Retaining wall failure modes

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Retaining walls are support works — generally permanent — used to manage the passage between two elevations when there isn't enough space for a sloped excavation. They are continuous support structures that receive the full force of the earth's pressure across their whole surface of contact with the ground. They're commonly used in landscaping and construction to create level surfaces and support vertical changes in elevation.

Types of retaining wall (by height)

According to AS 4678, there are two broad categories: civil retaining walls up to 15.0 m, and landscape retaining walls up to 800 mm in height (which fall outside the standard). They can be built from stone or brick masonry (including sandstone logs), concrete, reinforced concrete, or precast components such as crib/lattice walls or gabions.

• Up to ~1.5–2.3 m: a gravity wall is often used — brick or concrete masonry, sandstone logs, or gabion baskets.
• From ~2 to 7 m: a cantilever wall in reinforced concrete is usually the right choice.
• Over 7 m: a mixed approach — a wall with foundation toes, soil anchors and counterfort wings — often provides a workable, affordable solution.

Retaining walls are used to prevent erosion on sloping land, create level terraces, support structures built on sloping sites, or create usable space (such as parking) on a steep incline.

The three main failure modes

There are three primary ways a retaining wall can fail: sliding, overturning and bearing pressure.

1. Sliding stability

This means the horizontal forces pushing the wall out of equilibrium exceed the forces resisting them. For stability, the ratio of resisting (vertical-derived) to driving (horizontal) forces must generally be ≥ 1.3.

2. Overturning stability

The horizontal and vertical forces acting on the wall create both overturning and stabilising moments. For stability, the ratio of stabilising to overturning moments (calculated about the toe) must generally be ≥ 1.5.

3. Bearing pressure stability

The footing pressure on the ground must stay below the allowable bearing pressure and remain above zero (no uplift). It's worth noting there are other failure modes to consider depending on wall type — such as structural strength of the section and failure due to hydrostatic pressure.

Good design starts with good investigation

Designing a retaining wall is a critical task that requires careful consideration of the wall's geometry, materials, loads and potential failure modes. To do it properly, engineers carry out an appropriate scope of work along with geotechnical and topographical investigations. These provide the critical information about location, safety, serviceability, stability and strength needed to produce a design that meets all the necessary requirements and standards.