The Unified Soil Classification System (USCS) was developed by Arthur Casagrande, an Austrian-born civil engineer and geotechnical engineer. Casagrande's work in soil mechanics and geotechnical engineering significantly contributed to the development of soil classification systems.

- Development of the USCS:
Casagrande developed the USCS in the 1940s while working at the Harvard University School of Engineering. He recognized the need for a standardized system to classify soils based on their engineering properties. The USCS was first published in 1948 and has since become widely accepted and used in geotechnical engineering.

- Purpose of the USCS:
The USCS is designed to classify soils based on their physical and engineering properties, providing a standardized method for engineers and geologists to communicate and interpret soil characteristics. It is particularly useful in geotechnical investigations, foundation design, and construction projects.

- Classification criteria:
The USCS classifies soils into three major groups: coarse-grained soils, fine-grained soils, and highly organic soils. Each group is further divided into various subgroups based on their particle size distribution, plasticity, and organic content.

- Coarse-grained soils:
Coarse-grained soils include gravel (GW), sand (SW), and a combination of gravel and sand (GP, SP). They are classified based on their grain size distribution and can further be divided into well-graded (W) or poorly graded (P) subgroups.

- Fine-grained soils:
Fine-grained soils include silts (ML) and clays (CL). They are classified based on their plasticity, which is determined by the Atterberg limits. The plasticity index (PI) is used to further divide the fine-grained soils into low plasticity (L), medium plasticity (M), and high plasticity (H) subgroups.

- Highly organic soils:
Highly organic soils, such as peat (PT) and muck (MT), have a high percentage of organic matter. They are classified separately due to their unique engineering properties.

- Importance and applications:
The USCS is widely used in geotechnical engineering for site investigations, foundation design, slope stability analysis, and earthwork construction. It helps engineers and geologists understand the behavior and properties of soils, enabling them to make informed decisions and recommendations for construction projects.

In conclusion, the Unified Soil Classification System (USCS) was developed by Arthur Casagrande to provide a standardized method for classifying soils based on their physical and engineering properties. It has become widely accepted and used in geotechnical engineering for various applications.