If the group index of soil is high, what does this indicate?

A high group index of soil indicates a higher content of fine particles, particularly clay. The group index is a measure used in soil classification systems, particularly in the context of geotechnical engineering and highway construction. A higher group index means that the soil has a greater propensity to swell, shrink, or retain water, characteristics that are typically associated with clayey soils. Clay has the ability to hold moisture and can undergo significant volume changes with fluctuations in moisture content, leading to a higher group index value.

In contrast, sandy soils tend to have a lower group index due to their larger particle sizes and lower plasticity. Gravelly soils also generally have a low group index because they consist of larger particles that do not retain water well. Silty soils can have intermediate properties, but they usually do not reach the same high values for the group index as clayey soils. Therefore, a high group index is strongly correlated with clay content, making it a valuable indicator in soil classification and engineering assessments.