped retaining walls are similar to freestanding walls however, the top of the walls are reinforced using additional steel reinforcement bars designed to provide lateral restraint, creating a wall that is fixed at the top and bottom. For perspective, if you were to drive a wooden post into the ground and push it down at the top of the post whilst attempting to move it sideways, it won’t move or bend as it is propped at the top. This additional reinforcement in the top of the walls forms a ’ring beam’. This type of retaining wall relies on the strength of the ring beam, rather than the weight of soil over the base. In turn, this means that propped walls may not require a base, reducing the amount of reinforced concrete and excavation required. Propped walls are preferred where there is limited space for excavation of a large base, or where the wall will have a more complex shape e.g. a lightwell.
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Retaining walls that are improperly designed or built have many modes of failure that are categorised as a local or global failure. Local failure occurs when localised regions of the wall cannot sustain the imposed loading and, for example, cracks due to excessive movement. This can occur if the wall is not built properly or is insufficiently reinforced.
Global failure concerns the overall stability of the wall and surrounding soil, it occurs to the large wall sections at once as opposed to one localised region. Global failure is broken down further into specific types of wall failure. Overturning failure arises when the soil pressure and wall shape cause it to overturn at its lowest corner. Sliding failure refers to when the soil pressure causes the wall, in its entirety, to slide sideways in the direction of the load due to insufficient weight or resistance to movement. Bearing failure is similar to overturning failure, it causes the wall to rotate about its bottom corner. The cause of bearing failure is that the soil under the wall is overloaded and it compresses due to a sheer failure.
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Retaining walls are useful structures that are ubiquitous throughout the modern residential industry and come in a variety of shapes and sizes; they are used to retain soil and allow for a difference in levels on either side of them. In residential projects, they are mostly used when constructing basements and lightwells. The most common types of walls on this scale are freestanding walls that have an unrestrained top and propped walls that utilise a strengthening ring beam. Improperly designed walls can fail globally, where whole sections of the wall move or rotate excessively. If poorly built, the wall can fail locally resulting in undesirable cracking, moving, and bulging.
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