IBC Soils and Foundations: Bearing Capacity and Foundation Types

TL;DR — Key Takeaways

• IBC Chapter 18 requires a geotechnical investigation for most new buildings unless presumptive bearing values (Table 1806.2) are approved by the building official for the site conditions.

• Presumptive bearing values in Table 1806.2 range from 1,500 psf (clay) to 12,000 psf (bedrock) — these are conservative defaults available without site testing.

• The geotechnical report must address soil classification, bearing capacity, depth to groundwater, expansive soils, liquefaction potential (SDC D–F), and foundation type recommendation.

• Spread footing, mat, and deep foundations (piles, drilled shafts) are all recognized; Chapter 18 provides design requirements for each.

• Expansive soils require special foundation design to resist the heave forces — typical mitigation includes deepened footings, pier-and-grade-beam systems, or post-tensioned slabs.

• Excavation and fill requirements under §1804 govern how adjacent structures are protected during excavation; a sheeting, underpinning, or lateral support plan is required where the excavation could destabilize adjacent construction.

• Special inspections for soils under §1705.6 require continuous inspection during placement of concrete for footings and pile driving.

The Geotechnical Investigation Requirement (§1803)

IBC §1803.1 requires a geotechnical investigation before foundation design is completed for any building where:

• The loads are significant (essentially all commercial and multi-family buildings)

• Expansive, compressible, or unstable soils are suspected

• The building is in Seismic Design Category C, D, E, or F (where liquefaction assessment is required)

• Fill is present on the site

Contents of the geotechnical report (§1803.6):

1. A description of the site including surface and subsurface conditions

2. Soil classification per ASTM D2487 (Unified Soil Classification System)

3. Allowable bearing pressure recommendations

4. Groundwater depth and seasonal variation

5. Recommendations for foundation type and depth

6. Fill requirements if the site requires grading

7. For SDC C and higher: liquefaction, lateral spreading, and differential settlement evaluation

The geotechnical engineer of record stamps the report. The structural engineer of record uses the recommendations to design the foundations.

Presumptive Bearing Values (§1806.2 and Table 1806.2)

Where an investigation has not been performed and the building official approves, IBC Table 1806.2 provides conservative presumptive bearing values by soil or rock class:

Material	Allowable Bearing Value (psf)
Bedrock (crystalline rock)	12,000
Bedrock (sedimentary/foliated)	4,000
Sandy gravel or gravel	3,000
Sand, silty sand, clayey sand (dense/compact)	2,000
Clay, sandy clay, silty clay (medium stiff)	1,500

Presumptive values are intended for small to medium structures on clearly identifiable soil types. For major structures, tall buildings, or uncertain soil conditions, a full geotechnical investigation is always the better approach.

Foundation Types Under IBC Chapter 18

Shallow Foundations: Spread Footings and Mat Foundations (§1808)

Spread footings — individual column footings or continuous wall footings — are the most common foundation type. Requirements:

• Minimum depth: Footings must extend below the frost line per §1809.5 (depth varies by jurisdiction — local frost depth tables govern; typically 12–42 inches in continental U.S.)

• Minimum width: Footings supporting masonry or concrete walls: 12 inches minimum (§1809.8)

• Bearing pressure: Total load ÷ footing area must not exceed the allowable bearing pressure from the geotechnical report or Table 1806.2

• Footings on rock: Must be keyed or anchored where the rock surface is sloped per §1809.3

Mat foundations (§1808.6) — continuous reinforced concrete slabs supporting the entire building footprint — are used where spread footings would be impractically large or where differential settlement must be minimized. Mat foundations are common for high-rise buildings on soft soil and for buildings in SDC D–F where the mat provides a uniform base for the seismic-force-resisting system.

Deep Foundations: Piles and Drilled Shafts (§1810)

Where bearing soils at shallow depth are inadequate, deep foundations transfer loads to stronger strata at depth.

Driven piles (§1810.3): Steel H-piles, precast concrete piles, or timber piles driven to refusal or to a design tip elevation. Require pile load testing per §1810.3.3.1 or use of engineering formulas with driving logs.

Drilled shafts (§1810.3.9): Also called caissons or bored piles — drilled holes filled with reinforced concrete. Common in urban construction where vibration from pile driving is not permitted.

Pile caps (§1808.2.3): All piles must be connected by a pile cap (a reinforced concrete beam or slab) that distributes column loads to the pile group.

Minimum pile capacity: IBC §1810.3.1 requires that each pile be designed based on approved load testing or geotechnical analysis, with a safety factor against both bearing failure and uplift.

Expansive Soils (§1808.6.2)

Expansive soils — clays with high plasticity that swell when wet and shrink when dry — can exert uplift pressures of several thousand psf. They are prevalent in the central and western U.S. (Texas, Colorado, Wyoming, California).

IBC §1808.6.2 requires that foundations on expansive soils either:

1. Be designed to resist the anticipated uplift forces (structural approach — piers extending below the zone of moisture variation, grade beam foundation), OR

2. Remove or replace the expansive soil within the zone of moisture variation (remediation approach), OR

3. Use a post-tensioned slab-on-grade designed to resist differential heave (PT slab approach — common in Texas and Colorado residential construction, less common for commercial)

The geotechnical report for an expansive soil site must provide the plasticity index, swell pressure, and recommended mitigation approach.

Excavation and Adjacent Structure Protection (§1804)

Where excavation is required for foundations or below-grade construction, IBC §1804 governs protection of adjacent structures.

§1804.1: The person responsible for the excavation must protect adjacent structures from damage due to excavation. This is a legal and code obligation — if the excavation causes damage to an adjacent building, the party responsible for the excavation is liable.

Sheeting and bracing (§1804.1): Where an excavation extends deeper than the foundation of an adjacent structure and within the angle of repose (typically 45° from the base of the adjacent footing), temporary sheeting, underpinning, or lateral bracing must be provided.

Notice to adjacent property owner: In most jurisdictions, a legal notice requirement exists to give adjacent owners the opportunity to monitor the condition of their structures during excavation.

Survey of existing conditions: Best practice (and required by some jurisdictions) to document the pre-excavation condition of adjacent structures with photographs and measurements.

Fill and Site Preparation (§1804.4)

Fill placed under foundations must be compacted and tested. IBC §1804.4 requires:

• Fill supporting foundations must be engineered fill (compacted to 90% of maximum dry density per ASTM D1557 or equivalent)

• Organic material, trash, debris, and expansive soils must be removed from below foundations

• Compaction testing (ASTM D1556 or nuclear densometer) is typically required at regular intervals — frequency specified by the geotechnical report

Uncontrolled fill — fill placed without inspection, testing, or documentation of materials — is not an acceptable foundation material under IBC and must be removed or stabilized before foundations are placed.

Special Inspections for Soils (§1705.6)

Chapter 18 foundation work triggers special inspection requirements under Chapter 17:

• Concrete footing placement: Continuous special inspection per §1705.3 for structural concrete in footings

• Pile driving: Continuous inspection during driving with monitoring of driving logs, tip elevations, and set

• Drilled shaft construction: Continuous inspection during drilling and concrete placement

• Compacted fill: Periodic inspection and testing during fill placement

Research Foundation Requirements for Your Site

Foundation requirements vary significantly by soil type, frost depth, seismic zone, and local amendments. Melt Code lets you search IBC Chapter 18 provisions and your jurisdiction's geotechnical requirements together.

Search foundation requirements for your site on Melt Code Try Melt Code →

Frequently Asked Questions

Q: Is a geotechnical report always required by IBC?

Not always — §1803.1 allows the building official to waive the investigation for light structures on clearly identifiable soil types using presumptive values from Table 1806.2. In practice, most commercial projects require a geotechnical report because the building official and structural engineer both need the soil data, and lenders often require it.

Q: How deep must footings be?

Below the frost line at minimum (§1809.5). The local frost depth is specified in the jurisdiction's adopted code or local amendment — commonly 12 inches in southern states and up to 42 inches or more in northern states (Minnesota, Wisconsin, Maine). Footings on expansive soils must extend below the zone of moisture variation, which can be much deeper.

Q: What is the difference between a pile and a drilled shaft?

A pile is driven into the ground by impact (hammer) or vibration. A drilled shaft (caisson) is drilled as a hole and then filled with reinforced concrete. Driven piles are faster but generate more noise and vibration. Drilled shafts are used in urban settings or where vibration would damage adjacent structures.

Q: When is liquefaction analysis required?

IBC §1803.5.12 requires a liquefaction hazard assessment for buildings in SDC D, E, or F. Liquefaction — the temporary loss of soil bearing capacity during an earthquake due to excess pore water pressure — is a risk in saturated, loose, fine-grained soils. The geotechnical engineer performs the analysis using subsurface investigation data.