Which aspect does the Unified Soil Classification System consider?

The Unified Soil Classification System (USCS) is a widely used system for classifying soils based on their physical properties, specifically grain size and plasticity. This system helps in understanding the behavior of soils under various conditions, particularly in engineering and construction projects.

Grain size classification involves distinguishing between different types of soil particles, such as gravel, sand, silt, and clay, based on their size. Plasticity refers to a soil's ability to be molded or deformed when wet and how it behaves upon drying. Together, these two factors determine the soil characteristics that affect its suitability for construction and other engineering applications.

The other options focus on singular aspects of soil characteristics—compaction is just one consequence of soil density and does not encompass the broader aspects the USCS looks at; moisture content is important but does not influence classification in the same way grain size and plasticity do; and organic matter percentage, while relevant in other contexts, is not a primary factor in the Unified Soil Classification System. Thus, grain size and plasticity are the central elements considered in the USCS.