SOIL STUDY

A Guide to Soil Classification in Geotechnical Engineering
Why We Classify Soils

Engineers, primarily geotechnical engineers, classify soils according to their engineering properties. This classification is essential for determining a soil's suitability for use as foundation support or as a construction material. 

Modern engineering classification systems are designed to provide a smooth transition from field observations to preliminary predictions of soil's engineering properties and behaviors.

The Unified Soil Classification System (USCS)

The most common engineering classification system for soils in North America is the Unified Soil Classification System (USCS). 

The USCS has three major classification groups:

• Coarse-grained soils (e.g., sands and gravels)
• Fine-grained soils (e.g., silts and clays)
• Highly organic soils (referred to as "peat")

The USCS further subdivides these three major classes to provide a more detailed and descriptive classification.

Other Classification Systems

While the USCS is dominant, other engineering soil classification systems are also used in the United States and internationally. 

These include the AASHTO Soil Classification System (commonly used in road and highway construction) and the Modified Burmeister system.

Beyond Classification: The Geotechnical Report

A soil's classification is just one component of a full analysis. A comprehensive geotechnical engineering report will also include many other properties of the soil, such as:

• Color
• In-situ moisture content
• In-situ strength (derived from field tests)

Additional details on the material properties of the soil that are not covered by the USCS code alone.