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 flocculated structure, where particles are randomly oriented, often edge-to-face, creating a relatively open internal arrangement with larger, more interconnected voids. To achieve the target dry density on the dry side, a greater compactive effort is usually required to overcome the high inter-particle friction. The resulting compacted soil is typically stiff, rigid, and more brittle, meaning it is prone to cracking or fracturing under stress or minor deformation. Furthermore, due to its open, flocculated structure, it generally exhibits higher permeability, allowing water to pass through more easily. Most critically, because of the larger interconnected void spaces, this dry-side compacted soil has a greater potential for future swelling if it later becomes exposed to moisture, as water can easily enter and expand the soil structure, leading to volume changes and potential distress to overlying structures.

Conversely, compacting cohesive soil on the “wet side” of optimum means the moisture content is slightly greater than the OMC. In this scenario, the increased water content acts as an effective lubricant for the soil particles, significantly reducing inter-particle friction. This lubrication allows the soil particles to slide past each other more easily under compactive effort and achieve a more dispersed structure, where particles tend to orient themselves in a more parallel or face-to-face arrangement. Even if the same target dry density is achieved as on the dry side, the wet-side compaction results in a soil that is more workable during the compaction process and requires less compactive effort to reach that density. The resulting compacted soil is generally more plastic and ductile, meaning it can deform or yield more without fracturing, making it less prone to cracking under stress, settlement, or minor differential movements. The dispersed structure of wet-side compacted soil creates smaller, less interconnected voids, leading to significantly lower permeability, which reduces water flow through the soil. Crucially, being closer to saturation, this soil has a much lower potential for future swelling because it already contains more water and its void spaces are smaller and less interconnected, limiting further water absorption and subsequent volume change. This also often leads to a reduced potential for future shrinkage upon drying.

Therefore, achieving a target dry density slightly above the optimum moisture content is often preferred because it yields a compacted cohesive soil with superior engineering properties for long-term performance. This includes enhanced ductility and reduced brittleness, making the soil more resistant to cracking; lower permeability, which provides better resistance to water infiltration; and, most importantly, a significantly reduced potential for future volume changes such as swelling or shrinkage due to moisture fluctuations, leading to a more stable and durable foundation or embankment.