Terzaghi bearing capacity: what the three terms mean.

Terzaghi's classical shallow-footing equation gives a transparent way to estimate gross ultimate bearing pressure for a simple footing. It is a starting model, not a replacement for a geotechnical report or a code-specific design procedure.

Start with the gross ultimate equation.

For a rectangular footing with width B, length L, cohesion c, soil unit weight γ, and embedment depth D_f, the CivilKit screen uses:

The first term represents cohesion, the second the overburden pressure at the base, and the third the soil wedge contribution below the footing. The screen then divides q_ult by the entered factor of safety to report a preliminary allowable gross pressure.

Why the shape factors are shown.

A strip-footing equation is not identical to a rectangular-footing equation. For this screen, the width-to-length ratio is used in the classical rectangular factors: s_c = 1 + 0.2B/L, s_q = 1 + 0.1B/L, and s_γ = 1 - 0.4B/L. These factors are deliberately shown with the result so the user can see how a wider footing changes each contribution.

Choose the bearing-capacity factors carefully.

For friction angle φ greater than zero, N_q = e^πtanφtan²(45° + φ/2), N_c = (N_q - 1)/tanφ, and the classical Terzaghi N_γ = 1.5(N_q - 1)tanφ. At φ = 0, the screen uses N_c = 5.14, N_q = 1, and N_γ = 0.

What this quick check does not decide.

• It does not select drained versus undrained strength, soil layers, groundwater corrections, or the governing failure mechanism.
• It does not include code-specific depth, inclination, slope, base-inclination, shape, or load-eccentricity factors beyond the simple rectangular shape factors stated above.
• It does not calculate settlement, differential movement, sliding, overturning, structural reinforcement, or serviceability.