Factor of Safety Against Sliding

A gravity or cantilever retaining wall has three ways to fail, and sliding along its base is the one that catches people out because it has nothing to do with how strong the concrete is. The resisting force is friction, W tan δ, where δ is the friction angle between the base and the soil beneath — typically two thirds of the soil's φ for a cast-against-earth concrete base, or the full φ for a rough, poured-in-place footing on granular soil. A wall pressing down with 300 kN per metre on a base with δ = 25°, resisting a thrust of 90 kN per metre, has FS = 300 × 0.466 ÷ 90 = 1.55, which just clears the usual 1.5 requirement.

The trap is counting on passive resistance in front of the wall to make the numbers work. That soil is usually shallow, often disturbed, and may be excavated for a service trench the year after handover, so most codes require the wall to pass on base friction alone or allow only a fraction of the passive term. The second trap is water: an uplift pressure under the base reduces the effective W and therefore the friction, exactly as it does under a dam, which is why drainage layers and relief wells are stability elements rather than plumbing. Check overturning and bearing too — sliding is only one of the three.