Which two criteria are deemed essential for proper soil classification using the Unified or AASHTO systems?

The correct answer focuses on grain-size distribution and Atterberg limits, which are indeed fundamental for proper soil classification in both the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials (AASHTO) system.

Grain-size distribution provides critical information about the relative proportions of different particle sizes within a soil sample. This is essential for understanding how the soil will behave under different conditions, such as load-bearing capacity and drainage properties. It helps in determining whether the soil is granular (like sand and gravel) or cohesive (like clay), which influences construction and engineering decisions.

Atterberg limits, on the other hand, are used to assess the plasticity and consistency of fine-grained soils. These limits include the Liquid Limit (the water content at which soil changes from a plastic to a liquid state) and the Plastic Limit (the water content at which soil changes from a plastic to a solid state). These values offer insights into the soil's behavior under varying moisture conditions and are critical for determining descriptions of cohesive soils.

Together, grain-size distribution and Atterberg limits allow engineers to categorize soils effectively, providing a basis for predicting their behavior in various engineering applications, such as foundation design, earthworks, and