If a soil's Group Index (GI) is greater than 20, what does that signify?

A Group Index (GI) value is used to classify soil materials based on their strength and behavior, particularly in relation to their suitability as subgrade material in construction. A GI greater than 20 indicates that the soil has poor engineering properties, which can lead to unstable conditions and suboptimal performance when used as a subgrade.

In geotechnical engineering, soils are often evaluated using the GI to assess their potential limitations. Soils with a high GI are less desirable for construction because they may exhibit high plasticity, low strength, and a tendency to retain water, all of which contribute to reduced load-bearing capacity and increased risk of settlement or deformation under structural loads.

Therefore, a GI higher than 20 clearly signifies that the soil is poor for subgrade purposes, making it a critical consideration for engineers when designing foundations and other structural components.