Which material is typically not assumed to have a presumptive load bearing capacity for foundations?

Organic silt is typically not assumed to have a presumptive load-bearing capacity for foundations primarily due to its variable and often compressible nature. Unlike other materials such as granite, which is a hard and durable rock, or sand, which may demonstrate reliable drainage and support under certain conditions, organic silt contains decaying plant material and has inherently lower strength and stability. This organic content leads to unpredictable behavior under load, particularly when saturated, where it can experience significant consolidation and settlement issues.

Clay, while it can also present challenges due to its expansive properties, is more commonly analyzed and characterized for its load-bearing capability than organic silt. Therefore, in foundation design and soil analysis, organic silt is generally treated with caution, often requiring additional testing and consideration to determine its suitability for supporting structures.