Terzaghi Bearing Capacity Explained

Practical Overview

Terzaghi's bearing capacity equation remains useful because it shows the physical pieces of a shallow foundation bearing problem. It is not a universal design engine. It is a transparent starting point for simple shallow footings when its assumptions are reasonably close to the field condition.

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

The strip footing form is often written qu = cNc + gamma Df Nq + 0.5 gamma B Ngamma. Nc, Nq, and Ngamma are bearing capacity factors based mainly on friction angle. The equation assumes a relatively shallow foundation, uniform soil, level ground, concentric vertical load, and a general shear mechanism.

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

For clean sand with c = 0, the cohesion term drops out. Increasing footing width increases the gamma term, while deeper embedment increases the surcharge term. In clay under undrained conditions, phi is often taken as zero for short-term total stress screening and the cohesion term becomes dominant.

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 whether the soil layer is uniform through the depth influenced by the footing.
• Use effective stress parameters for drained long-term conditions and total stress parameters for appropriate undrained short-term cases.
• Apply groundwater corrections where the water table affects effective unit weight.
• Use more complete methods when the footing is on or near a slope, excavation, property line, or layered profile.

Common Mistakes

• Using Terzaghi factors without checking the assumed failure mode.
• Forgetting shape, depth, load inclination, or groundwater corrections where they matter.
• Using apparent cohesion in sand as if it were reliable long-term strength.
• Treating the equation as a substitute for settlement analysis.

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.