Final answer:
The ASTM Unified Soil Classification System classifies soils into four major groups: Gravel, Sand, Silt, and Clay. These classifications are based on particle size and plasticity characteristics, subdivided by fines content and Atterberg limits.
Step-by-step explanation:
The American Society for Testing and Materials (ASTM) Unified Soil Classification System (USCS) classifies soils into four major groups based on their particle size and plasticity characteristics. The system uses a set of laboratory tests to define these characteristics and classify the soil accordingly. The four major groups in the USCS are:
- Gravel (G)
- Sand (S)
- Silt (M)
- Clay (C)
Each of these groups is further subdivided based on the fines content (percentage of particles smaller than 0.075 mm in diameter) and their Atterberg limits, which measure the plasticity of the soil. For example, a soil classified as 'CL' would be a clay with low plasticity, while a 'GW' classification stands for well-graded gravel with good compaction characteristics. This classification system is used in civil engineering and geotechnical fields to predict soil behavior for construction and foundation design.