Which material is least likely to have a predictable load-bearing capacity for foundations?

Which material is least likely to have a predictable load-bearing capacity for foundations?

• A. organic silt
• B. crystalline bedrock
• C. sandy gravel
• D. sedimentary and foliated rock

Answer: (A)

Load-bearing capacity is most predictable when the soil or rock has consistent strength, drainage, and little time-dependent change. Organic silt stands out because it varies a lot in composition, moisture, and organic content. The organic material can decompose, absorb or lose water, and bioactivity can alter strength over time, causing large changes in bearing capacity and settlement that are hard to foresee. This makes it highly variable and unreliable for foundations. In contrast, crystalline bedrock is very strong and behaves in a predictable way under load, giving stable bearing capacity. Sandy gravel is a granular, well-draining material that tends to compact or shear in a predictable manner, allowing standard calculations to estimate capacity fairly reliably. Sedimentary and foliated rocks can have fractures or layering, which introduces some variability, but engineers can often assess and account for those properties with rock testing and site investigation. Because of its inherent variability and time-dependent behavior, organic silt is the least likely to provide a predictable load-bearing capacity for foundations.

Load-bearing capacity is most predictable when the soil or rock has consistent strength, drainage, and little time-dependent change. Organic silt stands out because it varies a lot in composition, moisture, and organic content. The organic material can decompose, absorb or lose water, and bioactivity can alter strength over time, causing large changes in bearing capacity and settlement that are hard to foresee. This makes it highly variable and unreliable for foundations.

In contrast, crystalline bedrock is very strong and behaves in a predictable way under load, giving stable bearing capacity. Sandy gravel is a granular, well-draining material that tends to compact or shear in a predictable manner, allowing standard calculations to estimate capacity fairly reliably. Sedimentary and foliated rocks can have fractures or layering, which introduces some variability, but engineers can often assess and account for those properties with rock testing and site investigation.

Because of its inherent variability and time-dependent behavior, organic silt is the least likely to provide a predictable load-bearing capacity for foundations.