Definition
Bearing capacity is the maximum load per unit area that soil can support without failing or experiencing excessive settlement. It is measured in pounds per square foot (psf) or kilopascals (kPa). Bearing capacity determines the required size of footings and foundations. Factors affecting bearing capacity include soil type, density, moisture content, and water table level.
Formula
The safety factor for foundations is typically 2-3. If soil testing shows an ultimate bearing capacity of 6,000 psf, the allowable capacity with a safety factor of 3 is 2,000 psf. This is the value used for footing design calculations.
Example
Soil Bearing Capacity Example
Typical bearing capacities: dense gravel - 8,000-12,000 psf, sandy gravel - 4,000-8,000 psf, sand - 3,000-5,000 psf, clay - 1,500-3,000 psf. For a 40,000 lb column load on soil with 2,000 psf allowable capacity, the footing must be at least 20 square feet (e.g., 5 ft x 4 ft).