Soil Bearing Capacity IBC: Presumptive Values, Table 1806.2, and When Geotech Is Required (2026)

IBC 2024 Table 1806.2 sets allowable vertical foundation pressure for five soil classes: from 1,500 psf for clay and silt up to 12,000 psf for crystalline bedrock. For most low-rise buildings on identifiable soil, these presumptive values let you size footings without a geotechnical investigation. Eight conditions in §1803.5 eliminate that option.

What soil bearing capacity numbers does the IBC specify?

IBC Table 1806.2 lists presumptive load-bearing values by soil class. These are the maximum vertical foundation pressures usable without a geotechnical report.

IBC 2024 Table 1806.2. Conversion: 1 psf = 0.0479 kPa.

DataDrivenAEC’s analysis of IBC Table 1806.2 across all five classes shows an 8× span, from 1,500 psf at Class 5 clay to 12,000 psf at Class 1 bedrock. Lateral bearing values scale proportionally: 100 psf/ft for clay soils, 1,200 psf/ft for crystalline rock.

Two soil types carry no presumptive value. Mud, organic silt (OL), organic clay (OH), peat (Pt), and undocumented fill are excluded from Table 1806.2 under §1806.2. No bearing capacity may be assigned to these materials without a geotechnical investigation. Using Table 1806.2 on organic soil or undocumented fill violates §1806.2 regardless of the bearing value selected. Class 4 sand and silty soils at 2,000 psf represent the most common condition in US residential and light commercial practice.

What does soil bearing capacity measure?

Soil bearing capacity is the maximum pressure a foundation can exert on supporting soil without causing excessive settlement or shear failure. IBC §1806.2 operates within an allowable stress design (ASD) framework. The presumptive values already include safety factors and are applied directly with ASD gravity load combinations.

Soil class is determined at the bearing elevation, not at grade. A site with sandy gravel at the surface and clay beginning at three feet bears at the clay value if the footing sits in that clay. The ASTM D2487 Unified Soil Classification System (USCS) symbols in Table 1806.2 (GW, GP, CL, CH) correspond to standard sieve analysis and plasticity index tests performed at the bearing stratum.

Lateral resistance uses two mechanisms: friction (a coefficient times dead load) for granular soils, and cohesion (a unit value times contact area) for clay soils. These may be combined under §1806.3.1. For clay soils, cohesion-based lateral resistance cannot exceed one-half the dead load per §1806.3.2.

When can you use presumptive values without a geotechnical investigation?

Any building on soil classifiable under one of the five Table 1806.2 classes, where none of the §1803.5 conditions apply, may use presumptive values without a geotech report. The building official may also waive the investigation when satisfactory data exists from adjacent sites under §1803.2 Exception.

One significant benefit applies under §1806.1. Presumptive values may increase by one-third when used with ASD load combinations that include wind or earthquake loads. A Class 5 clay site rated at 1,500 psf for gravity becomes 2,000 psf effective for combined wind or seismic load cases. This increase applies only to the combined load case, not to gravity-only design.

Minimum footing dimensions apply regardless of bearing capacity. IBC §1809.4 sets the floor at 12 inches of depth below undisturbed ground surface and 12 inches of footing width. Frost protection under §1809.5 requires footings to extend below the local frost line; in northern US states, this depth can reach 3–4 feet and governs over the 12-inch minimum.

What triggers a required geotechnical investigation under IBC §1803.5?

Eight conditions in §1803.5 make a geotechnical investigation mandatory. Any single trigger eliminates the presumptive value path. The building official cannot waive these in the way §1803.2 permits for standard investigations.

SDC D, E, and F add a second requirement. IBC §1803.5.12 mandates a liquefaction and landslide hazard assessment on top of the standard investigation. SDC C may avoid the liquefaction step if the risk is determined to be low after site review.

Expansive soils trigger §1803.5.3 when all four conditions are met: Plasticity Index ≥ 15 (ASTM D4318), more than 10% of particles passing the No. 200 sieve (75 µm), more than 10% of particles below 5 µm (ASTM D6913), and expansion index greater than 20 (ASTM D4829). Conditions 1–3 may be skipped if Condition 4 is tested directly.

How is foundation area calculated from bearing capacity?

The core formula: required footing area (sq ft) equals total ASD design load (lbs) divided by the allowable bearing pressure from Table 1806.2 (psf).

Foundation area (sq ft) = ASD design load (lbs) / Allowable bearing (psf)

A simple example: a bearing wall carries 30,000 lbs per foot of dead load. The soil is sandy gravel (Class 3, 3,000 psf presumptive). Required footing width = 30,000 ÷ 3,000 = 10 inches per foot of wall. IBC §1809.4 minimum of 12 inches governs instead.

Real projects add live load, wind and seismic load combinations, frost depth, and footing self-weight. Variable soil layers across the building footprint change the result.

Lateral resistance uses Table 1806.2 columns 4 and 5. Class 3 sandy gravel carries 200 psf/ft of lateral bearing. At 3 feet of embedment: 3 × 200 = 600 psf. Each foot adds 200 psf, capped at 15 times the tabular value (3,000 psf total) per §1806.3.3. Isolated poles receive double the tabular lateral bearing under §1806.3.4.

How do US presumptive values compare to European building codes?

IBC Table 1806.2 is a US-specific feature. European and German codes do not provide presumptive bearing values. A site-specific geotechnical investigation is required under those systems regardless of building size or soil familiarity.

The practical gap is wide. A US architect designing a two-story wood-frame building on Class 4 sandy soil at 2,000 psf has no geotech requirement. A European architect on the same project must commission a boring program regardless of building scale.

Schematic-design timelines reflect this difference. On a US IBC project, a soil classification from a site visit or local building records may be sufficient to begin footing design. European practice requires boring logs and lab results before structural calculations begin.

What are the most common soil bearing capacity mistakes?

Six errors appear consistently in foundation documents reviewed against IBC Chapter 18.

1. Using presumptive values on excluded soils. IBC §1806.2 excludes mud, organic silt (OL), organic clay (OH), peat, and undocumented fill from Table 1806.2. No presumptive value exists for these materials. Assigning any bearing capacity without test data violates §1806.2 and creates structural risk.
2. Missing the wind and seismic one-third increase. Presumptive values may increase by one-third under ASD combinations that include wind or earthquake per §1806.1. Missing this benefit oversizes footings on lateral load cases.
3. Skipping geotech near groundwater. §1803.5.4 mandates a geotechnical investigation when the lowest below-grade floor sits within 5 feet of the water table. Waterproofing alone does not satisfy this requirement.
4. Applying presumptive values on compacted fill. A footing on compacted fill deeper than 12 inches requires a geotech report under §1803.5.8, regardless of fill material or compaction density.
5. Exceeding the 15× lateral bearing cap. Lateral pressure may increase by one tabular value per foot of depth but stops at 15 times the tabular value per §1806.3.3.
6. Classifying soil at the surface, not at bearing elevation. Gravel at grade over clay at depth means the footing bears in clay. The clay value governs.

Frequently asked questions

What is the presumptive soil bearing capacity for clay under IBC 2024?

Clay, sandy clay, silty clay, and silt (USCS classes CL, ML, MH, CH) carry 1,500 psf vertical bearing pressure under IBC 2024 Table 1806.2. This value may increase by one-third when used with ASD load combinations that include wind or seismic loads per §1806.1. Organic clays (OH) and peat have no presumptive bearing value and require geotechnical testing before any capacity can be assigned.

When does IBC require a geotechnical investigation?

Eight triggers in §1803.5 mandate a geotech report: uncertain soil classification, claimed bearing values above Table 1806.2, expansive soils (Plasticity Index ≥ 15 or expansion index > 20), groundwater within 5 feet of the lowest below-grade floor, deep foundations, variable rock strata, compacted fill deeper than 12 inches, and Seismic Design Category C through F. Any single condition is sufficient. The building official cannot waive these mandatory triggers.

What is the minimum footing depth and width under IBC?

IBC §1809.4 sets minimums at 12 inches below undisturbed ground surface and 12 inches of footing width. Frost protection under §1809.5 often governs in cold climates: footings must extend below the local frost line, which can reach 3–4 feet in northern US states. These minimums apply regardless of the calculated bearing capacity.

Can lateral bearing pressure increase with footing depth?

Yes. IBC §1806.3.3 permits lateral bearing pressure to increase by the tabular value for each additional foot of depth below grade. The cap is 15 times the tabular value. For Class 3 sandy gravel at 200 psf/ft, the maximum is 3,000 psf at 15 feet of depth. Isolated poles receive double the tabular lateral bearing value under §1806.3.4.

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