Bearing capacity

Bearing capacity is the pressure a footing can apply before the soil beneath fails in shear. Terzaghi's theory treats the soil under a strip footing as a wedge pushed down, displacing failure zones sideways and upward.

The ultimate bearing capacity combines three terms: one for cohesion, one for the surcharge of soil above the founding level, and one for the soil's own weight below the footing. Each is multiplied by a bearing capacity factor that depends only on the friction angle. Shape, depth and inclination factors adjust the basic strip-footing result. The water table matters because submerged soil has a lower effective unit weight.

The safe bearing capacity divides the net ultimate value by a factor of safety, commonly about 2.5 to 3 in working-stress practice. Separately, settlement must be checked; the allowable bearing pressure is the lower of the two limits.

For saturated clay under quick loading (φ = 0), the factors simplify, and the net ultimate capacity of a strip footing is about 5.14 c_u by Prandtl's solution, while Terzaghi's original solution for a rough strip base gives a somewhat higher 5.7 c_u; the two come from different assumed failure mechanisms, not from rounding.