Effects of sand content and grain size on the compression behaviour of clayey sand

While extensive studies have been conducted on the compressibility of clay and sand, soils commonly encountered in natural environments or hydraulic reclamation projects are often gap-graded mixtures of coarse and fine particles, such as sand-clay or granular-fine mixtures. However, research on the compressibility of such special soils remains limited—particularly regarding the influence of sand particle size. To address this issue, sand-clay mixtures with varying sand-to-clay ratios, water contents, and three distinct sand size fractions (fine, medium, coarse) were prepared and subjected to one-dimensional consolidation tests to investigate their compression behavior and clarify the formation mechanism of the coarse-grained skeleton. The results indicate that the compression characteristics of the mixtures differ from those of conventional cohesive soils. Although the normalized index, the void index Iv, commonly used to evaluate the compressibility of pure clay, can be extended to sand-clay mixtures, distinct differences exist in the normalized compression behavior between pure clay and the mixtures, demonstrating its limited applicability. This discrepancy is fundamentally due to the formation of a sand skeleton in the mixtures under load. Before skeleton formation, compression is controlled by the clay, whereas after formation, it is governed by the sand skeleton. Using an established four-phase system for sand-clay mixtures and the parameters of clay void ratio and sand void ratio, this study quantifies, for the first time, how sand particle size influences skeleton formation thresholds. The critical stress for skeleton initiation increases with sand particle size, while the corresponding sand void ratio at formation decreases. SEM imaging provides microstructural validation of these particle size effects. The findings contribute to understanding the compression mechanism of sand-clay mixtures and offer valuable insights for settlement prediction in such soil foundations, particularly where sand gradation varies.