Every structure transfers its weight to the ground through foundations, and the soil must be strong enough to carry that load without excessive settlement or failure. Soil bearing capacity, the maximum pressure the soil can support, varies enormously: soft clay may handle only 1,000 PSF, while bedrock can carry over 12,000 PSF. Getting this number right determines whether you pour a 2-foot square footing or a 5-foot one.
For most buildings, the International Building Code Table 1806.2 provides presumptive bearing values based on soil classification. These values are conservative and do not require a geotechnical investigation. For larger structures, heavily loaded foundations, or questionable soil, a site-specific geotechnical report is essential. This guide covers the code table approach, when to call a geotechnical engineer, and how to size footings once you have a bearing value.
IBC 1806.2 Presumptive Bearing Values
The International Building Code provides presumptive bearing values for common soil types when a geotechnical investigation is not performed. Crystalline bedrock: 12,000 PSF. Sedimentary rock: 4,000 PSF. Sandy gravel or gravel (GW, GP): 3,000 PSF. Sand, silty sand, clayey sand (SW, SP, SM, SC): 2,000 PSF. Clay, silty clay, sandy clay (CL, ML, MH, CH): 1,500 PSF.
These values assume the footing bears on undisturbed native soil or properly compacted fill. They include a built-in safety factor of roughly 3:1. If the soil has been recently disturbed, is waterlogged, or contains organic material (topsoil, peat), the presumptive values do not apply. Strip the topsoil and verify you are bearing on competent native material or engineered fill compacted to at least 95 % standard Proctor density.
Soil Bearing Capacity Estimator
Estimate presumptive soil bearing capacity per IBC Table 1806.2 and size spread footings.
Sizing Spread Footings
Footing area = total column load ÷ allowable soil bearing pressure. A column carrying 30,000 lbs on soil rated at 2,000 PSF needs at least 15 square feet of footing area. A 4-foot square footing (16 ft²) would work. For continuous (strip) footings under walls, divide the wall load per linear foot by the bearing capacity to get the required width. A wall loading 2,400 PLF on 2,000 PSF soil needs a 1.2-foot wide footing, so round up to 1.5 feet (18 inches).
Footing thickness must be adequate for concrete shear and bending. A common rule of thumb for isolated spread footings: thickness equals the distance from the face of the column to the edge of the footing, with a minimum of 12 inches for column footings and 8 inches for wall footings per IBC. The structural engineer sizes the reinforcing based on the cantilever moment from soil pressure acting on the footing projection beyond the column.
Soil Bearing Capacity Estimator
Estimate presumptive soil bearing capacity per IBC Table 1806.2 and size spread footings.
When You Need a Geotechnical Report
The IBC presumptive values work for simple, lightly loaded structures on typical soils. You need a site-specific geotechnical investigation when: the structure is larger than 3 stories or has column loads exceeding 50,000 lbs; the soil is clay with high plasticity (CH or MH); the site has fill of unknown origin or quality; there is a high water table; the site is on a slope or near a slope; or the building official requires one.
A geotechnical report includes borings with soil classification at various depths, laboratory testing for strength and consolidation properties, groundwater observations, and site-specific bearing capacity recommendations. It will also address settlement estimates, something the IBC presumptive values do not cover. Clay soils can have adequate bearing capacity but still settle several inches over months or years due to consolidation.