Silty sands and poorly graded sand-silt mixes fall under which type of soil classification?

Silty sands and poorly graded sand-silt mixes are classified under the "SM" designation in the Unified Soil Classification System (USCS). This classification is specifically meant for soils that are a combination of sand with finer particles like silt, which are retained by a No. 200 sieve.

The "S" in "SM" stands for sand, while the "M" indicates that the soil contains silt, thus capturing the characteristics of silty sands effectively. These soils typically exhibit behavior that is influenced by both the sand's granular properties and the binding effects of the silt, which can affect drainage and stability. Proper identification of these types of soils is essential in geotechnical engineering and construction, as their mechanical properties can significantly influence the performance of foundations and other structures.

In contrast, the other classifications indicated represent different types of soils with distinct characteristics. For instance, "GW" is used for well-graded gravels, "SP" identifies poorly graded sands that do not contain silt, and "SC" designates clayey sands. Understanding these classifications helps professionals predict soil behavior under load and during environmental conditions.