When compacting a cohesive soil, why is achieving a moisture content slightly above the optimum often preferred over slightly below the optimum, if the target dry density is met?

When compacting a cohesive soil, the process involves mechanically increasing its dry density by expelling air from the voids. Cohesive soils, like clays, are characterized by fine particles that exhibit plasticity and stickiness when wet. Moisture content, which is the ratio of water mass to dry soil mass, significantly influences how these soils compact. There is an Optimum Moisture Content (OMC) at which a specific compactive effort achieves the maximum dry density for a given cohesive soil. The relationship between moisture content and dry density is typically represented by a compaction curve, which peaks at OMC. The question asks why, if the target dry density is met, a moisture content slightly above OMC (on the “wet side”) is often preferred over one slightly below OMC (on the “dry side”). This preference stems from the distinct engineering properties and internal structures developed by the cohesive soil on either side of the optimum, even when achieving the same target dry density. Compacting cohesive soil on the “dry side” of optimum means the moisture content is less than the OMC. In this condition, there is insufficient water to fully lubricate the soil particles and reduce inter-particle friction. As a result, the soil particles tend to form a....