Notes Extensions, chapter 4 of 4
Garden retaining walls: deeper is not stronger
A house foundation is deep because of what it stands on. A retaining wall is wide because of what is leaning on it.
Why does a garden wall need a wider base than the house?
Here is one of the questions we are asked most often, and it usually arrives in a slightly wounded tone. My house foundations are nearly two metres deep. Why does the garden retaining wall need a base a metre wide?
Carrying a wall down
The load comes straight down. The base has to reach ground that can hold it and stay still.
The underside of the base is still inside the zone that dries out and swells with the seasons. The load is pressed into a narrow strip of ground.
Holding soil back
The same base, asked to retain soil on one side. The push is sideways, and it wants to rotate the wall about its toe.
The wall has begun to rotate about its toe.
Illustrative. Not a calculation and not a design.
It takes the base down. It does not change the height of soil held back, so the retaining wall stays as it is.
This settles the wall holding soil back. Carrying a wall down, it changes how hard the load presses.
No numbers, on purpose: it shows why the two shapes differ, not what either should be.
Diagram drawn for this site, not to scale. One mechanism only: overturning about the toe, resisted by the width of the base and the weight on its heel. Sliding and the pressure under the toe are just as real.
Diagram drawn for this site. Not to scale, not a calculation and not a design. What a retaining wall needs is worked out for the soil, the height and the drainage on that site.
Key to the drawings
- Brickwork, new Single diagonal hatch. The outer leaf, and new brickwork in elevation.
- Blockwork, new Cross-hatch, so it can never be read as brick beside it.
- Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
- Undisturbed ground Basket weave, as a band that follows the ground line and fades out.
- Granular fill Open rounded stones: free-draining fill behind a retaining wall.
- Insulation board The looped line, touching both faces, inside the board outline.
- Reinforcement A line along the bar, a dot where a bar is cut.
- DPC Heavy dashed line, as Approved Document C draws it.
- Load path Where the load goes, from the roof to the ground; dashed where it runs beyond the cut.
- Load marks Arrows at the symbol weight: loads down onto a member, bearing up under a foundation.
- Hidden, beyond or removed Out of the cut, beyond it, or taken out.
- Named dimension A quantity named, never numbered.
- Break line The drawing stops; the building does not.
The two walls are asked different questions. A house foundation answers a vertical one: find ground that can carry this, below the ground that shrinks and swells. and watch the left-hand wall.
A retaining wall answers a sideways question as well: soil leans on it, trying to tip it and slide it. The push is set by the height of soil held back, and it grows faster than the height. Taking the base deeper leaves that height where it was.
What answers it is the base, sideways. The toe is the point the wall would rotate about, so width sets the wall’s weight, and the soil on its heel, further from it. and the wall stands square.
Depth is still necessary: the base must reach ground that can carry it. It is simply not what resists the push.
What actually brings garden retaining walls down?
Usually water. pushes considerably harder than . pushes harder too: a drive, a shed, a heap of spoil left over a winter. Drainage, and what may stand above the wall, belong in the design.
Illustrative. Not a calculation and not a design.
Diagram drawn for this site. Not to scale, not a calculation and not a design.