Bearing capacity of soil under a footing

A footing spreads a building load over soil. The ground must support that pressure without a bearing failure or settlement that the structure cannot tolerate. “Bearing capacity” describes this support limit, but a soil name by itself is not enough to choose a footing size. The site conditions, foundation geometry, loads, code, and geotechnical information all matter. Check both strength and movement.

Ultimate capacity and allowable pressure

Ultimate bearing capacity refers to the pressure associated with soil shear failure under a foundation. Allowable bearing pressure is a design value reduced or limited so the foundation has an appropriate margin against shear failure and settlement remains acceptable. FHWA's shallow-foundation guidance describes allowable capacity as the lesser of the pressure limited by shear failure and the pressure that would cause the specified settlement.

These terms are related but not interchangeable. A soil may resist shear at a high pressure yet compress enough to cause excessive settlement first. Differential settlement, where one part of a foundation moves more than another, can distort the structure even if the soil has not experienced a dramatic bearing failure.

Calculate average pressure under a footing

For a simple, centered vertical load on a rectangular footing with uniform contact pressure, average pressure q = P ÷ A. P is the supported load and A is the footing's soil-contact area. A 6-foot by 6-foot footing has an area of 36 square feet. If the centered load is 72,000 pounds, the average pressure is 72,000 ÷ 36 = 2,000 pounds per square foot.

This is a simplified average-pressure calculation, not a complete foundation design. Eccentric loads, moments, uplift, inclined forces, footing self-weight, groundwater, layered soils, nearby slopes, and nonuniform contact pressure may change the analysis. A plan or exam question may specify which loads and assumptions to include.

What determines the allowable value

• Soil type and condition, including density, strength, moisture, compressibility, and whether fill is engineered or undocumented.
• Footing width, embedment, shape, and the depth and arrangement of soil layers below it.
• Groundwater level and how it affects effective stress and soil behavior.
• The magnitude, direction, and eccentricity of structural loads.
• Settlement limits and the sensitivity of the supported structure to movement.
• The geotechnical investigation, applicable building code, local amendments, and the building official's requirements.

Presumptive code values are limited

Some building codes provide presumptive load-bearing values for listed soil or rock classes as a simplified route for eligible conditions. The International Building Code, for example, says its presumptive values apply to materials with similar physical characteristics and that higher values require supporting data and approval. It also cautions that mud, organic soils, peat, and unprepared fill do not automatically receive a presumptive capacity.

A presumptive value is not a universal property of every site with a similar-sounding soil description. Local jurisdictions may adopt different code editions or amendments, and the building official may require an investigation if classification, strength, or compressibility is uncertain. Use the value specified by the problem or project authority; do not substitute a memorized number for site evidence.

Bearing failure versus settlement

Bearing failure occurs when soil cannot resist the applied stress and shears, allowing the footing to punch, tilt, or sink. Settlement is vertical movement as soil compresses under load. Some settlement is normal, but total or differential movement beyond the design tolerance can crack finishes, distort frames, or damage utilities. Foundation evaluation must consider both strength and serviceability.

A practical field and plan review

1. Read the foundation plans for footing dimensions, founding elevation, reinforcement, and design notes.
2. Confirm the assumed soil-bearing value and its source: geotechnical report, code table, or project specification.
3. Check whether excavation exposes soil consistent with the documented bearing stratum and whether unsuitable material or undocumented fill is present.
4. Look for water, disturbed soil, loose material, or changed conditions that should be reported before concrete placement.
5. Confirm that any required testing, inspection, or engineer review is complete and recorded.

A contractor or field supervisor should not increase a footing's assumed capacity or approve a design change from visual soil appearance alone. Follow the plans, specifications, geotechnical recommendations, and direction of the responsible design professional and authority having jurisdiction.

Exam distinctions

• Average soil pressure for a simple centered load is load divided by contact area.
• Allowable bearing pressure must account for both shear failure and tolerable settlement.
• Ultimate capacity is not automatically an allowable design value.
• A presumptive code value is conditional on the code edition, material description, and site requirements.
• Soil classification helps describe material but does not alone establish foundation capacity.