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

Organic silt is not assumed to have a presumptive load-bearing capacity for foundations due to its inherently variable and often weaker physical properties. Organic materials, such as the decomposed vegetation present in organic silt, can significantly impact the stability and strength of the soil. As the organic materials decay, they can lead to changes in moisture content and density, making the material less reliable for supporting structures.

In contrast, sand, silt, and clay are generally more predictable in terms of load-bearing capacity, depending on their composition and soil mechanics. Sand typically provides good drainage and has a stable structure, while clay and silt can exhibit differing load capacities based on their moisture content and consolidation characteristics. Organic silt, however, lacks this consistency and is often treated with caution in construction due to its potential for settlement and instability when subjected to load. Therefore, engineers and contractors avoid assuming a presumptive load-bearing capacity for organic silt in foundation design and planning.