Residential Building Inspector B1Footings and FoundationsMedium
A residential building is being constructed on a site where the soil is classified as 'clay, sandy clay, silty clay, clayey silt, silt, or sandy silt.' According to the 2021 IRC, what is the presumptive load-bearing value of this soil type?
- A2,000 psf
- B1,500 psf
- C3,000 psf
- D4,000 psf
Show answer & explanationAnswer & explanation
Correct answer: B. 1,500 psf
According to IRC Table R401.4.1, 'Presumptive Load-Bearing Values of Foundation Materials,' soils classified as 'Clay, sandy clay, silty clay, clayey silt, silt, sandy silt' have a presumptive load-bearing value of 1,500 psf.
Why the other options are wrong
- A. Incorrect. 2,000 psf is typically for sandy gravel or gravelly sand.
- C. Incorrect. 3,000 psf is for gravel, sandy gravel, gravelly sand, or silty gravel.
- D. Incorrect. 4,000 psf is for crystalline bedrock.
Presumptive Soil Load-Bearing Value (Clay/Silt)
The assumed maximum allowable vertical pressure that a specific type of soil can safely support, as listed in the IRC, without requiring a geotechnical analysis.
- IRC Table R401.4.1 provides these values.
- Used for sizing footings in the absence of a site-specific soils report.
- Clay/silt soils generally have lower bearing capacities.
- Values are conservative to ensure safety.
Memory trick: Clay and silt are weak, so remember 'fifteen hundred' for their feet.