Soil Bearing Capacity Explained
Soil bearing capacity is the pressure the ground can carry under a footing without failing or settling too much. Footings are sized so the applied pressure stays below the allowable value.
Ultimate versus allowable
Ultimate bearing capacity is the pressure at which the soil fails in shear. Allowable bearing pressure applies a safety factor (often around 3) and also considers settlement limits, which frequently control design on softer soils.
Bearing capacity depends on soil strength, footing width and depth, and groundwater. The same soil can support a higher pressure under a deeper or wider footing, and saturated soil supports less than dry soil.
Presumptive code values
IRC Table R401.4.1 lists presumptive load-bearing values: 1,500 psf for clay, sandy clay, silty clay, clayey silt, silt and sandy silt; 2,000 psf for sand, silty sand, clayey sand, silty gravel and clayey gravel; 3,000 psf for sandy gravel and gravel; 4,000 psf for sedimentary and foliated rock; and 12,000 psf for crystalline bedrock. The IBC has a similar table (Table 1806.2).
These values may be used when the building official accepts them. Organic soils, soft clays, uncontrolled fill and expansive soils require investigation.
How it sizes a footing
Required footing area equals the load divided by the allowable bearing pressure. A 30,000 lb column load on 2,000 psf soil needs 15 ft² of footing, about 3 ft 10 in square. A geotechnical engineer may also recommend minimum footing widths and depths, frost protection and settlement estimates.
The estimate above ignores the weight of the footing and soil above it and assumes a centered load. Real designs account for these, plus eccentric loads, frost depth and concrete shear and reinforcement requirements, which is why foundations are designed by engineers.
Frequently asked questions
How do engineers measure bearing capacity?
Through borings with Standard Penetration Tests, cone penetration tests, lab tests and sometimes plate load tests, interpreted by a geotechnical engineer.
Can bearing capacity be improved?
Yes, by compaction, removing and replacing poor soil, or ground improvement such as aggregate piers.
Why does settlement matter if the soil does not fail?
Even without failure, uneven settlement can crack structures, so allowable values often reflect settlement limits.
Sources
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