UBC Full Form — Ultimate Bearing Capacity

UBC stands for Ultimate Bearing Capacity — the gross pressure at which the soil beneath a footing fails in shear and the foundation punches into the ground. It is the ceiling; the working value engineers actually design to, the safe bearing capacity, is this ultimate value divided by a factor of safety. This page covers the definition, the Terzaghi equation behind it, and how UBC relates to SBC.

What UBC means

Load a footing higher and higher and the soil eventually gives way along shear surfaces — the footing sinks suddenly. The pressure at that point is the ultimate bearing capacity. It is a failure value, never used directly for design; instead it is reduced by a factor of safety (typically 2.5 to 3) to give the safe bearing capacity that footings are actually sized to.

Safe bearing capacity = Ultimate bearing capacity ÷ Factor of safety

The Terzaghi equation

For a shallow strip footing, Terzaghi's classic equation gives the gross ultimate bearing capacity:

qu = c·Nc + q·Nq + 0.5·γ·B·Nγ

where c is the soil cohesion, q = γ·Df is the overburden pressure at founding depth, γ is the unit weight of soil, B the footing width, Df the founding depth, and Nc, Nq, Nγ are bearing-capacity factors that depend on the soil's angle of internal friction (φ). The three terms are the contributions of cohesion, surcharge and the footing width. The soil bearing capacity calculator evaluates this equation for you.

Net vs gross ultimate bearing capacity

• Gross UBC (qu) is the total pressure at failure, including the overburden.
• Net UBC subtracts the existing overburden pressure (γ·Df) that the soil already carried before excavation: qnu = qu − γ·Df. It is the additional pressure the new footing may impose.

Design works from the net values and always cross-checks settlement, because a soil can be safe against shear failure yet settle more than the structure can tolerate.

UBC vs SBC — the difference

So UBC is the raw strength of the soil, and SBC is the usable fraction of it. For how the safe value sizes a footing, see SBC full form and the footing size estimator.

Frequently asked questions

What is the full form of UBC? UBC stands for Ultimate Bearing Capacity — the gross soil pressure at which a foundation fails in shear.

What is the difference between UBC and SBC? UBC is the pressure at which the soil actually fails. SBC (safe bearing capacity) is UBC divided by a factor of safety, so it is the working value with a built-in margin that footings are designed to.

What is the Terzaghi bearing capacity equation? qu = c·Nc + q·Nq + 0.5·γ·B·Nγ, where c is cohesion, q is the overburden pressure at founding depth, γ is the soil unit weight, B the footing width, and Nc, Nq, Nγ are factors depending on the friction angle.

What factor of safety is used on ultimate bearing capacity? A factor of safety of 2.5 to 3 is typically applied to the ultimate bearing capacity to obtain the safe bearing capacity used in design.

What is net ultimate bearing capacity? Net ultimate bearing capacity is the gross ultimate value minus the overburden pressure the soil already carried (γ·Df) — the extra pressure a new footing may add beyond what the ground held before.