How the two differ in kind
A retaining wall has to satisfy several independent checks, and it can pass some and fail others. Understanding which is which is the difference between diagnosing a wall that has moved and guessing at it.
SIZING THE FOOTING is a bearing capacity question. The wall and the soil it retains impose a load on the base, distributed unevenly because the earth pressure applies a moment as well as a force — so the pressure is higher at the toe than at the heel, and can in some geometries lift the heel entirely. The check asks whether the maximum pressure is within the ground's allowable bearing capacity and whether the resultant falls where the design requires. A wall that fails it settles, and because the pressure is greatest at the toe it settles unevenly and tilts forward.
STABILITY is a different set of questions, treating the wall as a rigid body. OVERTURNING asks whether the earth pressure's moment about the toe exceeds the restoring moment from the wall's weight and the soil on its heel. SLIDING asks whether the horizontal thrust exceeds the friction available along the base plus any passive resistance in front. And GLOBAL stability — the one most often omitted — asks whether a failure surface can develop through the soil BENEATH the whole wall, so that the wall rotates outward as an intact object sitting on ground that failed under it. That last mode is common on sloping sites and in soft ground, and no amount of making the wall stronger addresses it.
One factor changes the loading in every one of those checks, and it is the reason most retaining walls fail: DRAINAGE. A drained backfill imposes earth pressure. A saturated one imposes earth pressure plus full hydrostatic pressure, which can roughly double the thrust — a load the wall was never designed for, appearing years later when a drain silts up.
The factors that actually differ
Show
| Footing sizing (bearing) | Stability checks | |
|---|---|---|
| Question it asks | Can the ground carry the pressure under the base? | Does the wall overturn, slide, or rotate out on a surface beneath it? |
| Failure it prevents | Settlement and forward tilt as the toe punches into the ground. | The wall going over, sliding forward, or the ground beneath it failing as a mass. |
| What it depends on | The ground's bearing capacity and the pressure distribution under the base. | Geometry, weight, the friction available at the base, and the soil's strength beneath and behind. |
| The mode most often omitted | Not applicable. | GLOBAL stability — a failure surface passing under the whole wall, which is not a property of the wall at all. |
| Fix when it fails | A wider base, better ground, or a different foundation such as piles. | Overturning and sliding: a wider base, a shear key, more weight. Global: a different solution entirely. |
| Sensitivity to drainage | High — saturation raises the thrust, which raises the toe pressure. | High, and this is the usual cause of failure in service. |
| Sensitivity to surcharge | Direct. A load on the retained ground increases the pressure under the base. | Direct, and it is frequently added after construction — a drive, a parked vehicle, a building. |
| Sloping ground | Modest effect. | Large. A wall at the top of a slope or on sloping ground is where global stability governs. |
| Passive resistance in front | Not applicable. | Counted in the sliding check — and lost if the ground in front is excavated later, which happens. |
| Which governs | Soft ground, heavy walls, narrow bases. | Sloping sites, soft layers beneath, and any wall where the thrust is high relative to the weight. |
Which one, and when
Choose footing sizing (bearing) when…
- Establishing whether the ground can carry the wall — the first check on any wall on soft or unknown ground.
- Diagnosing a wall that has settled or tilted forward without sliding.
- Choosing a base width, which bearing and overturning jointly decide.
- Assessing whether a deeper or different foundation is needed under an otherwise sound design.
Choose stability checks when…
- Any retaining wall, because overturning and sliding are not optional checks.
- A wall on a slope, at the crest of one, or with soft ground beneath — where global stability can govern.
- A wall that has moved outward or rotated rather than settled.
- Before adding any surcharge behind an existing wall — a drive, a building, stored material.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- What is global stability and why is it missed?
- It is the check that asks whether the ground beneath and behind the wall can fail as a mass, along a curved surface passing under the whole structure — with the wall riding out on top of it, intact and undamaged, having been a perfectly adequate wall sitting on ground that gave way. It is missed because it is not a property of the wall: no amount of reinforcement, base width or weight addresses it, and it does not appear in the structural calculations that size the wall itself. It is a slope stability analysis, requiring knowledge of the soil strata and their strengths beneath and behind, and it is most likely to govern exactly where retaining walls are most often built — on sloping sites, at the crest or toe of a slope, and where a soft layer underlies a competent one.
- Why is drainage the usual cause of failure?
- Because it changes the load rather than the resistance, and it changes it enormously. A retaining wall with a drained backfill carries lateral earth pressure from the soil. Let that backfill saturate and the wall carries the earth pressure PLUS full hydrostatic pressure from the water standing behind it — and water is heavy, so the total thrust can be roughly double what was designed for. No wall designed for the drained case is adequate for the undrained one. The failure appears years after construction, when the drainage that was installed correctly has silted up, been crushed, or was never connected to an outfall. That is why a drainage layer, a perforated drain at the base with a filter, and an outlet that actually discharges somewhere are as much a part of a retaining wall as its reinforcement.
- What does a shear key do?
- It increases sliding resistance by forcing any sliding surface down into undisturbed soil rather than along the smooth underside of the base. A downstand cast beneath the footing, usually near the heel, means that for the wall to slide the soil itself has to shear along a deeper plane where it is stronger, and it mobilises passive resistance against the key's face. It is a common and effective remedy when the sliding check is the one that fails, and it is cheaper than widening the base or adding mass. It is not a remedy for overturning, which is a moment problem rather than a friction one, and it does nothing at all for global stability. Which check has failed therefore determines whether a key is the right answer, which is the point of distinguishing them.
- Why does the heel sometimes lift?
- Because the earth pressure applies a moment that shifts the resultant force toward the toe, and if it moves outside the middle third of the base, the pressure distribution can no longer be compressive across the whole width — the heel goes into tension, which soil cannot provide, so it lifts and the whole load concentrates on a narrower strip near the toe. That raises the maximum bearing pressure substantially, which is why the middle-third rule appears in retaining wall design: keeping the resultant within it guarantees compression everywhere and a manageable pressure distribution. A wall whose resultant falls outside it is not automatically failing, but it is a design that needs the bearing check done on the reduced effective base width rather than the full one, and it is much less tolerant of any increase in thrust.
- How much does a surcharge behind the wall matter?
- A great deal, and it is the change most often made to an existing wall without anybody checking. A load applied to the retained ground — a driveway, a parked vehicle, stored material, a building, or fill placed to level a garden — increases the lateral pressure on the wall over its whole height, and it affects every check at once: more thrust means a larger overturning moment, more sliding force, and higher pressure under the base. Walls that have stood for decades and then move usually did so because something was put on the ground behind them. A vehicle is a large surcharge on a domestic wall. Anyone adding a hard standing, an extension or a substantial amount of fill behind a retaining wall should treat it as a change to the wall rather than a change to the garden.
- What if the ground in front is excavated?
- The wall loses passive resistance it may have been relying on, and the sliding check can fail as a result. Passive pressure from the soil in front of the base and any shear key contributes to resisting sliding, and where it was counted in the design, removing that soil removes the resistance — so digging a trench for a drain, lowering a path, or excavating a bed in front of a wall can destabilise it without touching it. The same applies to undermining the toe through erosion or a scoured watercourse. Many designs deliberately ignore passive resistance for exactly this reason, on the grounds that the soil in front cannot be relied upon over the wall's life, which is a conservative assumption and a sensible one. Whether it was ignored or counted is the question to ask before excavating.
- Which check usually governs?
- It depends on the geometry and the ground, which is why all of them are performed rather than the one that seems most likely. Sliding often governs on a wall with a smooth base on a low-friction soil, particularly a clay. Overturning governs on a tall wall with a narrow base. Bearing governs on soft ground, or where the resultant has shifted toward the toe and concentrated the pressure. Global stability governs on slopes and where a weak layer underlies the site. A useful pattern for diagnosis: a wall that settled and tilted forward failed bearing; one that moved outward at the top while the base stayed put failed overturning; one that translated forward bodily failed sliding; and one that rotated out with the ground in front heaving up failed globally.
- Do these checks apply to a small garden wall?
- In principle yes, in proportion, and the two that most often bite on small walls are drainage and surcharge rather than the structural checks. A low garden retaining wall built dry-laid or in blockwork with no drainage behind it and a lawn above may stand indefinitely; the same wall with a saturated backfill after a blocked drain, or with a new parking space built behind it, is carrying a load nobody calculated. Most jurisdictions set a height above which a retaining wall requires design and approval, which is the threshold at which these checks become formal rather than a matter of judgement — and the threshold is lower than people expect. Below it, the two things worth doing regardless are providing drainage with a real outlet and not loading the ground behind.