Below ground, a retaining wall relies on footings to anchor it into stable soil and resist movement. Soil pressure, moisture changes, and ground conditions constantly act on the wall. If footings are too shallow or soil behaviour is not accounted for, the wall can slowly move, crack, or fail over time.
Why is soil movement critical to retaining wall stability?
Soil movement changes the pressure acting on a retaining wall. Wet soil becomes heavier and pushes harder, while dry soil can shrink and shift. If a wall is not designed for these movements, stability is gradually reduced even if the wall looks fine at the surface.
Can a retaining wall fail even if it looks strong above ground?
Yes. Many retaining wall failures start below ground. Inadequate footings, poor drainage, or unstable soil can cause slow movement that is invisible for years before cracking or leaning appears.
Why the Most Important Part of a Retaining Wall Is Invisible
When people look at a retaining wall, they focus on what they can see: sleepers, panels, finishes, and straight lines. In reality, the long-term performance of a retaining wall is determined almost entirely by what happens below ground. Footings, soil conditions, and drainage systems do the real work. If these elements are wrong, even the strongest wall above ground will eventually fail. In Perth, where soil types vary widely and seasonal moisture changes are common, below-ground design is critical.
Understanding Footings and Their Role
Footings are the part of the retaining wall that transfers the load into the ground. They resist sliding, overturning, and rotation caused by soil pressure. A properly designed footing must be deep enough to reach stable soil and large enough to counteract the forces acting on the wall. Shallow footings may perform adequately in the short term, but as soil conditions change, they lose effectiveness. In sandy soils common across much of Perth, footings must be deeper to resist erosion and softening. In clay soils, they must account for expansion and contraction caused by moisture changes.
How Soil Really Behaves Over Time
Soil is not static. It constantly moves in response to moisture, load, and environmental conditions. After rainfall, the soil absorbs water and becomes heavier. This increases lateral pressure against the retaining wall. During dry periods, some soils shrink, creating voids that reduce support under footings. Over repeated wet-dry cycles, this movement gradually weakens stability. This is why walls often fail years after construction rather than immediately. The movement is slow, cumulative, and largely hidden.
The Impact of Poor Soil Preparation
Before a retaining wall is built, the soil beneath and behind it must be properly prepared. This includes excavation to the correct depth, removal of loose material, and correct compaction. If soil is not compacted correctly, it will settle over time. This settlement reduces support beneath the footings and creates uneven pressure behind the wall. Settlement often goes unnoticed until cracks or leaning appear. Poor preparation is one of the most common causes of delayed retaining wall failure.
Why Drainage Protects the Ground Below
Drainage is not just about removing water from behind the wall; it protects the soil supporting the wall. Without drainage, water saturates the soil, increasing its weight and reducing its strength. Saturated soil loses bearing capacity, meaning it cannot support footings as effectively. This allows the wall to rotate or slide forward. Proper drainage keeps soil conditions consistent and predictable, which is essential for long-term stability.
Footing Depth and Wall Height Are Linked
As retaining wall height increases, so do the forces acting below ground. Taller walls require deeper and stronger footings to counteract increased pressure. A common mistake is building a wall with adequate visible height but insufficient footing depth. This imbalance causes slow rotation at the base of the wall. The wall may remain upright for years before movement becomes noticeable. Correct footing design must always match retained height and soil conditions.
How Loads Above the Wall Affect What’s Below
Anything placed near or above a retaining wall adds load to the soil. This includes fences, driveways, sheds, vehicles, garden beds, and future landscaping changes. These loads increase pressure behind the wall and beneath the footing. If they were not considered in the original design, the soil below the footing may gradually compress or shift, reducing stability. This is why retaining walls must be designed not just for soil, but for realistic long-term use of the site.
Why Repairs Often Don’t Fix the Real Problem
When a retaining wall shows signs of movement, repairs often focus on what is visible. Cracks are patched, panels are replaced, or braces are added. These fixes rarely address the underlying issue below ground. Footing depth, soil instability, or drainage failure remain unchanged. As a result, movement usually continues. This is why many failed retaining walls ultimately require full replacement rather than repair.
Designing for Below-Ground Performance
A retaining wall designed to last decades is engineered from the ground up. Footings are sized for soil conditions, drainage is designed to protect bearing capacity, and loads are realistically assessed. In Perth’s variable soils, this approach is essential. Walls designed only to look strong above ground rarely perform well long-term.
Build on Solid Ground
If you’re planning a retaining wall or replacing one that has failed, understanding what happens below ground is essential. Getting the unseen elements right is the key to long-term performance. Contact Structural Retaining Walls today to discuss a retaining wall solution designed for stability from the ground up.