Bearing Capacity Calculation Example

Practical Overview

This example shows how to organize a preliminary shallow footing bearing calculation. The numbers are illustrative and use a simplified Terzaghi-style approach.

This article is written for preliminary engineering understanding, study, field review, and calculation checking. It should not be used as a substitute for project-specific subsurface exploration, local code requirements, or review by a licensed professional engineer.

Soil Mechanics Or Design Basis

Assume a strip footing with B = 4 ft, Df = 3 ft, gamma = 120 pcf, c = 0 psf, phi = 30 degrees, and FS = 3. For phi = 30 degrees, representative bearing factors are approximately Nc = 30.1, Nq = 18.4, and Ngamma = 22.4.

The important habit is to name the soil model before selecting numbers. For geotechnical topics, that usually means asking whether the problem is drained or undrained, total stress or effective stress, short-term or long-term, and whether the soil profile is uniform enough for the simplified method being used.

Worked Mini Example

The surcharge term is gamma Df Nq = 120 x 3 x 18.4 = 6,624 psf. The width term is 0.5 gamma B Ngamma = 0.5 x 120 x 4 x 22.4 = 5,376 psf. With no cohesion term, qu is about 12,000 psf and qall is about 4,000 psf before settlement or groundwater adjustments.

The purpose of the example is not to create a universal design value. It shows how to organize the calculation, keep units visible, and interpret whether the result is controlled by strength, serviceability, water, construction, or uncertainty.

Field Checks And Practical Clues

• Check groundwater and settlement before accepting the answer.
• Confirm phi and unit weight from project data.
• Use the same unit system throughout.
• State whether pressure is net or gross.

Common Mistakes

• Using the calculation outside its assumptions.
• Ignoring groundwater or drainage.
• Using parameters without checking their source.
• Reporting a precise answer from uncertain inputs.

How To Use This On Civil Geo Tools

The related calculator, /calculators/bearing-capacity-calculator, can be used as a transparent worksheet after the assumptions are understood. Start with the sketch or geometry, enter conservative but realistic parameters, read the step-by-step output, and compare the result against the limitations on the page.

Engineering Interpretation

For real projects, the calculation is only one part of the decision. Review the boring logs, groundwater observations, lab data, construction sequence, drainage, loading, and consequence of poor performance. Where uncertainty is high, sensitivity checks are often more useful than a single polished number.

References And Further Reading

• Das, B. M. Principles of Foundation Engineering.
• Coduto, Yeung, and Kitch. Geotechnical Engineering: Principles and Practices.
• FHWA geotechnical engineering manuals and design circulars.
• ASTM and AASHTO test standards where project specifications require them.

FAQ

Can this article be used for final design?

No. It is educational and useful for preliminary screening, but final design requires project-specific data, applicable standards, and professional judgment.

What is the most important input to verify?

Usually the soil or water condition that controls the mechanism. For many geotechnical problems, groundwater, drainage, weak layers, and construction disturbance matter as much as the headline formula.

Why do different engineers sometimes get different answers?

They may be checking different failure modes, using different drainage assumptions, selecting different strength parameters, or applying different safety and serviceability criteria. The assumptions should be compared before the final numbers are compared.