Overturning and sliding: why a retaining wall's stability checks aren't about material strength
A retaining wall can be built from more than strong enough concrete and steel and still fail — by rotating forward over its own toe, or by sliding bodily across its base — long before any material comes close to its capacity.
Lateral earth pressure behind a retaining wall creates an overturning moment about the wall's toe (its front, downhill edge). Resisting it is the wall's own self-weight plus the weight of any soil sitting on top of its heel, both acting through their own lever arms back toward the toe. The check compares destabilizing and stabilizing moments — as a ratio in factor-of-safety-style practice, or as a direct comparison of factored actions in EC7's partial-factor approach — and it's a stability check, not a strength check at all: a wall poured from arbitrarily strong concrete can still overturn if it's simply too narrow or too light for the earth pressure pushing on it.
Sliding is a separate check entirely: whether the wall's base can develop enough friction — and, if included, passive resistance from soil in front of the wall — to resist the horizontal component of earth pressure trying to push the whole wall forward across its foundation. The resisting friction depends on the vertical load pressing the base down into the soil and the friction coefficient between the base and the soil beneath it, so the same design moves that help overturning (more self-weight, a wider heel) also tend to help sliding. The two checks are still verified independently, though — a wall can comfortably pass one while failing the other, since they resist genuinely different force components.
Passive resistance is available, but only cautiously
Passive soil resistance in front of the wall's toe can help both overturning and sliding — but because full passive pressure needs more wall movement to mobilize than most designs are comfortable relying on (and because the soil there is often disturbed, or gets excavated later for a utility trench), it's common practice to either ignore it entirely or credit only a reduced fraction of it. Retaining Wall Design makes this explicit: passive resistance is opted into deliberately, with its own mobilization-reduction factor, rather than assumed automatically (see the related tool below).