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 because it is a material that often contains decomposed plant and animal matter. This organic content can lead to significant variability in soil properties, including compressibility, settlement behavior, and overall stability under load. As a result, organic silt may not provide a reliable or predictable foundation support compared to other soil types.

In contrast, other materials like clay, granite, and sand can exhibit more consistent engineering properties. For example, granite is a strong, competent rock that is known for its high load-bearing capacity. Sand, while it can be influenced by moisture content and grain size, generally provides good drainage and can also be evaluated for its bearing capacity based on its density and compaction. Similarly, clay can offer substantial support, although it requires careful consideration of various factors such as swelling and shrinkage characteristics.

Thus, organic silt's inherent properties make it less reliable for structural support, whereas other mentioned materials are more commonly used in foundation design calculations.