Soil Bearing Capacity: IBC 2024 Presumptive Values and Footing Checks

IBC 2024 Table 1806.2 lists presumptive load-bearing values for five classes of soil and rock near the surface. The vertical values range from 1,500 psf for the listed clay and silt class to 12,000 psf for crystalline bedrock.

Those values are not a blanket permission to skip a geotechnical investigation. Section 1803 governs when and how an investigation is conducted and when the building official may waive one based on satisfactory adjacent-area data. Section 1806 limits use of the presumptive values, and §§1808 and 1809 still require capacity, settlement, load, supporting-soil, footing-dimension, frost, and other foundation checks.

IBC 2024 Table 1806.2 values

The Table 1806.2 USCS symbols for Class 4 are SW, SP, SM, SC, GM, and GC; Class 5 uses CL, ML, MH, and CH. One pound per square foot equals 0.0479 kPa, and one pound per square foot per foot equals 0.157 kPa/m.

The vertical-pressure, lateral-bearing, friction, and cohesion columns answer different design questions. Do not use a vertical bearing value as a lateral earth-pressure value or treat the friction coefficient as an allowable soil pressure.

What Table 1806.2 does—and does not—authorize

Section 1806.2 says supporting-soil and rock values near the surface must not exceed Table 1806.2 unless substantiating data are submitted and approved. It also says the presumptive values apply to materials with similar physical and engineering characteristics.

The table does not assign a presumptive capacity to mud, organic silt or organic clay (OL or OH), peat (Pt), or undocumented fill unless data substantiating the value are submitted. The section contains a building-official exception for lightweight or temporary structures where the official deems the value adequate; therefore, “no value can ever be assigned” is also too absolute.

Use the material at the actual bearing elevation. Surface appearance alone does not establish the bearing stratum, and the table does not replace the ASTM classifications required by §1803.5.1 or an investigation required by the building official.

Geotechnical investigation is a separate decision

Section 1803.2 requires geotechnical investigations in accordance with §§1803.3 through 1803.5. Its exception permits the building official to waive the investigation where satisfactory data from adjacent areas demonstrates that an investigation is unnecessary for the specified conditions in §§1803.5.1 through 1803.5.6, 1803.5.10, and 1803.5.11.

That is a building-official determination, not an automatic low-rise-building exemption. It is also incorrect to reduce §1803.5 to “eight triggers.” The 2024 section contains twelve investigated-condition subsections:

The operative language differs across these subsections. For example:

• under §1803.5.2, the building official is permitted to require an investigation where the soil or rock is in doubt or a superior bearing value is claimed;
• §1803.5.3 requires soil tests in areas likely to have expansive soil;
• §1803.5.5 requires a deep-foundation investigation unless sufficient design and installation data are otherwise available;
• §1803.5.7 requires a registered design professional’s assessment where excavation will reduce foundation support;
• §§1803.5.8 and 1803.5.9 require investigations for the specified compacted-fill and CLSM conditions; and
• §1803.5.11 requires an investigation for structures assigned to Seismic Design Category C, D, E, or F, with additional content under §1803.5.12 for Categories D through F.

Do not turn that condition-specific language into a single yes/no checklist. The §1803.6 report also needs project- specific recommendations, including bearing capacity, settlement, groundwater, expansive-soil mitigation, and other applicable content.

Expansive-soil and groundwater checks

Under §1803.5.3, soils are considered expansive where all four listed criteria are met:

1. plasticity index of 15 or greater under ASTM D4318;
2. more than 10 percent passing the No. 200 sieve under ASTM D6913;
3. more than 10 percent smaller than 5 micrometers under ASTM D6913; and
4. expansion index greater than 20 under ASTM D4829.

Tests for Items 1 through 3 are not required where the Item 4 expansion-index test is conducted. The section does not say that either PI 15 or expansion index 20, standing alone, is always the complete trigger.

Section 1803.5.4 requires an investigation to determine whether groundwater is above or within 5 feet below the lowest floor where that floor is below the adjacent finished ground level, and whether groundwater depth will affect design and construction. The earlier guide reduced this to one universal five-foot trigger and omitted the second condition.

Preliminary bearing-area calculation

For a concentrically loaded isolated footing in a simplified preliminary check:

required bearing area = applicable ASD service load / allowable vertical foundation pressure

For a continuous footing carrying a wall line load:

required footing width = ASD line load per foot / allowable vertical foundation pressure

Example: a wall line load of 3,000 lb/ft over Class 3 material at 3,000 psf produces a preliminary bearing width of 1 foot, before checking the rest of Chapter 18 and the structural design.

The earlier version used 30,000 lb/ft and reported 10 inches. The arithmetic actually gives 10 feet:

30,000 lb/ft ÷ 3,000 lb/ft² = 10 ft

This quotient is only a bearing-pressure check. The final footing must also account for load combinations, footing and fill weight where applicable, eccentricity, overturning, sliding, punching and one-way shear, flexure, reinforcement, settlement, adjacent foundations, groundwater, frost, expansive soil, and project-specific geotechnical criteria.

Load combinations and the one-third increase

Section 1806.1 requires Table 1806.2 values to be used with the ASD load combinations in ASCE 7 §2.4 or the alternative ASD combinations in IBC §1605.2. The table’s vertical foundation pressure and lateral bearing pressure may be increased by one-third only where used with the alternative ASD load combinations of §1605.2 that include wind or earthquake loads.

Do not apply that increase to gravity-only cases or describe it as available under every ASD combination. Also do not apply it automatically to friction or cohesion; §1806.1 names vertical foundation pressure and lateral bearing pressure.

Lateral resistance

Where Table 1806.2 is used for lateral resistance:

• §1806.3.1 permits combining resistance from lateral bearing pressure and lateral sliding resistance;
• §1806.3.2 limits cohesion-based sliding resistance for the listed clay and silt materials to one-half the dead load;
• §1806.3.3 permits the lateral bearing pressure to increase by the tabular value for each additional foot of depth, up to 15 times the tabular value; and
• §1806.3.4 permits twice the tabular lateral bearing pressure only for the specified isolated-pole applications and movement condition.

These are not general retaining-wall design values. Foundation and retaining walls also require the applicable §1610, §1807, groundwater, stability, and seismic checks.

Minimum footing dimensions and frost protection

Section 1809.4 sets a minimum footing depth of 12 inches below the undisturbed ground surface and a minimum footing width of 12 inches, with §1809.5 applying where frost protection is required.

Section 1809.5 provides three frost-protection methods: extend below the local frost line, comply with ASCE 32, or bear on solid rock. It also contains a limited exception for qualifying freestanding Risk Category I buildings. Do not publish a generic three- or four-foot frost depth; the frost line is local and another permitted protection method may apply.

Common mistakes

1. Treating the presumptive table as a geotechnical waiver. Investigation and waiver decisions are governed by §1803, not by Table 1806.2 alone.
2. Calling §1803.5 an eight-trigger checklist. The section has twelve condition-specific subsections with different operative language.
3. Using a value for an excluded material without substantiation. Mud, OL/OH organic materials, peat, and undocumented fill have no assumed presumptive value under the baseline rule.
4. Misclassifying the bearing stratum. Apply the value to the material supporting the foundation, not merely the visible surface soil.
5. Dividing a wall line load without carrying the units. Pounds per foot divided by pounds per square foot produces feet of footing width.
6. Applying the one-third increase too broadly. It is tied to the alternative §1605.2 ASD combinations that include wind or earthquake and to the named pressure values.
7. Checking only bearing pressure. Section 1808 also requires differential settlement to be minimized, and the complete structural and site design can govern.
8. Using a generic frost depth. Verify the local frost line or another permitted §1809.5 method.

Frequently asked questions

What is the IBC presumptive bearing value for clay and silt?

Table 1806.2 lists 1,500 psf for its Class 5 clay and silt materials (CL, ML, MH, and CH). That does not include organic clays classified OH and does not establish that the site material has been correctly classified or that an investigation can be omitted.

Does Table 1806.2 let a low-rise building skip a geotechnical report?

Not automatically. Section 1803.2 establishes the investigation requirement and a building-official waiver based on satisfactory adjacent-area data for the specified conditions. Local amendments and the actual conditions must be checked.

How is continuous-footing width estimated from bearing pressure?

Divide the applicable ASD wall line load in pounds per foot by the allowable vertical foundation pressure in pounds per square foot. The result is feet of preliminary footing width. Then perform the remaining foundation, structural, and geotechnical checks.

Can the presumptive pressure be increased for wind or earthquake?

Section 1806.1 permits a one-third increase in vertical foundation pressure and lateral bearing pressure when used with the alternative IBC §1605.2 ASD combinations that include wind or earthquake loads. It is not a gravity-only increase.

Scope and limitations

This guide explains the unamended 2024 IBC model-code sequence. It is not a soil classification, geotechnical investigation, foundation design, or site-specific capacity determination. The adopted code, local amendments, building-official decisions, subsurface conditions, settlement, groundwater, seismic hazards, structural loads, and engineered foundation design must be verified for the project.

Primary source

• IBC 2024, Chapter 18: Soils and foundations

Correction history

• 2026-08-31 — Revision 1: Corrected the 30,000-lb/ft example from 10 inches to 10 feet; replaced the false blanket geotechnical exemption and eight-trigger model with the §1803 investigation/waiver sequence; corrected expansive-soil, groundwater, one-third-increase, frost, excluded-material, and lateral-resistance scope; and removed unsupported US– Europe comparison and automation claims.