Retaining walls are support works, in general, permanent works, used to ensure the passage between two elevations when the space is not sufficient for a sloped excavation.
Retaining walls are continuous support structures that receive the full force of the earth's pressure throughout their whole surface of contact with the ground. They are commonly used in landscaping and construction projects to create level surfaces and provide support for vertical changes in elevation.
According to AS 4678, there are two different types of retaining walls: civil retaining walls up to 15.00 m and landscape retaining walls up to 800 mm in height, which are not included in the standard.
Stone or brick masonry, including sandstone logs, concrete, reinforced concrete, and precast components (types of lattice or gabions), is used to construct the supporting walls.
Gravity walls are often constructed up to 1.5–2.3 metres in height from materials such as brick (clay or concrete) masonry, though they can also include sandstone logs, concrete, or gabion baskets. From 2 to 7 m, a cantilever wall made of reinforced concrete is usually the right choice.
Over 7 m, a retaining wall with foundation toes, soil anchors, and many wings can often be employed in a mixed approach to provide a workable, affordable solution.
Retaining walls can be used in a variety of applications, such as to prevent erosion on sloping land, to create level terraces for gardening or other activities, to support the weight of a structure built on a sloping site, or to create space for parking or other activities on a steep incline.
There are mainly 3 main failure modes of a retaining wall; those are:
Sliding, overturning, and pressure.
Stability Sliding: This means that the forces that are pushing out of equilibrium the retaining wall are greater than the forcing that opposes.
The equation below represents the stability equations and states that the ratio between the vertical and horizontal forces must be greater than or equal to 1.3 to satisfy the stability equations. The failure phenomenon is the partial or total displacement of the wall until the supported loads reach an equilibrium state, satisfying the condition from the equation once again.
2. Stability Overturning: The horizontal and vertical forces that are pushing or stabilizing the retaining walls create overturning and stabilizing moments. The below equation represents the stability equations and means that the ratio between the stabilising and overturning moments calculated in the centre of rotation must be greater or equal to 1.5 to satisfy the stability. The failure phenomenon is represented by the total or partial rotation of the wall around the rotation point situated at the base of the foundation at the most extreme point of the toe.
3. Stability Pressure: Footing pressure on the terrain has to be under the value of the allowable bearing pressure ABA and above 0. The failure of the foundation layer is evident through the settlement of the foundation layer and the displacements and rotations of the wall, which may result in the wall encountering one of its initial two failure modes: sliding or overturning.
It should be mentioned that there are other failure modes that need to be considered depending on the retaining wall type, such as the strength of the section, failure due to hydrostatic pressure, exceptional forces such as impact forces, etc.
Designing a retaining wall is a critical task that requires engineers to carefully consider the wall's geometry, materials, loads, and potential modes of failure.
To accomplish this task effectively, engineers must conduct a proper scope of work and a minimum of geotechnical and topographical investigations. These investigations provide critical information about the location, safety, serviceability, stability, and strength of the retaining wall, which allows engineers to create a design that meets all necessary requirements and standards.