What defines the optimum moisture content of soil?

The optimum moisture content of soil is defined as the moisture level that allows for the greatest compaction density. When soil is compacted, the particles need to be arranged in a manner that minimizes the voids and maximizes the density. At this specific moisture content, the soil particles can slide past one another and fill in the voids more effectively due to the right amount of water acting as a lubricant.

If the moisture content is too low, there won't be enough water to allow effective compaction, resulting in lower density. Conversely, if the moisture content is too high, the excess water may create voids or lead to reduced inter-particle contact, also resulting in lower density. Thus, the optimum moisture content is a critical parameter in geotechnical engineering and construction, as it directly impacts the mechanical properties and stability of the soil.

This concept is distinct from the other options. For example, the maximum water that soil can hold refers to the soil's saturation point, which is a different measure than optimal compaction. The moisture level where soil starts to erode relates to the stability of the soil under various moisture conditions but does not pertain to compaction. Similarly, saturation occurs at 100% moisture content, which does not represent