Which of the following shall not be assumed to have a presumptive load bearing capacity for foundations?

Which of the following shall not be assumed to have a presumptive load bearing capacity for foundations?

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

Answer: (A)

Presumptive load bearing capacity means a soil type is assumed to reliably support a foundation without detailed site-specific strength testing. Organic silt cannot be treated this way because its strength is highly variable and it is very compressible. The organic content leads to poor shear strength and significant, time-dependent settlement, and its behavior changes with moisture, making it unreliable to rely on a presumption of bearing capacity. In contrast, crystalline bedrock, sandy gravel, and sedimentary/foliated rock are typically strong, stiff, and predictable under foundations, so they are appropriate to treat as having presumptive bearing capacity under standard conditions.

Presumptive load bearing capacity means a soil type is assumed to reliably support a foundation without detailed site-specific strength testing. Organic silt cannot be treated this way because its strength is highly variable and it is very compressible. The organic content leads to poor shear strength and significant, time-dependent settlement, and its behavior changes with moisture, making it unreliable to rely on a presumption of bearing capacity. In contrast, crystalline bedrock, sandy gravel, and sedimentary/foliated rock are typically strong, stiff, and predictable under foundations, so they are appropriate to treat as having presumptive bearing capacity under standard conditions.