The compaction behavior of sandy materials diverges significantly from that of clayey materials. Sands and gravels exhibit two distinct states of maximum density: completely dry and completely saturated. In the intermediate moisture content range, the achievable dry density falls below these maxima.
This anomaly is attributed to the influence of capillary forces within the sand matrix. When voids between soil particles are partially filled with water, these forces create a bonding effect, akin to elastic ties cementing the particles together. As noted by Lars Forssblad (1981), this capillary action hinders compaction efforts at intermediate moisture levels.
In contrast, the typical compaction curve for clay, which depicts a peak dry density at an optimal moisture content, is applicable to most soil types except sands and gravels. This is because even a small amount of clay renders soils impermeable, significantly altering their compaction behavior. Clay particles, due to their shape and surface properties, tend to bind together more readily with water, leading to a well-defined peak dry density at a specific moisture content.
Understanding these distinct compaction characteristics is crucial in geotechnical engineering. By recognizing the unique behavior of sandy and clayey materials, engineers can tailor their compaction strategies to achieve optimal density and stability, ensuring the long-term performance and durability of earthworks and foundations.