Guide · 6 min read
Retaining Walls Explained
A retaining wall holds back ground where levels change. Reinforced concrete, mass-block and gabion walls each suit different loads and sites, but all three fail for the same reason: water pressure building up in the retained ground with nowhere to escape.
The short answer
Retaining walls resist the lateral pressure of soil trying to move to its natural angle of repose. That pressure rises sharply when the retained ground is saturated, so every retaining structure needs a drainage path as well as structural capacity.
For anything more than a low garden step — and for any wall with a driveway, road or building above it — the design should come from an engineer.
Reinforced concrete walls
A reinforced concrete wall is cast on a footing, with reinforcement sized by the engineer for the height of retained ground and any surcharge above it. It gives the highest capacity in the thinnest profile, which matters on tight boundaries.
The footing is often wider than people expect, because it is what stops the wall rotating. That footing width is a common reason a wall needs more excavation than the finished wall suggests.
Mass-block and masonry walls
Mass-block and masonry retaining walls rely on their own weight and a footing to resist soil pressure. They suit modest level changes and give a consistent finished face, and are common on house sites, patios and terraced gardens.
Gabion walls
Gabions are wire baskets filled with hard stone. They are free-draining by nature, tolerate small amounts of ground movement without cracking, and are frequently the practical choice on embankments, road edges and rural sites where a rigid wall would be over-specified.
Drainage: the detail that decides everything
Behind a retaining wall you want free-draining granular backfill, a way for water to leave — weep holes or a land drain at the base — and no clay or spoil tipped in to trap water against the wall.
Almost every leaning retaining wall in the country has the same history: no drainage, clay backfill, and a wet winter.
- —Granular backfill placed and compacted in layers
- —Land drain or weep holes at the base of the wall
- —Surface water directed away from the top of the wall
- —No unsuitable spoil used as backfill
Frequently Asked Questions
How high can a retaining wall be without engineer design?+
There is no single universal figure — it depends on the ground, the wall type and what sits above it. Any wall retaining meaningful depth, or carrying a surcharge such as a driveway or structure, should be designed by an engineer.
Do retaining walls need planning permission?+
It depends on height, location and whether the wall is part of a development already permitted. Check with the local authority, particularly for walls at boundaries or on road frontages.
Why is my existing retaining wall leaning?+
Most commonly water pressure behind it, an undersized footing, or backfill with unsuitable material. Any repair should establish which of the three it is before rebuilding, or the same failure repeats.