Understanding Fine-Grained Soil Particle Size

Soil is classified into different types based on the size of its particles. This classification is fundamental in geotechnical engineering as particle size significantly influences soil properties like drainage, strength, and compressibility.

Soils are broadly categorized into coarse-grained soils and fine-grained soils. Coarse-grained soils include gravel and sand, while fine-grained soils consist of silt and clay.

The key difference between coarse-grained and fine-grained soils is the particle size boundary. This boundary is internationally recognized and is based on standard sieve sizes.

The particle size that separates fine-grained soil (silt or clay) from coarse-grained soil (sand) is 0.075 mm.

• Particles larger than 0.075 mm are generally considered part of the coarse-grained fraction (sand and gravel).
• Particles smaller than 0.075 mm are classified as fine-grained soil, which includes silt and clay.

This specific size, 0.075 mm, corresponds to the opening of the U.S. Standard No. 200 sieve. Therefore, soil particles that pass through a No. 200 sieve are considered fine-grained.

Here is a general representation of soil particle size ranges:

Based on this classification, soil with a particle size less than 0.075 mm is defined as fine-grained soil (silt or clay). This cutoff is crucial for determining the soil's behavior under various engineering loads and conditions.

Revision Table: Soil Particle Size Limits

Reviewing the standard particle size limits helps reinforce the definition of fine-grained soil.

Additional Information on Fine-Grained Soil Classification

While 0.075 mm is the upper limit for fine-grained soil, silt and clay are further differentiated, usually at 0.002 mm. However, distinguishing between silt and clay is often based on plasticity characteristics rather than solely on particle size due to the challenges in accurately measuring such small particles.

• Silt: Fine-grained particles generally ranging from 0.002 mm to 0.075 mm. Silt feels like fine sand or flour when dry and gritty when wet. It is typically non-plastic or slightly plastic.
• Clay: Fine-grained particles smaller than 0.002 mm. Clay particles are often plate-like and exhibit plasticity, meaning they can be molded into shapes when wet and retain their shape upon drying. Clay feels smooth and sticky when wet.

The 0.075 mm size is a critical division point in soil classification systems like the Unified Soil Classification System (USCS) and the AASHTO system, used widely in civil and geotechnical engineering.