What is the significance of the percentage of soil passing through a No. 200 sieve?

The percentage of soil passing through a No. 200 sieve is a crucial parameter in soil classification systems, specifically in the Unified Soil Classification System (USCS) and the AASHTO classification system. This sieve corresponds to a mesh that allows particles smaller than 0.075 mm (75 micrometers) to pass through, which helps to distinguish between fine-grained and coarse-grained soils.

When evaluating soil for engineering purposes, understanding the amount of silt and clay in a sample is vital, as these fine particles significantly influence the soil's engineering properties, such as shear strength, compressibility, and permeability. Therefore, by determining the percentage of soil passing through the No. 200 sieve, engineers can classify the soil within a specific group that reflects its behaviour and suitability for various applications, including construction and foundation design.

This classification plays a critical role in predicting how the soil will perform under loads, its compaction characteristics, and its drainage capabilities, which are essential considerations in civil engineering practice.