IBC Foundation Types: Minimum Depths, Bearing Capacities, and Concrete Strengths (2024)

12,000 psf. That is the presumptive bearing capacity of crystalline bedrock under IBC Table 1806.2. Clay gets 1,500 psf. Organic soil and peat get no assumed value. IBC Chapter 18 governs foundation selection from minimum footing depth to concrete strength by seismic design category. Here is what the code requires before foundations are designed, specified, or permitted.

What are the IBC Chapter 18 foundation code numbers?

IBC Chapter 18 is the governing standard for foundation design in the United States, covering both shallow foundation systems (spread footings, mat slabs) and deep foundation systems (driven piles, drilled shafts, and micropiles). The numbers below appear on structural drawings, geotechnical reports, and permit applications. Knowing the specific thresholds prevents the most costly plan review corrections.

Per IBC §1806.1, presumptive values may be increased by one-third where used with the alternative allowable stress design load combinations of §1605.2 that include wind or earthquake loads (not the ASCE 7 §2.4 combinations). Mud, organic silt and clay (OL, OH), peat, and undocumented fill are not to be assumed to have a presumptive capacity unless data substantiating a value are submitted, per IBC §1806.2. Values above Table 1806.2 also need submitted and approved data, and the building official may accept presumptive capacity for lightweight or temporary structures.

What determines which foundation type is required?

Foundation type selection is determined by four variables under IBC Chapter 18: soil classification, seismic design category, structural loads, and frost depth. Shallow foundations (spread footings, continuous footings, and mat slabs) are appropriate where surface soils carry adequate bearing capacity and differential settlement stays within structural tolerances. Deep foundations (driven piles, cast-in-place concrete piles, drilled shafts, and micropiles) are required where shallow soils cannot support structural loads without excessive movement. Per IBC §1803, soil classification must be based on borings, test pits, or other subsurface exploration before any foundation system is specified. Buildings in SDC C through F require a geotechnical investigation that evaluates slope instability, liquefaction, total and differential settlement, and surface displacement from faulting or lateral spreading (IBC §1803.5.11). In SDC D through F the investigation adds dynamic seismic lateral earth pressures on walls retaining more than 6 feet of backfill and the potential and consequences of liquefaction and soil strength loss (IBC §1803.5.12). Expansive soil is triggered by the site, not the SDC (IBC §1803.5.3). The geotechnical report drives every downstream structural decision about foundation type, depth, and concrete specification.

Why does IBC set different requirements by soil type and seismic category?

Presumptive bearing values are the code’s proxy for soil strength: IBC Table 1806.2 lists different values for each soil class because soil behavior varies by composition and particle structure. Crystalline bedrock, at 12,000 psf, is dense, continuous, and essentially incompressible under building loads. Sedimentary rock reaches 4,000 psf but is weaker due to layering and fracture planes. Sandy gravel and gravel rate at 3,000 psf because coarse granular soils consolidate rapidly and predictably under load. Clay and silt receive only 1,500 psf. Fine-grained soils consolidate slowly, generating long-term differential settlement that structural frames cannot absorb without damage. Organic soils and peat carry no presumptive value: water content and decomposition make bearing capacity entirely unpredictable without site testing. Seismic design categories add a second layer. Soft soils amplify earthquake shaking at frequencies matching building resonance. Soil amplification is why SDC D, E, and F trigger mandatory geotechnical evaluation beyond basic bearing capacity. Site Class F soils require a site-specific evaluation under ASCE 7.

What are the presumptive bearing capacities per IBC Table 1806.2?

Presumptive bearing capacity is the allowable vertical foundation pressure permitted by IBC §1806.1. IBC Table 1806.2 spans from 12,000 psf for crystalline bedrock to 1,500 psf for clay and silt, with sedimentary rock at 4,000 psf, sandy gravel at 3,000 psf, and sand at 2,000 psf. IBC Table 1806.2 also specifies lateral bearing values by soil class:

Presumptive values are not to be assumed for mud, organic silt and clay, peat, or undocumented fill unless data substantiating a value are submitted (IBC §1806.2). The vertical and lateral values may be increased by one-third where used with the alternative ASD combinations of §1605.2 that include wind or earthquake loads, per IBC §1806.1.

What are the minimum dimensions for shallow foundations?

Shallow foundation dimensions are set by IBC §1809.4, which establishes two universal minimums regardless of load or soil type: footings must extend at least 12 inches below undisturbed ground surface and minimum footing width is 12 inches. Footings must bear on undisturbed soil, compacted fill, or controlled low-strength material (CLSM) per IBC §1809.2. For light-frame construction, IBC Table 1809.7 provides prescriptive footing widths and thicknesses by number of stories supported:

Adjacent footings on granular soil must be proportioned so the line between lower footing edges does not slope steeper than 30 degrees from horizontal per IBC §1809.6. Buildings on slopes steeper than 1V:3H require slope clearance analysis per IBC §1808.7 before foundation depths are finalized. Mat foundations on expansive soils must be designed per WRI/CRSI or PTI DC 10.5 procedures under IBC §1808.6.2.

When does IBC require a geotechnical investigation?

Geotechnical investigation is a formal subsurface study (borings, test pits, or equivalent exploration) required by IBC §1803 before foundation design proceeds under specific site conditions. IBC §1803.5 identifies the conditions that require investigation. Buildings assigned to SDC C, D, E, or F require a report per IBC §1803.5.11. Sites likely to contain expansive soils require a report per IBC §1803.5.3. Deep foundation projects require a report that includes recommended foundation types, center-to-center spacing, field inspection procedures, and material suitability per IBC §1803.5.5. Shallow foundations bearing on compacted fill greater than 12 inches deep require investigation per IBC §1803.5.8. Designers applying in moderate-seismic jurisdictions often assume geotechnical reports apply only to SDC D and above. IBC §1803.5.11 makes no such exception.

What concrete strength do deep foundations require?

Deep foundation concrete strength is governed by IBC Table 1808.8.1, which sets minimums by seismic design category and foundation element type. The baseline for SDC A, B, and C is 2,500 psi. The same minimum applies to residential structures two stories or fewer in SDC D, E, and F (Group R or U, light-frame construction). For all other deep foundation elements in SDC D, E, and F, the minimum rises to 3,000 psi. Precast nonprestressed driven piles, socketed drilled shafts, and micropiles require 4,000 psi regardless of seismic design category. Precast prestressed driven piles require 5,000 psi. Unbraced deep foundation elements in fluid soils need lateral support: per IBC §1810.2.1, support is provided at 5 feet into stiff soil or 10 feet into soft soil. Deep foundations left in place from previously demolished structures cannot be reused unless load tested or redriven to verify current capacity per IBC §1810.1.2.

How do expansive soils affect foundation design?

Expansive soils are highly plastic clays (typically classified CH in USCS classification) that swell when wet and shrink when dry, generating differential movement that can crack both foundations and structural slabs. IBC §1808.6 requires one of four design approaches for buildings on expansive soil: bearing within the active zone but designed to resist volume changes; extending below the active zone entirely; removing the expansive soil to the depth of constant moisture content per IBC §1808.6.3; or chemical stabilization of the soil mass. Slab-on-ground construction on expansive soils must be analyzed for both center lift and edge lift loading conditions. Per IBC §1808.6.2, analysis must use WRI/CRSI methods for non-prestressed slabs or PTI DC 10.5 for post-tensioned slabs. Soil testing comes before any design approach is selected: in areas likely to have expansive soil, IBC §1803.5.3 has the building official require soil tests to determine where such soils exist.

What does frost protection require for exterior footings?

Frost protection is a requirement under IBC §1809.5 that foundations and other permanent supports be protected from frost by extending below the local frost line, by constructing in accordance with ASCE 32 (frost-protected shallow foundations), or by erecting on solid rock. Footings cannot bear on frozen ground unless the frozen condition is permanent permafrost. The frost protection requirement extends beyond building perimeter walls. Per IBC §1809.5.1, exterior landings of required exits with outward-swinging doors also need frost protection, but only to the extent needed to keep the exit door able to open unobstructed. A limited exception applies to freestanding Risk Category I structures: buildings of 600 square feet or less (light-frame) or 400 square feet or less (all other construction) with eave height at or below 10 feet are exempt from frost protection per IBC §1809.5. All other structures must comply, and so must the exterior landings of required exits with outward-swinging doors. Frost depth varies by climate zone. Designers must confirm local frost depth from the authority having jurisdiction before specifying footing depths, since IBC minimums do not substitute for AHJ-confirmed frost lines.

The part everyone gets almost right — DataDrivenAEC

Using presumptive bearing values on organic soils, peat, or undocumented fill is a recurring foundation code violation in structural plan review. IBC §1806.2 says presumptive values are not to be assumed for these materials unless data substantiating a value are submitted, yet the mistake appears regularly when structural drawings are started before the geotechnical report arrives. Ignoring frost at exit door landings is another recurring error. The exterior landing at a required exit with an outward-swinging door needs frost protection under IBC §1809.5.1, separate from building perimeter footings. Specifying 2,500 psi concrete for deep foundations in SDC D is a third problem: in SDC D, E and F, foundations for all structures other than Group R or U light-frame buildings of two stories or less require 3,000 psi minimum under IBC Table 1808.8.1. The minimum rises to 5,000 psi for precast prestressed piles. The fourth error is reusing existing deep foundations without current load testing. Prior driving records from demolished structures are not acceptable substitutes for a load test or redriving verification per IBC §1810.1.2.

How DataDrivenAEC automates structural compliance checking